Size home battery storage for an outage by energy and by power
Updated October 2026
List what you want to keep running (refrigerator, lights, internet, furnace blower, well or sump pump, maybe a window air conditioner) and find each one’s wattage on its nameplate or with a plug-in watt meter. Average load decides runtime: a refrigerator’s compressor cycles on and off, so its average is much lower than its running wattage. Largest load at once decides power: add up everything that could run at the same moment, including motor start-up surges. Then pick a battery (or enter a custom capacity and continuous power) and the hours of backup you want. Delivery efficiency covers conversion losses; 90% matches the AC round-trip figure on Enphase’s IQ 5P datasheet.
A battery has two limits. Capacity in kilowatt-hours decides how long it lasts; continuous power in kilowatts decides what it can run at the same time. The number of units you need is whichever limit requires more.
For a 1,000-watt average load over 24 hours at 90% efficiency: 1 kW × 24 h ÷ 0.9 = 26.67 kWh. Powerwall 3 stores 13.5 kWh, so energy needs 26.67 ÷ 13.5 = 1.98, rounded up to 2 units; a 4,000-watt peak is well under one unit’s 11.5 kW, so power needs 1. The answer is 2. One unit alone runs 13.5 × 0.9 ÷ 1 = 12.2 hours. With the 5 kWh IQ Battery 5P, energy needs 26.67 ÷ 5 = 5.3, so 6 units, and power needs 4 ÷ 3.84 = 1.04, so 2; the answer is 6.
| Plan | Average / peak load, hours | Energy needed | Powerwall 3 units | IQ 5P units | Runtime on one Powerwall 3 |
|---|---|---|---|---|---|
| Fridge, lights, internet, phone charging | 500 W / 3,000 W, 12 h | 6.7 kWh | 1 | 2 | 24.3 h |
| Essentials plus furnace blower or sump pump | 1,000 W / 4,000 W, 24 h | 26.7 kWh | 2 | 6 | 12.2 h |
| Essentials plus a window air conditioner | 1,500 W / 6,000 W, 24 h | 40.0 kWh | 3 | 8 | 8.1 h |
| Most of the house, central air cycling | 2,500 W / 9,000 W, 12 h | 33.3 kWh | 3 | 7 | 4.9 h |
| Whole house with electric cooking | 3,000 W / 12,000 W, 24 h | 80.0 kWh | 6 | 16 | 4.0 h |
Loads here are examples, not measurements; your own appliances may draw more or less.
Tesla’s Powerwall 3 stores 13.5 kWh and delivers up to 11.5 kW continuously. Enphase’s IQ Battery 5P stores 5 kWh usable and delivers 3.84 kVA continuously, with short peaks of 7.68 kVA for 3 seconds or 6.14 kVA for 10 seconds for motor starts; its datasheet lists 90% AC round-trip efficiency. Units are stacked to reach the size you need. Solar panels can recharge batteries during a long outage, which can stretch backup from hours into days.
Using nameplate watts as the average. A 700-watt refrigerator might average a fraction of that over a day, so nameplate totals oversize the battery for energy. Ignoring start-up surge. Well pumps and air conditioners can briefly draw several times their running watts and trip a battery that has plenty of energy left. Skipping the efficiency loss overstates runtime by about 10%. Assuming whole-home backup by default. Many installations back up a critical-loads panel, not every circuit. Planning on full capacity for years. Capacity fades with use; the IQ 5P warranty, for example, covers the battery staying above 60% capacity for up to 15 years or 6,000 cycles.
It does not design the system. A licensed electrician or certified installer decides which circuits are backed up, how the battery isolates your home from the grid during an outage, whether your panel needs changes, and what permits and utility approvals apply. Ask for a load calculation based on your actual circuits before buying.