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To size a home battery, decide which loads must run, add up their kW (power at one moment), then multiply the average load by the hours you need to get kWh (energy). Buy enough units to cover both numbers. This guide gives the method, worked examples with assumed loads, and how many Tesla, Enphase and FranklinWH units each example needs. We did not test these batteries; specs are from manufacturer documents as of October 24, 2026. See how we pick products.
Want the arithmetic done for you? Use our home battery size calculator.
Step 1: Decide the goal
- Essential-load backup: refrigerator, lights, internet, some outlets, maybe a furnace blower or a well pump.
- Whole-home backup: everything, including air conditioning and an electric range.
- Bill management: shifting solar or off-peak energy into the evening on a time-of-use rate.
The goal changes the answer by a factor of several.
Step 2: List loads and kW
Use nameplates, a plug-in watt meter, a circuit-level monitor or your utility’s interval data. The loads below are assumed round numbers for illustration, not measurements. Your equipment will differ.
| Load (assumed) | Running kW | Notes |
|---|---|---|
| Refrigerator | 0.15 | Average running draw; the compressor start is higher |
| Lights and electronics | 0.30 | |
| Internet and a laptop | 0.10 | |
| Furnace blower | 0.50 | |
| Well pump | 1.00 | Starting surge is higher |
| Total if all run at once | 2.05 |
Add a margin for motor starts. A compressor, well pump or air conditioner needs a short surge above its running power. Manufacturer starting-current data is the right input; Tesla’s datasheet lists the ability to start loads up to 185 LRA, and FranklinWH says one aPower 2 can start a 5-ton air conditioner, so use your equipment’s own data and ask your installer.
Step 3: Convert to energy
Formula: kWh needed = average kW × hours.
Do not use the simultaneous total from step 2 as your average if loads cycle. A refrigerator runs part of the time. For a rough estimate use measured daily kWh, or estimate average kW at 50 to 70 percent of the simultaneous total for cycling loads, and say so.
Example A (essential loads, assumed): 1.2 kW average × 12 hours = 14.4 kWh.
Example B (larger, assumed): 2.5 kW average × 24 hours = 60 kWh.
For context, EIA’s residential consumption data show average annual electricity purchased per US residential customer of 10,791 kWh in 2022, about 899 kWh a month. Our math: 10,791 ÷ 365 = 29.6 kWh a day. EIA notes these are electricity purchases, which may understate use for homes with on-site solar, and the average varies widely by state (Louisiana 14,774 kWh; Hawaii 6,178 kWh).
Step 4: Adjust for real-world losses
- Usable vs nominal. Enphase states 5.0 kWh usable for the IQ Battery 5P; Tesla’s datasheet states 13.5 kWh nominal and does not state usable energy. Treat nominal as an upper bound.
- Round-trip losses. Energy you store is not all energy you get back. We use no single efficiency here; ask for the datasheet figure.
- Solar recharge. If panels cannot refill the battery on a cloudy day, plan for the battery as one-shot energy.
- Reserve. Leaving some capacity unused is common practice; ask how each system’s reserve setting works.
Step 5: Match the battery
Specs used (manufacturer sources, Oct 24, 2026):
| Battery | Capacity | Continuous kW |
|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh (nominal) | 11.5 |
| FranklinWH aPower 2 | 15 kWh | 10 |
| Enphase IQ Battery 10C | 10 kWh | 7.08 |
| Enphase IQ Battery 5P | 5.0 kWh | 3.84 |
Number of units = round up of the larger of (kWh needed ÷ capacity) and (peak kW ÷ continuous kW).
Example A (14.4 kWh, 2.05 kW peak, no surge margin):
- Powerwall 3: 14.4 ÷ 13.5 = 1.07 → 2 units (one unit comes up slightly short on nominal energy; a bit of solar recharge or lighter loads might make one workable, so ask).
- aPower 2: 14.4 ÷ 15 = 0.96 → 1 unit on paper, with little margin.
- IQ Battery 10C: 14.4 ÷ 10 = 1.44 → 2 units.
- IQ Battery 5P: 14.4 ÷ 5.0 = 2.88 → 3 units; power: 3 × 3.84 = 11.5 kW, plenty.
Example B (60 kWh, 2.5 kW average, assume 8 kW peak with surges):
- Powerwall 3: 60 ÷ 13.5 = 4.44 → 5 units; power 5 × 11.5 = 57.5 kW.
- aPower 2: 60 ÷ 15 = 4 → 4 units; power 4 × 10 = 40 kW.
- IQ Battery 10C: 60 ÷ 10 = 6 → 6 units; power 6 × 7.08 = 42.5 kW.
Notice that in Example B energy, not power, decides the count. A whole-home, multi-day goal is mainly a kWh problem, which is why solar recharge and load shedding matter more than a bigger battery. Also check whether the manufacturer limits how many units can stack.
Quick sizing summary
| Goal | Starting question | Typical constraint |
|---|---|---|
| Overnight essentials | Average kW × 12 hours | kWh |
| Central air during outage | Starting surge and running kW | kW |
| Multi-day | Daily essential kWh × days, less solar recharge | kWh and solar |
| TOU shifting | kWh moved into peak window | kWh and spread |
Who should skip oversizing
- If outages last a few hours, a larger battery buys little. See Do solar panels work in a power outage?.
- If you only want peak shaving, size to the peak window’s kWh, not the day.
Incentives
The IRS says the residential clean energy credit is not available for property placed in service after December 31, 2025, so more capacity is paid for at full price unless a state or utility program applies. Check Rates by State.
Checklist
- Choose a goal and list loads.
- Record running kW and starting surges.
- Convert to kWh with hours.
- Adjust for usable energy and losses.
- Divide by capacity and round up on both kW and kWh.
- Confirm stacking limits and electrical panel capacity with an installer.
- Compare quotes. See how to compare solar quotes and our earlier battery sizing guide.
Limitations
Loads in the examples are assumed, not measured. Capacities are manufacturer-stated, partly nominal. Sizing is not engineering advice; have a licensed installer verify your electrical system.
Sources
- EIA, average residential electricity consumption (FAQ) (2022 data)
- Tesla Powerwall 3 datasheet · FranklinWH aPower 2 · Enphase 10C · Enphase 5P
- IRS, Residential Clean Energy Credit
Related: Best home batteries for backup power
Frequently asked questions
How big of a home battery do I need?
It depends on the loads you want to run and for how long. Multiply the average kW of those loads by the hours of backup you want to get kWh, and check that the battery's continuous kW is at least your simultaneous load plus starting surges.
What is the difference between kW and kWh for a battery?
kW is power, how much the battery can deliver at one moment. kWh is energy, how much it stores. A battery with 13.5 kWh and 11.5 kW can supply 11.5 kW at most and, at a 1.5 kW load, roughly 9 hours before any recharge.
How many batteries does an average house need?
There is no single answer. EIA reports average residential electricity purchases of about 29.6 kWh a day (2022 data), but most backup systems are sized to cover essential loads, which are a fraction of that.
Do I need to size the battery for my solar system too?
Yes, if you want it to recharge daily. A battery recharges from solar only to the extent your panels produce more than the home uses, so a small array may not refill a large battery after a cloudy outage.
