Our homes are more important than ever: they are our offices, schools, entertainment, and, at times, the only sit-down restaurant in town. So, having a home that powers itself, even when there’s a power outage, equates to huge peace of mind. How is this possible? By pairing rooftop solar and backup battery systems. Assessing and upgrading your home’s energy system with batteries can improve resilience, security, and comfort, and increase home value. If you’re considering or already have a solar photovoltaic (PV) system, a backup battery system can be the final piece of the energy puzzle.

When paired with a rooftop solar system, backup batteries allow homeowners to avoid peak usage and power their homes during outages. Gif animation courtesy Lillian Shobe.


Cool Davis is hosting a webinar on Battery Backup Systems Sept 29, 2020, 5:30pm to 7:00pm. We will go over the basics, feature expert panelists, and open up opportunities for Q&A. Register today then check out other solar and electric vehicle events during the Cool Davis Driving on Sunshine Week Sept 28 through Oct 2, 2020. Co-hosted by the Davis Electric Vehicle Association and the City of Davis.

  • Get evolving program details for all the goings on for Driving on Sunshine Week

Rooftop and battery systems 101

Generally, the energy produced by residential solar systems is immediately routed into your home to meet demand. When the sun is shining during the middle of the day — especially in the summer — rooftop solar systems may produce more energy than your home needs. Normally, this excess is exported to the grid at wholesale prices and added to your overall energy balance.

When homeowners also install a backup battery system along with rooftop solar, excess energy not needed by the home is used to charge the battery system. Any remaining energy is then exported to the grid. So far, so good.

But what if an outage occurs and energy cannot be received or transmitted to the grid? Without a battery system, you have no power for your home and the potential energy from your solar system goes to waste. Because battery systems allow your solar panels to continue to power your home during a grid outage, both planned and unplanned power outages are no longer the worry they once were. This is a huge advantage!

The other big benefit is that battery systems also allow you to draw from the battery on a daily basis when solar panels aren’t producing, like during cloudy periods or in the evening or nighttime. This last feature means that battery systems can also be used to reduce peak energy usage, helping us all keep the grid up and running during high demand periods. Reducing peak energy usage can also save you money by avoiding peak energy pricing associated with time-of-use rates, increasing the value and usefulness of the up-front investment of a battery system even more. What’s a time-of-use rate? Read on.

Two objectives: avoid the peak and power your home during outages

According to local solar installer Chris Soderquist of RepowerYolo, “Battery systems are sold to meet two potentially competing objectives: provide power to the home and grid during the most expensive, peak time periods and to provide backup power when the grid fails.”

Generally, homeowners want batteries to discharge during peak times on a regular basis, but you also want to reserve the ability to save up energy for unplanned power outages. Smart controls allow some systems to satisfy both objectives, as long as you have sufficient warning of upcoming outages. Phone-based apps give homeowners the power to view and control when batteries discharge. If you know about the outage ahead of time, you can reserve the power you would normally use during peak time to help you get you through the outage, depending on battery capacity and the energy you consume.

During the summer, monitoring the reserve is not as important. During the winter, depending on whether you have an electric heating system or a heavy electric load generally, it’s more important to keep batteries charged up because solar panels don’t produce as much.

Think of a battery as a water tank with a valve at the bottom. The water in the tank represents the amount of energy available and the valve opening represents the load or demand. When solar system production starts declining in the evening, the battery can kick in, so the lower the demand (and the lower the flow rate through the valve), the longer the battery will last.

Two types of systems: whole home and partial home

Systems are generally designed in one of two ways: whole home or partial home. A whole home battery system is designed to power all circuits and may require multiple batteries depending on a home’s power requirements. Whole home backup systems are most effective when homeowners actively monitor and balance energy usage (for example, avoiding running loads like dryers and air conditioners at the same time and managing loads to preserve battery capacity and output).

Partial home or “essential load” backup systems are designed to power only the most important fixtures and appliances (typically refrigerators, charging outlets, and smaller amperage circuit breakers). These systems are generally less expensive than whole house backup systems. Essential load systems are “right-sized” to meet homeowner expectations without a tremendous level of interaction during outages.  However, this type of backup generally requires a separate subpanel, which may add expense.

Higher amperage circuits like your air conditioner, dryer, and large appliances may be included in the system but there are tradeoffs. Powering an air conditioner with a partial home battery system may be possible but will deplete a battery system very quickly.

Avoiding peak energy rates and flattening the energy curve

So, we already know that backup battery systems keep the juice flowing during an outage, which is a huge advantage. But battery systems can also provide critical energy on a daily basis, especially in the summer, allowing you to avoid peak energy rates, save money, and flatten the energy demand curve.

During summertime, in our Central Valley climate, we use most of our daily electricity in the early evening (4 to 9 pm) running air conditioners and fans and gathering as a family. Even homeowners with solar panels end up relying on the electricity grid to power their homes during this time of day.

New Time-of-Use (TOU) rates reflect this higher demand by charging more during peak times and less during off-peak times. Peak summer electricity rates are 20 to 60 percent more expensive then off-peak. When the grid cannot produce enough electricity to meet demand, rotating outages (planned outages) or even rolling outages (unplanned) can occur. It’s a good idea to aim to do laundry, run dishwashers, and pre-cool your home in the morning because off-peak rates are much lower and the grid has more available energy.

