The average home cited by Eversource uses about 10,400 kWh per year, which works out to roughly 28 kWh a day. Your battery should not cover all of that. Size it around the electricity you use after sunset, then make sure your solar panels can refill it the next day.
Quick answer
Add up your evening, overnight, and backup loads in kWh. Divide that number by the battery’s usable-capacity percentage. Then size your panels so they cover daytime use and also recharge the battery.
- Best for outages: size for critical loads only
- Best for bill savings: size for overnight use
- Start with your bill or smart-meter data, not a generic size range.
- Battery capacity (kWh) is stored energy. Inverter power (kW) is how much you can run at once. You need both.
- Nominal capacity is bigger than usable capacity. Plan for depth of discharge, losses, and a reserve.
- Your panels must produce enough daytime surplus to refill the battery.
- Backup for critical loads costs far less than whole-home backup.
- Start with your bills and meter data
- How big a solar battery do I need for my home?
- How do I calculate the battery capacity I need in kWh?
- Worked example: 20 kWh a day
- Capacity vs. usable capacity
- Power rating vs. energy capacity
- Whole house or essential appliances?
- How big a solar panel system do I need?
- How many solar panels do I need to charge a battery?
- Future loads
- Power outages and off-grid use
- Questions before you sign
- What do owners say?
- Frequently Asked Questions
Start with your bills and meter data
Your electric bill shows your use in kilowatt-hours (kWh). Smart-meter data is even better, because it shows when you use power. Both are the preferred starting point for sizing a system.
Pull 12 months of bills. Write down the monthly kWh, then divide the total by 365 for your daily average. Note your price per kWh too. The U.S. Energy Information Administration shows state averages from roughly 12 to 40 cents, with the national average near 17 cents. This guide uses 17 cents.
Don’t have smart-meter data? Estimate from appliances. One kilowatt-hour is 1,000 watt-hours, so multiply watts by hours of use. A 100-watt device running 10 hours uses 1,000 Wh, or 1 kWh.
How big a solar battery do I need for my home?
Most homes need a battery sized to their after-sunset load, not their whole day. That is often 5 to 15 kWh of usable storage, but your own bill decides it. A quiet home with gas heat and a small evening load needs much less than a house running electric heat all night.
Battery size mainly depends on four things: your solar system size, your early-morning and overnight use, your outage goals, and your budget. You also don’t have to store every spare unit of solar. The battery just needs to charge from daytime solar and cover the load you picked.
The table below compares the four common approaches. The kWh figures are planning assumptions for a typical household, not product ratings.
| Goal | What it covers | Typical usable kWh (assumed) | Inverter need |
|---|---|---|---|
| Critical-load backup | Fridge, lights, router, phone charging, furnace fan | 3 to 6 kWh per day | Low (about 2 to 5 kW) |
| Overnight shifting | Evening and overnight use, to cut grid purchases | 6 to 12 kWh | Medium |
| Whole-home backup | Nearly everything, including big appliances | 20 kWh or more per day | High, with surge capacity |
| Multi-day backup | Days without grid or sun | Several days of critical load, plus extra panels | Medium to high |
How do I calculate the battery capacity I need in kWh?
Work out how many kWh you want to use from the battery, then divide by the share of the battery you can actually use. Add a margin for losses and a reserve.
First, find the target load. For overnight use, add the kWh you use from about sunset to sunrise. Smart-meter data makes this easy. Without it, list the appliances running at night, multiply watts by hours, and add them up.
A sample critical-load list, using assumed figures:
- Refrigerator: about 1.5 kWh per day
- Lights: about 0.6 kWh
- Router and modem: about 0.3 kWh
- TV and laptops: about 0.5 kWh
- Furnace fan: about 1.0 kWh
That totals about 3.9 kWh, so call it 4 kWh per day. Check your own appliance labels, since real numbers differ.
Worked example: a home using 20 kWh a day
Take a household using 20 kWh per day, with 8 kWh used overnight. These are the assumptions: 95% inverter efficiency, a 10% reserve, and 90% usable depth of discharge. Check your battery’s datasheet, since some makers already list usable capacity.
