A battery can make a rooftop solar system much more useful during a blackout. It can also make that system much more expensive without saving enough on electric bills to pay for itself. Those are separate questions, and a sales quote that combines panels and storage into one payback figure can hide the difference.
Start with the solar-only proposal. Then ask the installer for the added, installed price of the battery and the extra savings it is expected to produce. If you’re still deciding on panels, work out solar payback and export credits first. Judge storage on what it adds: backup power, the ability to use cheap electricity later, and any additional bill savings.
Decide what the battery must do
Most homeowners are buying storage for one or both of two jobs:
- Backup: Keep selected circuits running when the utility goes down.
- Bill management: Save solar production or lower-priced grid electricity for hours when buying power costs more.
The distinction matters because the best settings can conflict. A battery that discharges every evening to cut the bill may have little charge left for an overnight outage. Setting aside a backup reserve protects against that risk, but leaves less capacity for daily savings.
Nor does having solar automatically give you backup. Most grid-connected rooftop systems shut down during an outage for safety. To run when the grid is down, the solar-and-storage system needs equipment configured to operate independently. Ask the installer to show which circuits will stay on, how the system disconnects from the grid, and whether the panels can recharge the battery during an outage.
Size backup around essential loads, not the whole electric bill
A household’s monthly electricity use is a poor shortcut for battery sizing. During an outage, you may need the refrigerator, some lights, internet equipment, a medical device or a well pump—but not the oven, clothes dryer and EV charger.
Make a list of the loads you would actually run. For each, estimate its energy use in kilowatt-hours (kWh) over the backup period. Separately, note which devices may run at the same time and which motors need a burst of power to start. This gives you two different sizing tests:
| Battery specification | What it tells you | Question to ask |
|---|---|---|
| Usable capacity, in kWh | How much stored energy can be discharged under the stated conditions | How many hours will my essential loads run? |
| Continuous output, in kW | How much power the system can supply at once | Can it run my selected circuits together? |
| Starting or peak output | Whether it can briefly handle a motor starting | Will it start the pump or air conditioner I want backed up? |
A 10 kWh usable battery serving loads that average 0.5 kW has a simple starting estimate of 20 hours. If you keep 20% in reserve, that falls to about 16 hours. Actual runtime depends on the equipment’s losses, battery condition and what turns on during the outage. Ask whether the quoted “usable” figure is measured at the battery or at the home’s AC output so you don’t count conversion losses twice.
Capacity won’t fix a power shortfall. A battery with plenty of kWh may still be unable to start a large pump or run several heavy appliances together. Manufacturers distinguish between energy and output: Powerwall 3 specifications list 13.5 kWh of nominal battery energy alongside separate, configurable AC power ratings. The installed configuration—not the largest number in a brochure—is what matters.
For a multi-day outage, also ask what happens after the first night. Solar may replenish the battery when there is enough sun, but shade, bad weather and household demand can limit charging. If the goal is to protect a few essentials, backing up a smaller set of circuits may be more useful than paying for enough capacity to run the entire house. If outages are rare and brief, compare that cost with other backup arrangements before assuming a whole-home battery is necessary.
Calculate the savings from electricity you can actually shift
A battery does not create electricity. It changes when electricity is used, and some energy is lost along the way. The value of storing solar depends mainly on what you give up by not exporting it and what you avoid paying when you use it later. Export compensation varies by state and utility, so use your own tariff rather than an advertised national savings figure.
Here is a simplified daily example. Suppose 10 kWh of midday solar would earn $0.08 per kWh if exported. Instead, you store it and later receive 9 kWh for household use—a 90% round trip. If evening electricity costs $0.35 per kWh:
- Avoided evening purchase: 9 kWh × $0.35 = $3.15
- Forgone export credit: 10 kWh × $0.08 = $0.80
- Added value from that charge and discharge: $2.35
That is the battery’s incremental value for this cycle, not the value of the solar generation itself. A 90% round-trip figure is published for one Enphase battery under specified test conditions; real-world performance and the measurement method can differ. Don’t assume every battery will deliver nine usable kWh from ten put in.