Flatten the Energy Demand Curve: The summer peak (red zone, 4-9 pm) puts a lot of stress on the energy grid and is the most expensive time to use electricity. Stay out of the red zone to save money on time of use rates and to reduce stress on our energy grid. Image courtesy Kristin Heinemeier Frontier Energy, Davis CA.

As mentioned, residents can also use batteries to power their home and avoid peak pricing. When your solar system is producing less than your energy demand, a battery system can power your home until it is depleted. If you are hyper-conscious about load-shifting (shifting energy use to another time of day), you may not need any additional electricity from the grid during that time.

Avoiding electricity use during peak times is also a great way to help “flatten the energy demand curve” and allow the energy utility to avoid exceeding capacity and potentially using dirty auxiliary energy sources. If we all conserve energy during Flex Alerts, typically called due to anticipated high use of air conditioners during heatwaves, we all benefit by avoiding outages.

Battery systems also help us reach our community greenhouse gas (GHG) emissions reduction goals by reducing the use of fossil fuels and lowering our carbon footprint. Energy used during peak times is often produced from the dirtiest sources of energy. Keeping our energy off-peak can reduce the amount of energy generated by non-renewable sources.

Gamify your battery-equipped home to flatten the curve

Katrina Sutton, a solar and battery owner, says she “gamifies” her house and uses her battery system app to help her keep her energy low during peak times. “PG&E’s peak hours are from 4 and 9pm, so that means that any electricity used during those hours is significantly more expensive. I don’t charge the car, for example, between 4 and 9, I try really hard not to run the AC, the dishwasher’s not running, the laundry’s not running, the ice maker’s not running. I don’t charge my phone (then), you know anything little I can think of I just don’t do it. I usually go on a walk between 7 or 8pm, so I’m not even home.”

Katrina has an Enphase brand battery at her house, but there are several brands to choose from even in this emerging market. Energy Sage is a good source for technical details and reviews of batteries (see More Reading).

Pre-cooling is another way to load-shift

Pre-cooling your home before peak time is another way to help you coast through the peak time. In our area, we can use the Delta Breeze to cool our homes significantly just by opening our windows in the evening and closing them again in the morning. See the Cool Davis video on evening cooling using your windows and the cool Delta Breeze.

If conditions are smoky, pre-cooling is still a useful strategy, but windows and doors should be kept shut and heating and cooling system filters should be changed often.

Kristin Heinemeier, Cool Davis board member and an engineer at Frontier Energy, flattens her energy demand during the peak period by pre-cooling her house from 12 to 4pm. Watch how she does it in this Cool Davis pre-cooling video. Pre-cooling is also useful during Flex Alerts when the California Independent System Operator (CAISO) (our grid operator) asks us to voluntarily reduce energy usage to avoid blackouts (see More Reading).

Both Kristin and Katrina like to take walks in the early evening because the easiest way to conserve electricity during peak time is to “do nothing” from 4 to 9pm. You could also go for a swim or some other non-electric activity. If you washed and hung out your laundry in the morning, evening is a great time to bring it in and put it away. Need tips? We got you covered with this line-drying video.

Battery systems are still costly

Battery systems are new and still pretty expensive. There are incentives for certain communities and consumers, but Davis homes are highly unlikely to qualify (see More Reading for links to PG&E incentives webpage).

Battery pricing depends on capacity and power output. Capacity is the amount of electricity a battery can store, measured in kilowatt hours (kWh). Power output is the amount of electricity that a battery can deliver at one time, measured in kilowatts (kW). Energy Sage points out that “a battery with a high capacity and a low power rating would deliver a low amount of electricity (enough to run a few crucial appliances) for a long time. A battery with low capacity and a high power rating could run your entire home, but only for a few hours.”

Batteries run $560-750 per kWh of capacity with labor doubling that. A single-battery system runs $12,000 to $16,000 installed providing 10 to 14 kWh of energy (approximately two hours of air conditioning with other standard loads). The average air conditioning unit uses about 4000 watts (4 kilowatts) or 4 kWh when it operates for one hour.

Batteries help you finish your home energy puzzle

While battery backup systems add more expense to your home’s energy puzzle, keep in mind that battery systems also allow rooftop solar systems to continue to produce energy during outages, resulting in both cost savings and a huge reliability factor.

More importantly, batteries allow us to all to contribute to a cleaner and more secure energy system now and in the future. For further information about solar PV systems, battery storage, energy conservation, and other energy security and resilience measures, keep an eye on the Cool Davis Rooftop Solar and Home Heating and Cooling webpages.

Written by John Walter with contributions from Chrissy Backman, Kerry Loux, and Leslie Crenna

More Reading

Cool Davis Rooftop Solar webpage (https://www.cooldavis.org/cool-solutions/rooftop-solar/)

Cool Davis Home Heating and Cooling webpage (https://www.cooldavis.org/cool-solutions/hvac-make-plan/)

FlexAlerts from the California Independent System Operator (CAISO) (our grid operator) (http://www.flexalert.org/)

Energy Sage article: “How to choose the best battery for a solar energy system”


PG&E backup battery incentive program webpage