How to size home battery backup
Plan backup around the appliances you need.
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Start with the appliances you need and the hours you want them to run. Check the system’s energy capacity, power output and ability to start motors. Your electric bill alone cannot tell you how many batteries you need.
The examples use assumed appliance loads and battery energy, not product ratings. Use your equipment’s data for sizing.
What do you want to run?
Choose how long you want backup: an evening, overnight or a full day. List the appliances you need and what can wait.
- Refrigerators, lights, internet, fans and outlets.
- AC, well pumps and other equipment you need during an outage.
- EV charging, pool pumps, laundry, electric water heating and cooking you can postpone.
Whole-home backup can include more circuits, but battery output still limits what runs at once. Load controls can pause selected appliances to prevent overload.
kW and kWh
kW measures power. kWh measures energy.
A 1 kW load uses 8 kWh in eight hours. A 4 kW load uses the same energy in two hours. The battery needs enough output to run the appliances and enough stored energy to keep them running. Electricity units
Calculate appliance use
Record each appliance’s running watts, quantity and hours of use. Use its specifications or measured consumption.
kWh = watts × quantity × hours running ÷ 1,000.
Refrigerators cycle on and off. AC use changes with the weather, thermostat setting and home. Count the hours the equipment runs at the listed wattage. For variable-speed equipment, use measured energy where possible. Estimating appliance use
Example: 12 hours without power.
| Equipment | Watts each | Quantity | Hours running | Energy |
|---|---|---|---|---|
| Refrigerator | 200 W | 1 | 4 | 0.8 kWh |
| Internet equipment | 25 W | 1 | 12 | 0.3 kWh |
| LED lights | 10 W | 6 | 5 | 0.3 kWh |
| Fans | 50 W | 2 | 8 | 0.8 kWh |
| Work and device charging | 100 W | 1 | 3 | 0.3 kWh |
| Air conditioning | 3,500 W | 1 | 4 | 14 kWh |
The five essential loads use 2.5 kWh. The AC adds 14 kWh, for 16.5 kWh total. Longer AC use requires more energy. Battery losses and the system’s own consumption must also be covered.
Check what runs at once
Add the running watts of appliances that could be on together.
The five essential loads above draw 0.485 kW together. With the AC, they draw 3.985 kW. Add a 4.5 kW water heater and 7.2 kW EV charger, and demand reaches 15.685 kW.
Pausing the water heater and charger brings demand back to 3.985 kW. The battery still has to start the motors and meet circuit limits.
A breaker rating does not tell you how much power an appliance uses. A 200-amp service does not tell you how much power a battery supplies.
Check AC and pump startup
Motors can draw a short surge when starting. Find the AC model and LRA, or locked-rotor amps, on its label. The installer must check startup current alongside other running loads.
Powerwall 3’s motor-start limit is 185 A LRA per unit. Other batteries have different limits. More storage alone does not increase inverter output or motor-start capacity. Powerwall backup-load requirements
Check electric heat strips, dryers and tankless electric water heaters separately. Their power demand differs from a heat-pump compressor or storage water heater.
Calculate backup time
Estimated hours = available kWh ÷ average kW.
With 10 kWh available to the appliances and no recharging:
| Average load | Runtime |
|---|---|
| 0.5 kW | 20 hours |
| 1 kW | 10 hours |
| 2.5 kW | 4 hours |
| 5 kW | 2 hours |
The table assumes a constant average load. Starting charge, temperature, age and system losses affect available energy. Do not deduct conversion losses twice if the capacity rating already includes them.
Powerwall’s Backup Reserve is saved for an outage and becomes available when the grid fails. Backup Reserve settings
Check solar recharge
Solar must cover daytime use before the surplus can charge the batteries. More battery capacity does not produce more solar energy.
20 kWh of solar minus 12 kWh of daytime use leaves 8 kWh before charging losses and equipment limits. That will not replace 15 kWh used overnight.
Check sunny and cloudy days. Weather, shade, storm damage and equipment affect recharge. Solar panel kW is not daily production in kWh. The backup system must support solar operation during an outage. Backup system design
How many batteries?
Divide the required kWh by the energy each unit can deliver, then round up. If each unit delivers 10 kWh to the appliances, the 16.5 kWh example needs two units for energy. Output, startup and installation requirements may change the count.
Compare capacity, continuous output, startup limits and controls. Batteries with the same kWh can run different loads.
Powerwall 3 Expansion adds storage through Powerwall 3’s inverter without increasing output. Compare it separately from Powerwall 3 or Enphase IQ Battery 10C. For existing Powerwall 2 systems, check upgrade compatibility.
What the proposal should show
- Which circuits are backed up, controlled or excluded.
- Power output and whether the AC and pumps can start.
- Available energy, starting charge and estimated runtime.
- Overnight use and solar recharge in poor weather.
- Controllers, installation work, warranties and expansion options.
Tell us what you want to run. We’ll size the system around it. Request a battery plan.