Water Heating

Definition

Commercially available integrated and split electric heat pump water heaters (HPWHs) meet the demands of residential households and small businesses, delivering hot water two to five times more efficiently than conventional electric resistance, standard gas water heaters, or fossil-fuel-fired water heaters. Innovations — including plug-in 120V and convertible electrical inputs, low-GWP refrigerants, load-shifting capabilities, and alternative form factors — offer increased utility and environmental benefits while supporting diverse retrofit applications.

Research Initiatives

Research Initiatives	Performance Validation Needs	Market Analysis Needs	Measure Development Needs	Program Development Needs
120V & Convertible 120/240V Residential
Small Commercial
240V Unitary
Split Systems & Small Form Factor
Low-GWP Refrigerant
Connectivity & Load Shifting

Opportunities

• Storage HPWHs are a critical residential and small commercial building decarbonization strategy, providing a cost-effective, electric water heating solution for load reduction and shifting peak during carbon-intensive utility periods.
  • There is enormous potential for load shifting and shaping, load management, and the resulting greenhouse gas (GHG) reduction from shifting the heating schedule to times when the electricity grid has a lower marginal emissions rate and cost to operate.
• Additional performance validation, measure, and program development will be important to accurately assess impacts and adoption pathways of split systems, 120V and convertible 120V/240V convertible unitary, and solar-assisted models.
  • It will also be necessary to evaluate the interaction of HPWHs with recirculation systems and optimizing designs, including HPWH selection, upsizing storage capacity, and utilization of integrated or master mixing valves. Standardization of first hour and recovery ratings for HPWHs and installer training regarding appropriate design and installation best practices to account for recovery rates of different HPWH solutions will be necessary to ensure satisfactory performance for all residential users.
  • Significant gains have been made in developing best-practices guides for the sizing and installation of HPWHs; however, additional research is required to assess design solutions and applications in common recirculation pump designs found in many California homes and small businesses.
• Shifting to low-GWP refrigerants, e.g., CO2, offers increased performance, higher water storage temperature capabilities, and other direct GHG emission benefits to eliminate the impacts of refrigerant leakage.
  • To support the market shift, new low-GWP performance validation, especially at higher storage temperatures, accompanied by mixing valves, will be necessary to inform updates to measure and program development.
• Planned zero-emission regulations on residential water heaters in the Bay area and statewide are based on reducing indoor exposure to combustion gases to households.
  • The assessment and attribution of non-energy benefits (NEBs) — such as health impacts from air quality and the development of innovative solutions for increasing equitable access to HPWHs — are important for supporting an accelerated transition away from gas water heaters.
• Identifying opportunities for cost-compression of equipment and installation costs is necessary for equitable, scalable HPWH market development. Additionally, in order to establish a sustainable and sufficiently capitalized incentive program that enables accelerated adoption of HPWHs, it is critical to assess existing and new, innovative, and equitable financing mechanisms, deployment interventions, and behavioral programs.
• Replacement of “easy access,” e.g., garages, gas electric resistance water heaters (seven percent of the California market) and the new construction market offer the quickest opportunities for market adoption and TSB. Program designs that support electric resistance to HPWH incentives and enhanced builder engagement strategies are needed to support these market sectors.
• Plug-in 120V HPWH models offer disadvantaged community (DAC) and hard-to-reach (HTR) renters and homeowners a lower-cost and efficient alternative to 240V HPWH fuel-switching retrofits, overcoming common financial barriers and avoiding costly electrical infrastructure and panel upgrades.
  
  

Barriers

HPWHs face many barriers, but most of them are due to market and installation practices, not shortcomings in the technology itself. HPWHs have installation challenges and operational features not found in common gas storage alternatives that can make fuel-switching challenging:

