To size a solar battery, multiply the power your chosen loads draw (in kilowatts) by the hours you want to cover. Then divide by the battery's usable fraction and its round-trip efficiency. The result is the nominal capacity to shop for, in kilowatt-hours (kWh). Separately, check that the battery and inverter can deliver enough power (kW) to start and run those loads at the same time. Capacity decides how long you last, and power decides what you can run.
Sizing for nights and sizing for outages are different problems. A night-time battery is cycled daily and sized around your evening and overnight usage. An outage battery sits mostly full and is sized around a short list of essential loads. This guide shows how to size for each, where the numbers mislead, and includes a calculator you can adjust.
The key factors
- Which loads you want to power. Whole-home backup and "fridge, lights, internet" are very different targets.
- How long you need coverage. One night, one evening outage, or several days without sun.
- Usable vs. nominal capacity. Not every kWh on the label is available to you.
- Efficiency losses. Energy is lost in conversion, so you must store more than you deliver.
- Power limits. Continuous output and motor start-up surges can cap you before capacity does.
Capacity vs. power: two different limits
Capacity is the amount of energy stored, measured in kWh. Think of it as the size of a fuel tank. Power is how fast energy can be delivered, measured in kW. Think of it as the width of the fuel line. A battery can have plenty of energy but be unable to run a large load, or deliver high power but run dry quickly.
A 10 kWh battery with a 5 kW continuous output can run a 1 kW load for roughly ten hours in ideal conditions. It cannot run a 7 kW load at all, however much energy remains. Always check both numbers against your plan.
Sizing for nights vs. sizing for outages
Nights: sizing for daily cycling
If the goal is to use stored solar energy after sunset, size around your typical usage from sunset until the next morning. Your electricity usage data (a smart meter, your utility's online portal, or a monitoring app) shows how many kWh you use in those hours. The battery gets charged by surplus solar during the day, so there is a second constraint: your solar array must produce enough extra energy to fill the battery after covering daytime loads. A large battery paired with a small array will rarely fill.
Outages: sizing for essential loads
For outages, you are not trying to reproduce normal life. You are choosing a short list of essentials and deciding how long they must run. Two questions drive the size:
- Which loads are truly essential (refrigeration, lighting, communications, a well pump, or equipment someone depends on)?
- How long are outages in your area, and can solar recharge the battery the next day?
With solar, a battery that is recharged each day can stretch coverage across multiple days, provided there is enough sun and the system is designed to operate while the grid is down. Not every solar-plus-battery installation does this by default, so confirm it with your installer.
Step-by-step sizing method
- List the loads you want to cover. Note each device's running watts and, for motors, its start-up (surge) watts. Nameplates, manuals, or a plug-in power meter give better numbers than guesses.
- Estimate hours of use per load. A refrigerator cycles on and off, so use its average draw over time, not its nameplate peak.
- Convert to energy. Energy (kWh) = power (kW) × hours. Add up all loads for the period you want to cover.
- Add a margin. Real usage varies, and estimates are usually optimistic. A cushion of 10–25% is a common planning habit, but it is a judgment call rather than a rule.
- Adjust for usable capacity and efficiency. Required nominal capacity = energy needed ÷ (usable fraction × round-trip efficiency).
- Check power. Add up the loads that could run at the same time. Compare the total, plus the largest start-up surge, against the battery and inverter's continuous and surge ratings.
- Check the recharge side. Make sure the solar array can realistically refill the battery between cycles, especially in the season with the least sun.
Why "usable" and "efficiency" matter
Manufacturers publish nominal capacity and, usually, a usable capacity. Many modern home batteries allow most of their nominal capacity to be used, but the exact figure varies by product and settings. Some systems also reserve a portion for backup, so the amount available for nightly use may be lower than the amount available in an outage. Round-trip efficiency, the share of stored energy you get back, is often quoted around the high-80s to mid-90s percent for lithium-based home batteries, depending on the product and conditions. Use the datasheet for the model you are considering rather than these general ranges.
Try it: battery size estimator
Enter the average power of the loads you want to cover and how many hours you want to cover them. The calculator estimates the nominal battery capacity needed and the continuous power to look for. The starting values are editable examples, not real data about your home.
If the result is larger than you expected, the fastest ways to shrink it are to remove the largest loads from the list or reduce the hours. Those two inputs move the answer far more than efficiency or margin.
Worked illustrative example
This is an illustrative example, not data from a real home. Suppose someone wants to cover a refrigerator, a few LED lights, a Wi-Fi router, and phone and laptop charging overnight for 12 hours.
- Assumed average draw of all these loads together: about 600 W (0.6 kW).