If the same 10 kWh would instead receive a full $0.35-per-kWh export credit that offsets evening purchases dollar for dollar, storing it would generally lose value: you would give up $3.50 in credits to avoid a $3.15 purchase. The exact result can change with credit-expiration rules, fixed charges and time-based export rates, but generous export terms often weaken the bill-savings case for storage.
A battery can also charge from the grid where the system and utility rules permit it. In that case, replace the forgone export credit in the calculation with the cost of charging. At $0.15 per kWh off-peak, buying 10 kWh costs $1.50; using 9 kWh during a $0.35 peak period avoids $3.15, leaving $1.65 before other costs. The peak period must also be long enough—and your household must use enough electricity during it—for that cycle to happen. When comparing time-of-use electricity plans and their charges, check the full rate schedule, not just the highest peak price.
Compare added cost with added savings
Ask for two otherwise comparable, itemized quotes: solar alone and solar with storage. The difference is your starting battery cost. For an existing solar system, get a battery retrofit quote that includes any inverter changes, backup controls, electrical-panel work, permits and installation. A battery-only price is not an installed backup-system price.
For scale, the Department of Energy’s first-quarter 2025 residential benchmarks model an 8 kW solar system at $2.95 per watt and the same-size system with a 13.5 kWh battery at $4.59 per watt, using modeled market-price costs expressed in 2024 dollars. That implies about $13,120 extra in the benchmark—not a quote for your home or a prediction of what a retrofit will cost.
Use the incremental price in a simple payback check:
Battery-only payback = net added installed cost ÷ realistic annual additional savings and payments
Suppose an installed battery adds $12,000 and achieves the $2.35 solar-shifting benefit above on 250 days a year. That is about $588 in annual savings and a simple payback of roughly 20 years. This illustrative estimate assumes the same rates, output and cycling pattern year after year; it excludes financing, repairs and changes in capacity. It is not evidence that a particular household will achieve 250 profitable cycles.
A better estimate uses at least a year of bills and, if available, hourly usage and solar-production data. Model seasons separately: a battery may fill regularly in spring but have less surplus solar in winter. Count any eligible utility payments or local incentives, but check their terms, including whether a program may dispatch your battery when you would prefer to preserve backup charge. Don’t count the same stored kWh once as avoided peak purchases and again as an export payment.
One tax detail can materially change an old sales calculation. The federal Residential Clean Energy Credit is unavailable for property placed in service after December 31, 2025. A 2026 battery quote should not simply subtract the former 30% homeowner credit. Check separately for applicable state or utility incentives.
Test the payback against battery aging and the warranty
Simple payback is only a screening tool. Batteries lose capacity over time, so a design that barely meets your backup needs when new may provide less runtime later. Read the proposed model’s warranty for its term, retained-capacity guarantee, covered uses and any cycle or energy-throughput limits.
For example, the U.S. Powerwall limited warranty specifies 70% energy retention at 10 years for the listed solar self-consumption, time-based control and backup applications, subject to its terms. That does not mean the battery stops working at year 10, nor does it promise full original capacity until then. It does show why a projected 20-year payback deserves close scrutiny.
Run a less favorable case before signing: fewer profitable cycles, a smaller gap between peak and off-peak rates, reduced battery capacity, and any loan interest or service costs. Compare that result with how long you expect to stay in the home. Backup has value too, but it is a personal benefit rather than an electricity-bill saving; keep it separate so the numbers remain honest.
A battery is easiest to justify when outages make reliable power valuable to you, export credits are low, and substantial household use falls in expensive hours. It is harder to justify on savings alone when exports are well compensated or the battery would seldom cycle. The most useful next step is an itemized solar-only versus solar-plus-storage quote, paired with a backup-load plan and a savings estimate built from your actual utility tariff.