• Cost-effectiveness of HPWH fuel substitution can be negatively impacted due to high HPWH equipment and installation costs, current electrical and gas tariff structure, customer concerns about electric cost trends, and/or grid integration incentives.
• Plumbing contractors generally lack awareness of HPWHs and expertise in the design and installation, including the disposal of condensate and ensuring adequate airflow and venting for proper performance.
• Electrical service or panel upgrades can be a significant cost barrier; market expertise and guidelines are needed to describe when load management strategies or 120V products may be deployed to mitigate the expense of an upgrade.
• Emergency replacements are the most common scenario for a new water heater installation in existing homes, which creates immediate cost, time, and complexity barriers to conversion from conventional gas water heaters, including a separate electrical contractor. Regional initiatives are exploring streamlining of permitting processes to mitigate this issue.
• Split-incentive issues between property owners and rental ratepayers complicate the costs and benefits of HPWHs.
• Space, comfort, and noise issues can undermine the suitability of unitary HPWHs, requiring a smaller form factor, split designs, or relocation.
• The introduction of new low-GWP natural refrigerants will require reevaluating their performance, as well as addressing permitting barriers and perceptions of safety risks with local inspectors.
• There is an absence of a savings claims infrastructure, including electronic Technical Reference Manual (eTRM) load shapes, rules for quantifying load shape benefits, rules for viable electric alternative measures to replace gas appliances, and the coordination on refrigerant leak reduction efforts.
• Mobile and manufactured homes, which are common in DAC and HTR communities, have unique challenges for the installation of HPWHs due to limited electric panel capacity (30A–100A), wiring concerns, and state agency rules on exterior installation.
• Typical water heater closets found in mobile homes have insufficient space (volume measured in cubic feet) and venting for the normal operation — let alone optimal performance — of HPWHs, which makes this technology unfeasible for fuel-switch scenarios without additional structural remediation.
• High water temperatures required by health and building codes for small commercial buildings may limit opportunities for HPWH applications and reduce performance. High setpoints promoted for demand flexibility may also reduce efficiency.

Definition

Domestic hot water (DHW) systems are among the largest end uses poised for decarbonization. HPWH options have a higher efficiency than electric resistance and gas alternatives and can achieve dramatic energy and GHG savings. This technology research area covers efficient, demand-flexible DHW systems for multifamily and nonresidential applications, such as offices, hotels, healthcare, and foodservice. Hot water systems under this group may include a primary heat source (e.g., heat pump), storage, distribution, recirculation, pumping, valves, controls, temperature maintenance systems, heat recovery, and alternative heat sources (e.g., solar or geothermal).

Research Initiatives

Research Initiatives	Performance Validation Needs	Market Analysis Needs	Measure Development Needs	Program Development Needs
Split HPWH
Unitary HPWH
Load Flexibility Controls
Dual Fuel Systems
Distribution System Optimization & Recirculation
Heat Recovery

Opportunities

There are many important targets of research, development, and market transformation across different technologies, designs, and market segments within this research area, each with their own needs. These projects could be executed as field demonstrations, technology development, lab studies, market studies, modeling, market transformation tools, or novel program delivery mechanisms. The state of understanding and research needs may differ, based on design configuration (e.g., integrated, split, central, or clustered), segment (e.g., education, hospitality, healthcare, office, foodservice, or multifamily), or building vintage (i.e., new construction or retrofit).

Opportunities to address include:

• Reducing DHW system energy use and improving efficiency, designing without backup or temperature maintenance electric resistance (such as return-to-primary configurations), and reducing the complexity of all-electric centralized HPWHs.
• Quantifying defrost derate for output capacity of split heat pumps to ensure designers are aware of specific needs depending on refrigerant type and split HPWHs models. Sufficient backup electric resistance or additional heat pumps may be necessary to ensure capacity is maintained on defrost design day.
• Exploring innovative program designs that can bring benefits of HPWHs to DACs. Programs could quantify and enhance the benefits of commercial HPWHs for DACs by passing benefits to tenants, improving air quality, innovating financing mechanisms, or adopting other innovations in program delivery.
• Developing incentive programs for medium and large low-GWP commercial HPWHs in nonresidential and multifamily buildings. Innovative program designs can ensure the multiple value streams of efficiency, decarbonization, and grid-integration are all actualized.
• Demonstrating overseas HPWH technologies that use new low-GWP refrigerants and other form factors — such as low-GWP integrated HPWH or 20- to 30-gallon integrated HPWHs — for distributed point-of-use applications.
• Reducing cost and space requirements through new designs, configurations, and equipment selection of HPWHs, temperature maintenance, and storage tanks. This could be achieved in multiple ways, including technology development, field demonstrations, or the design guideline development.
• Designing and field assessing dual-fuel DHW systems to address the needs of high-load, rapid-recovery applications — such as existing commercial kitchens, hotels, and multifamily buildings with gas water heaters — to back-up, trim, boost, and act in secondary to a primary HPWH. Dual-fuel hot water systems may present an opportunity to reduce first cost, deliver both gas and electric savings, and temper performance uncertainties by leveraging existing gas infrastructure during market transition, especially in the gas retrofit market.
• Demand flexibility controls demonstration and implementation guidance. Optimization of load flexibility controls to minimize energy costs and GHG emissions. Technical and program-required recommendations for streamlined onboard load shift programming for HPWH systems.
• Evaluating automated, algorithmic load shift controls based on input parameters — such as monitored system operation, system capacity, hot water loads, total building coincident electrical demand, and utility rates (real-time or scheduled time-of-use).
• Assessing new unitary commercial HPWH designs above 12 kW that offer more versatility for application in existing buildings, with features such as direct ducting options, minimum efficiency reporting valve (MERV)-rated inlet filter for grease and lint capture, additional operating modes that fully lock out the electric resistance operating mode, and integrated controls to optimize usage, cost, and GHG, based on time-of-use factors such as peak rate periods and utility flex alerts.
• Bringing clarity to designers for cost-effective drain water heat recovery scenarios and using recovered heat as a heat pump source reservoir.
• Incorporating integrated exhaust air or refrigerant heat recovery systems at the water heater or point-of-use equipment location and exploring capture and recovery of cooling effects of HPWHs to offset cooling loads, such as in a commercial kitchen.
• Optimizing distribution systems through novel recirculation and load-matching control strategies, such as automatic balancing valves, combined optimization of temperature modulation, variable speed pumps with integrated constant return temperature control or occupancy-based inputs, distributed isolating valves, and pipe insulation.
• Incorporating high-performance master mixing valves to increase thermal storage capacity and utilization, increase the tank temperature stratification with continuous recirculation, and reduce recirculation loop heat losses through precise control.
• Exploring innovative program pathways and strategies for supporting the remediation and upgrades of existing, recirculating DHW distributions systems, e.g., pipe insulation, pipe hangers, shower crossover repair, balancing, pump controls, and more.
• Improving system efficiency through clustered centralized systems.
• Enhancing energy density and load matching of solar thermal and photovoltaic (PV)-assisted water heater designs.
• Increasing thermal energy storage density by using phase-change materials for increasing renewable energy penetration and load flexibility while assuring reliability and durability.
• Assessing innovative utility rate structures or dedicated metering to facilitate decarbonization by mitigating operating cost burdens on building owners.
  
  

Barriers

Commercial HPWH systems are still in a nascent technological stage that continues to evolve. Physical space, electrical infrastructure, and installed costs are major upfront barriers that have slowed HPWH adoption in retrofit nonresidential and multifamily applications. Of particular concern are escalating operating costs and affordability, as the electricity-to-gas cost ratio per unit of energy is approximately six, significantly higher than recent years. Other limitations include product availability of low-GWP four-season heat pumps, weight, and noise.

Researchers and design firms have developed better sizing tools to right size heat pumps for mitigating cost and space requirements in multifamily buildings, but similar tools are needed for many other commercial HPWH applications. Current health department sizing requirements do not address the use of HPWH systems in commercial kitchens and do not account for storage volume as a factor in sizing water heater capacities. Recirculation systems, although important to improve hot water delivery time and minimize water waste, can heavily impact water heater performance in central multifamily and commercial buildings.

Barriers to address include:

• Lack of diverse commercial-duty integrated heat pump products, such as 120- to 200-gallon HPWHs, as well as lack of outside installation, ducting, and high-performance air inlet filter options.
• Lack of field performance data of various designs, configurations, and applications, including system reliability and cost-effectiveness, to enhance industry knowledge and confidence in various HPWH technologies, products, and use cases.
• Lack of easy-to-access case studies that span the diversity of buildings with commercial HPWHs (120V-integrated HPWHs, 240V-hybrid-integrated HPWHs, indirect central HPWHs, and HPWHs combined with complementing distribution strategies, including point-of-use heaters, heat recovery, master mixing valves, balancing, and others).
• Regulatory barriers to R290 HPWH development and adoption.
• Lack of statewide incentive programs for medium and large commercial HPWHs for businesses.
• Lack of design tools to select, appropriately size, and model HPWHs outside of multifamily applications.
• Minimal documentation and empirically determined hot water load profiles for various nonresidential building types.
• Ways to streamline electrical panel upgrades to support HPWHs or alternative technologies to minimize or eliminate the need for upgrades.
• Lack of HPWH familiarity for building permitting authorities and health departments.
• Lack of coordination between trades, e.g., engineering design, electrical, and plumbing.
• Changes in the tariff structure and grid integration incentives to mitigate cost-effectiveness concerns.
• Lack of code readiness activities to support electric ready code requirements for all nonresidential building types that utilize commercial water heaters.
• Lack of demonstration, guidance, and simplified implementation procedures of dependable demand flexibility and load-shifting controls.
• Lack of trusted distribution system software tools and design guides.
• Lack of trusted software tools and design guides to simplify solar hot water system designs.
• Lack of consistency among code requirements related to hot water setpoint temperatures.
• Lack of experience deploying drain water heat recovery, particularly with the variety of potential heat sources.
• Lack of experienced practitioners who can bring quality commercial-HPWH systems to building owners.
• Potential efficiency and reliability degradation with improperly designed or maintained recirculation systems served by large HPWHs.
• Legionella risk should be addressed through technological means to expand HPWH system use cases and confidence.
• Limited business case and value proposition for contractors to promote and install HPWHs due to higher installation complexity and costs.

Definition

Residential multifunction heat pumps (MFHPs) — also known as combi heat pumps, combination heat pumps, or integrated heat pumps — use an efficient compressor system to serve both the space conditioning and water heating requirements of a household. These systems may be air-to-air or air-to-water. MFHPs can come in multiple formats: two-function or combination heat pump systems — which serve space heating and water heating demands — and three-function MFHPs, which also provide space cooling.

Note: This technology family is cross listed with the HVAC TPM.

Research Initiatives

Research Initiatives	Performance Validation Needs	Market Analysis Needs	Measure Development Needs	Program Development Needs
Two-function: Water Heating & Space Heating
Three-function: Hot Water, Space Heating & Space Cooling
Selection Guidelines

Opportunities

Residential MFHPs offer a novel pathway to decarbonization, providing an efficient alternative to existing gas-fired equipment or to an arrangement of multiple heat pumps, e.g., HPWH and a packaged central heat pump. MFHPs can potentially replace space heating, space cooling, and water heating with a single system, depending on the configuration and design. MFHPs have the potential to provide much higher total system benefits by extending the benefits of thermal storage to space heating and potentially space cooling. In addition, the single heat pump may free up a home’s electrical panel capacity for other electrification uses, and some air-to-water systems may be deployed with lower overall refrigerant charge than current heat pump systems due to the replacement of the refrigerant line to the air handler and water heater tank with a hydronic line.

Defrost cycles may also benefit from a MFHP configuration using DHW, allowing a faster, more efficient heat pump defrost without cold air comfort issues. In some three-function systems, it is also possible to recover waste heat during space cooling to use for water heating.

MFHPs are relatively new to the US market and, as a result, there are many opportunities to improve the understanding of their performance and impact on the residential sector. Manufacturers are introducing products combining DHW and space conditioning outside of shared heat pumps, such as HVAC units using heat recovery from space cooling to supply DHW. Examples of opportunities for research include:

• Evaluating the performance of MFHP systems. The US market for MFHP systems is in a nascent state, with residential MFHP systems currently being developed or adapted to US homes. Work is needed to characterize the performance of these systems, including laboratory testing of MFHPs to evaluate system performance in various applications and field demonstration and performance validation of MFHPs in new construction and existing building applications.
• Assessing the market of MFHPs for California homes. MFHP systems may have unconventional installation requirements, for example, refrigerant lines between an outdoor unit and water heater. More information is needed about the opportunities and challenges of installing these systems, including cost and requirements associated with an MFHP installation in new construction and existing buildings.
• Conducting TSB analysis of MFHPs. More work is needed to understand the TSB of MFHP technologies compared to existing technologies that provide water heating and space conditioning. Examples include assessing the energy performance, demand flexibility, fuel substitution, and refrigerant emissions compared with the efficiency of a single-function separate heat pump, HVAC, and water heating equipment.
• Assessing the bill impacts and customer economics of MFHPs. Work is needed to understand economic impacts of residential MFHPs on the customer, including total cost of operation, operating costs under current rate structures, and increased value of load shed compared with the efficiency of a single-function separate heat pump, HVAC, and water heating equipment.
• Developing modeling tools to compare various MFHP types. Energy modeling is an important tool often used in comparative analyses of water heating and space conditioning equipment. Work is needed in this area to develop tools for modeling MFHP systems that will offer insight into their operation compared to other systems in a variety of scenarios, guide program development, and support early adopting market actors.
• Assessing the workforce needs. MFHPs are unique in their installation procedures and services requirements, often requiring both plumbing and HVAC installers. More research is needed to inform the training needs of the workforce that will be installing and servicing MFHPs, including upselling practices to customers, comfort level of installation, and maintenance needs.
• Developing best practices for residential MFHP commissioning, service, and maintenance. Work is needed to understand best practices to ensure customer comfort using MFHP systems in the United States, including validating customer comfort expectations and ensuring that proper hot water temperature and space temperatures can be met.
  