- Energy for 12 hours: 0.6 kW × 12 h = 7.2 kWh.
- With a 15% margin: about 8.3 kWh.
- If 90% of nominal capacity is usable and round-trip efficiency is 90%: 8.3 ÷ (0.9 × 0.9) ≈ 10.2 kWh nominal.
The same household adding a well pump with a high start-up surge would need to check power ratings first. Capacity alone would not tell them whether the system can start it.
Typical load ranges to start your list
These are broad, approximate ranges for planning only. Actual draw varies a lot by model, age and settings, so check your own appliance labels.
| Load | Typical running power (approx.) | Sizing note |
|---|---|---|
| LED lighting (per bulb) | A few watts to about 15 W | Small individually, but they add up across a house. |
| Wi-Fi router / modem | About 10–20 W | Runs continuously, so its hours add up. |
| Refrigerator | Roughly 100–200 W while the compressor runs | Cycles on and off. Use average draw, and allow for start-up surge. |
| Laptop / TV | About 30–150 W | Depends heavily on screen size and usage. |
| Well pump / large motor | Often several hundred watts or more | Start-up surge can be a multiple of running power. |
| Air conditioning, electric oven, electric water heating | Often in the kilowatt range | Often the loads that push a battery size up sharply. Many people leave them out of backup. |
Trade-offs by goal
| Goal | What sets the size | Main advantage | Main limitation |
|---|---|---|---|
| Shift solar to evenings | Evening and overnight energy use, and daytime solar surplus | Uses more of your own solar energy | A bigger battery than your array can fill adds cost without benefit |
| Cover short outages (hours) | A short list of essential loads | Smaller, cheaper battery | Does little if an outage runs for days with little sun |
| Cover multi-day outages | Daily essential energy, solar recharge, and worst-case weather | Greatest resilience | Highest cost, and still depends on sunshine and strict load discipline |
| Whole-home backup | Peak power across many loads, plus capacity | Little change in daily life during outages | Large heavy loads can force many batteries and a larger inverter |
Edge cases and limits
- Heavy loads change the answer. Air conditioners, electric cooking and water heating can dwarf everything else. If you include them, expect a much larger system, or choose to power them only for limited periods.
- Cloudy stretches. Daily recharging assumes enough sun. In a long storm, solar may recover only a small part of what you use, so the battery's stored energy becomes the main budget.
- Backup may not work by default. Some solar systems shut down when the grid fails for safety reasons. Whether your system keeps running depends on its equipment and wiring, so ask your installer directly how it behaves in an outage.
- Batteries age. Capacity declines slowly with use over the years, so a system sized with no margin may feel smaller later.
- Temperature and settings. Extreme heat or cold can affect performance, and a configured backup reserve reduces what is available for daily use.
- These numbers are estimates. Averages hide spikes. A kettle, microwave or pump starting at the same time as a fridge compressor can exceed power limits even when energy is plentiful.
- Critical equipment. If anyone relies on powered medical equipment, treat the plan as a safety question. Talk to the equipment provider or a healthcare professional about backup needs, and do not rely on an estimate alone.
Electrical work on homes should be done by qualified, licensed professionals following local codes. Rules and incentives vary by country and utility, so check with your local authority or utility for current requirements.
Common sizing mistakes
- Sizing from the monthly bill. Total monthly kWh does not tell you your evening or outage load. Use the hours you actually want covered.
- Treating nominal as usable. Skipping usable share and efficiency leaves you short of the runtime you expected.
- Ignoring start-up surge. Motors can draw several times their running power for a moment. A battery with enough kWh may still trip on start-up.
- Counting only energy, not power. Check both kWh and kW before buying.
- Oversizing relative to the solar array. A battery that never fills is capacity you paid for and never use.
- Assuming a battery equals backup. Some batteries are installed only for bill savings and are not wired to supply the home during an outage.
- Backing up everything by default. A dedicated essential-loads subpanel is often a more practical approach than backing up the whole house.
How to think about it
- Decide the purpose first: nights, outages, or both.
- Choose the loads that matter, and be honest about the big ones.
- Convert to kWh, add a margin, and adjust for usable share and efficiency.
- Check that power ratings can handle simultaneous loads and start-up surges.
- Confirm that your solar array can recharge the battery in your least sunny season.
- Ask installers to show their sizing assumptions, and compare quotes on the same assumptions rather than on battery size alone.
A good battery size is the smallest one that covers the loads you chose for the hours you chose, with a realistic margin and enough power to start them. Re-run the estimate whenever you change the load list, since small changes there move the result more than anything else.