- Overnight target: 8 kWh.
- Allow for inverter loss: 8 ÷ 0.95 = 8.4 kWh.
- Add a 10% reserve: 8.4 × 1.10 = 9.3 kWh.
- Divide by depth of discharge: 9.3 ÷ 0.90 = 10.3 kWh nominal.
So this home should shop for a battery of about 10 to 13 kWh nominal. A 10 kWh unit would run slightly short on the heaviest nights. The next size up leaves room to grow.
What does it save? At 17 cents per kWh, 8 kWh shifted from grid to battery is about $1.36 per day, or roughly $496 a year. That assumes you pay a flat rate and the battery fills every day. If your utility pays full retail for exported solar (net metering), the battery saves much less. If it pays low export rates or charges time-of-use prices, the battery saves more.
What is the difference between battery capacity and usable capacity?
Nominal capacity is the total energy a battery holds. Usable capacity is the part you are allowed to draw out. A 10 kWh battery with 90% depth of discharge gives you 9 kWh.
Three things shrink the number further:
- Depth of discharge (DoD): the share of the battery you can use before it stops, set to protect battery life.
- Round-trip efficiency: energy is lost when charging and discharging. If efficiency is 90%, 10 kWh of solar into the battery gives about 9 kWh back out.
- Reserve: some systems hold back a share for outages. If you set 20% for backup, only 80% is open for daily bill savings.
Compare batteries on usable kWh, never on the headline number.
Power rating vs. energy capacity
Capacity in kWh tells you how long the battery runs. Power in kW tells you how much it can run at once. Mixing them up is the most common sizing mistake.
A 10 kWh battery with a 3 kW inverter can power your fridge and lights all night. It cannot run an electric range, a dryer, and a heat pump together. Add up the watts of everything you want running at the same moment, then compare that to the continuous output rating.
Startup surge matters too. Motors in air conditioners, well pumps, and freezers pull several times their running watts for a moment. Check the battery’s surge or peak rating, and check the label on your motor loads.
Should I size the battery for my whole house or only essential appliances?
For most homes, essential appliances. Whole-home backup needs a much bigger battery and a stronger inverter, so the price climbs fast. Critical-load backup keeps the basics running for far less money.
If your goal is lower bills at night, then size for overnight use (6 to 12 kWh in most homes).
If your goal is surviving short grid outages, then choose a critical-load panel and a battery sized for 4 to 8 kWh per day of essentials.
If you need central air or an electric range during outages, then price whole-home backup and get an installer to confirm inverter surge ratings.
If you rely on medical equipment, then talk to your installer and your doctor about a dedicated backup plan.
How big a solar panel system do I need?
Divide your daily kWh by peak sun hours and by a system-loss factor. That gives the kW of panels you need to cover your use. Roof space, budget, and local utility rules then set the limit.
Here is the math for our example home. The assumptions: 20 kWh per day, 4.5 peak sun hours (a common mid-range US value; look up yours), and 80% efficiency after losses from heat, wiring, and the inverter.
- Each kW of panels makes 4.5 × 0.80 = 3.6 kWh per day.
- 20 ÷ 3.6 = about 5.6 kW of panels.
- With 400-watt panels, that is 14 panels.
A second method uses annual use. One US utility guide estimates that 1 kW of solar produces about 1,100 kWh per year and takes up roughly 100 square feet of roof. Our home uses 7,300 kWh a year (20 × 365), so 7,300 ÷ 1,100 is about 6.6 kW, or 17 panels. The gap between the two answers comes from sunshine: 1,100 kWh per kW reflects a cloudier region than 4.5 peak sun hours. Use the figure that matches your location. Eversource’s solar sizing page and the Vermont fact sheet on sizing grid-connected systems walk through the same logic.
Eversource also notes that systems between 5 and 10 kW are generally enough for many homes in its area. Real results vary with shade, roof angle, and design.
How many solar panels do I need to charge a battery?
Add the battery’s refill energy to your daytime use, then size panels for that total. A battery that never fully charges will not cover your night.