  

Barriers

As an emerging technology in the US market, there are many barriers to MFHP adoption that may be addressed. Understanding the performance of MFHPs in the context of US homes, the development of testing and installation standards, and the development of equipment selection guidelines are all necessary for understanding the efficacy of MFHPs in meeting California’s decarbonization goals and encouraging MFHP use in California. Specific barriers include:

• Absence of standardized testing procedures for MFHP evaluation. Test procedures for MFHPs are in the development phase, and it may be difficult to apply test standards to some MFHP systems, considering the variability in operational modes and system configurations.
• Lack of MFHP product offerings compared to international markets. International markets, particularly those where hydronic heating is common, have a greater number of MFHP products on the market. These systems often have a capacity range lower than that typically required in the US market and may use refrigerants that are not commonly used in the United States.
• Limited understanding of the capabilities of the MFHP system in managing occupant thermal comfort. More understanding is needed on the capability of MFHP systems to meet the thermal comfort of occupants in the United States, particularly in extreme climates.
• Absence of a standardized installation procedure and contractor knowledge. Contractors may be reluctant to recommend MFHP systems due to the lack of standardized installation procedures and workforce knowledge regarding these systems.
• Absence of understanding of the efficiency of MFHPs compared to independent systems. There are questions about the benefits of MFHP systems compared to systems independently providing water heating and space conditioning. More work is needed to evaluate whether programs should be developed to promote these systems.
• Need for a market assessment of MFHP for California homes. A lack of understanding of the cost and requirements associated with MFHPs in new construction and retrofits makes them less likely to be adopted in the market.
• Absence of MFHP modeling and design tools. The lack of modeling and design tools reduces understanding of these systems compared to alternatives.

Definition

Commercial multifunction air-to-water heat pumps (AWHPs) can serve water heating and space conditioning needs for multifamily or nonresidential buildings. This multifunction category includes combination AWHPs that provide DHW and space heating only, as well as systems that additionally provide space cooling. These systems use refrigerant to move thermal energy in air-to-hydronic and air-to-forced air space conditioning distribution systems. They typically can provide two or three functions simultaneously.

Research Initiatives

Research Initiatives	Performance Validation Needs	Market Analysis Needs	Measure Development Needs	Program Development Needs
Combination: DHW & Space Heating
Two-function: DHW & Space Cooling
Multifunction: DHW, Space Heating & Cooling
Sizing Methodology
Modeling & Software Tool Development
Test Method Development

Opportunities

Multifunction AWHPs that can provide multiple hydronic services to a building can address efficiency and decarbonization market needs across multifamily and nonresidential sectors.

Opportunities to address include:

• Testing laboratory applications and field demonstrating various multifunction AWHP systems in new construction and existing buildings.
• Developing sizing toolsbased on building load inputs and development of multifunction AWHP performance maps.
• Developing modeling and software tools to be validated with laboratory and field demonstration data.
• Assessing electrical infrastructure impacts, especially in retrofit applications. Will multifunction hydronic heat pumps reduce the need for electrical service or panel upgrades when decarbonizing existing buildings? Can a multifunction hydronic heat pump use existing chiller electrical service?
• Conducting studies on retrofitting existing buildings with variable refrigerant flow (VRF) systems with multifunction hydronic heat pumps. In applications where VRF systems are failing, multifunction hydronic heat pumps may be a cost-effective decarbonization solution with lower refrigerant charges.
• Assessing the benefits in space, cost, energy, peak power, and GHG emissions relative to decarbonization solutions that rely on separate heat pumps for DHW, cooling, and heating.
• Integrating thermal energy storage and quantifying of the amount of energy available for load flexibility.
  
  

Barriers

Barriers to address include:

• Manufacturers, researchers, programs, and regulators need standardized test methods for combination and multifunction AWHPs with native controls that mimic real world conditions and operation for all products and all configurations.
• Multifunction AWHP efficiency may not be as high as separate heat pumps for DHW, cooling, and heating. This requires improved understanding of the definition of multifunction system efficiency and how controls, heat recovery, and system design can increase efficiency in multifunction AWHPs.
• Impacts on occupant comfort are not known. This requires improved understanding of controls, capacity, and system design that can maintain occupant thermal comfort, e.g., determining whether simultaneous water heating and space heating loads are being met.
• Multifunction AWHP technologies are more popular in international markets, particularly those in which hydronic heating prevails. More research is needed to understand and address the barriers to entry and possibilities in the US market.
• Load flexibility of multifunction AWHPs has not been explored. Controls that incorporate function switching, thermal energy storage, and load up and shed require modeling, development, and testing.
• Early adopter approaches are often custom-engineered, site-built systems. Packaged designs are needed for design, equipment, installation, and commissioning cost compression.

Definition

This technology family encompasses electric pool heaters for residential and nonresidential pool markets; pool pumps and pool controls designed for the residential and nonresidential pool market to increase efficiency, performance, and enable load shifting; and alternative strategies for pool heating and maintenance. The technology family will help support the development of all-electric codes and ease pool heating loads to improve grid resiliency.

Research Initiatives

Research Initiatives	Performance Validation Needs	Market Analysis Needs	Measure Development Needs	Program Development Needs
Hot Tub Heat Pump Pool Heaters
Residential Heat Pump Pool Heaters
Commercial Heat Pump Pool Heaters
Commercial Variable Speed Pool Pumps

Opportunities

Opportunities in this technology family will increase efficiency with optimized equipment and designs, including optimized electrification of pool heating loads, pool operation controls and the incorporation of load shifting of electric pool loads. Prospective emerging technology studies should include controls solutions, design guides, or demonstrations that address:

• Projects that demonstrate the emergence of new technical innovations, such as smart controls, variable speed, hybrid units, low-temperature operability, and staggered start-up capability with heat pump pool heaters (HPPHs).
• Projects that demonstrate electrification of pool heating loads as part of home electrification service assistance. Projects may include those that have homes with PV solar or plans for PV to be installed soon.
• Projects that encourage HPPH or solar-assisted HPPH adoption in new home construction or newly constructed pools.
• Innovative pool cover projects that encourage the consistent use of pool covers to enable the wider adoption of HPPH. The project could include novel methods to reduce the installation and maintenance costs of automated pool covers.
• Innovative projects to address electrical system requirements when the existing pool heater uses natural gas.
• Innovative applications of HPPH to provide heating to inground spas. These projects would demonstrate the utility of hybrid HPPH to provide spontaneous heating.
• Innovative projects to demonstrate ability of electrical system to accommodate startup surges due to HPPH compressor operations and accommodation of other emergent loads on the home electrical system, such as heat pump and electric car charging.
• Projects that demonstrate the load-shifting potential for both pool heating and pumping in coordination with proposed flexible demand appliance regulations by the California Energy Commission.
  
  

Barriers

HPPH installation faces opposition where high electric rates discourage the adoption of electric heating.

HPPHs lose heating capacity as temperatures decrease. While not a concern for spring, summer, and fall heating seasons, many climate zones within California present challenges to economical heating from HPPHs during the winter months, due to an increased heating load and decreased heating capacity.

Alternative hot water design strategies are an important approach to decarbonize many hard-to-electrify water heating scenarios. Emerging technology investments in this technology family can help bring greater awareness and highlight alternative decarbonization pathways.

Potential barriers studies should address include:

• Inconsistent design software.
• Ongoing practice to oversize heaters and pumps.
• Learning curve on pool heating operation with a HPPH versus a gas-fired pool heater regarding set back temperatures.
• Limitations based on health code requirements.
• Roof space for solar thermal competing with PV.