Back to our example. The 14-panel system (5.6 kW) makes about 20 kWh per day on average. If the home uses 12 kWh during daylight, only 8 kWh is left over. The battery needs about 9.3 kWh of usable energy, plus roughly 10% lost to round-trip efficiency, so about 10.3 kWh of surplus. That is too tight, especially in winter.
Move up to about 6.5 kW (16 panels of 400 W). Output becomes 6.5 × 3.6 = 23.4 kWh per day. After 12 kWh of daytime use, you have 11.4 kWh of surplus, enough to refill the battery on an average day.
Use this panel-to-battery checklist before you buy:
- Daily load: average kWh per day, and the winter and summer extremes.
- Peak sun hours: your local value, ideally for the worst season you care about.
- Roof area: about 100 square feet per kW as a rough guide, minus shaded or awkward areas.
- Inverter rating: continuous kW and surge rating against your largest loads.
- Future appliances: an EV, heat pump, or induction range.
- Outage priorities: which circuits must stay on, and for how many hours.
Plan for future loads
Add up what you plan to buy in the next five to ten years. An electric vehicle can add several kWh a day, and a heat pump adds load all winter. Induction cooking adds short, high-power peaks that stress the inverter.
You don’t need to buy all the battery now. Many systems allow extra battery modules later. Ask the installer whether the inverter can support more capacity, and whether adding panels later will need a bigger inverter.
How large a solar battery do I need for power outages?
Multiply your critical-load kWh per day by the number of days you want covered, then apply the usable-capacity math above. For a 4 kWh-per-day critical load and one day of backup, a 5 to 6 kWh nominal battery is a reasonable start.
Remember that a battery used for backup must be paired with an inverter and isolation equipment that can operate safely when the grid is down. Standard grid-tied solar shuts off during an outage on purpose, to protect line workers. A battery system with proper islanding gear is what keeps your lights on. Never try to bypass that shut-off.
Duration changes the answer. For a short grid outage, a modest battery plus morning sun can carry you through. For multi-day or off-grid operation, you need more storage, more panels, and a plan for cloudy days. Panels must produce enough to run loads and refill the battery, so off-grid systems are usually much larger than grid-tied ones.
Installer questions, permits, warranties, and financial checks
Ask every installer the same questions so quotes compare cleanly:
- What usable kWh, continuous kW, and surge kW does the system deliver?
- How long will permits and utility approval take?
- What does the warranty cover, in years and in remaining capacity after that time?
- Who handles service if the system goes offline?
- What does my utility pay for exported solar, and does it offer time-of-use rates?
- Are there federal, state, or utility incentives, and are they still available? Rules change, so confirm before you sign.
Run your own payback math with your state’s price per kWh. A home at 12 cents saves far less per stored kWh than one at 35 cents. A licensed electrician or installer should confirm local code and permit needs.
What do owners say?
Owners like
- Reports focus more on problems than on praise. The main benefits owners cite are the ones the sizing math above is built to deliver: nighttime coverage and outage backup.
Common complaints
- Recurring reports describe delays with installation, permits, or utility approval.
- Owners frequently complain about batteries or systems going offline and difficulty obtaining responsive post-installation service.
Summarised from owner reviews and long-term user reports; individual experiences vary.
Frequently Asked Questions
How big solar battery do I need?
Start with your overnight use in kWh. If you use 8 kWh after sunset, plan for roughly 10 kWh nominal once you allow for depth of discharge, inverter losses, and a 10% reserve. For outage-only backup, size for your critical loads instead, often 4 to 6 kWh per day.
How big solar panel do I need?
Divide your daily kWh by peak sun hours and by 0.80 for system losses. A home using 20 kWh a day with 4.5 peak sun hours needs about 5.6 kW, or 14 panels at 400 W. Add roughly 15% more if the panels must also charge a battery.
How large solar battery do I need?
Use this formula: nominal capacity = target usable kWh ÷ inverter efficiency, plus reserve, ÷ depth of discharge. Then check the inverter’s continuous and surge kW against your biggest appliances. A battery with enough kWh but too little power will trip under heavy loads.
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