Backup power
Home battery backup calculator
Multiply your critical loads in kWh per day by the days of outage, then divide by inverter efficiency (about 90%). Check power too: the batteries must deliver your peak kW at once, which often decides the count before energy does.
8.9 kWhof battery capacity
- Energy your loads use
- 8.0 kWh over 1 day
- Units of 13.5 kWh wall battery
- 2 → 27.0 kWh, 10.0 kW continuous
- Units needed for energy / for power
- 1 / 2
Check: Power, not energy, sets the count here: 6.0 kW of simultaneous load needs 2 units at 5.0 kW each.
No solar recharge counted. With panels, a battery only has to cover the night and cloudy days.
The formula
energy needed (kWh) = daily kWh × days ÷ efficiency ÷ usable fraction units by energy = ⌈ energy needed ÷ unit kWh ⌉ units by power = ⌈ peak kW ÷ unit continuous kW ⌉ units = the larger of the two
A home battery has two limits. Energy (kWh) is how long it lasts. Power (kW) is how much it can deliver at the same moment. A 13.5 kWh battery rated at 5 kW can run a 1 kW load for over 12 hours, but it cannot start a 4-ton AC compressor at all. When people are disappointed by a battery, it is usually power, not energy.
Most current home batteries quote usable kWh, so leave depth of discharge at 100% unless your spec sheet gives total capacity. Round-trip efficiency (AC in to AC out) is 85–92% for lithium systems.
Typical outage scenarios
Computed for 13.5 kWh / 5 kW units, 90% efficiency. Last two columns: kWh needed and number of units.
| Scenario | kWh/day | Peak kW | 1 day | 3 days |
|---|---|---|---|---|
| Essentials: fridge, lights, internet, phones | 4 | 1.5 | 4.4 kWh · 1 | 13.3 kWh · 1 |
| Essentials + gas furnace blower (winter) | 9 | 3.0 | 10.0 kWh · 1 | 30.0 kWh · 3 |
| Essentials + well pump | 7 | 4.0 | 7.8 kWh · 1 | 23.3 kWh · 2 |
| Whole home, no AC, gas heat | 18 | 6.0 | 20.0 kWh · 2 | 60.0 kWh · 5 |
| Whole home with central AC (summer) | 45 | 9.0 | 50.0 kWh · 4 | 150.0 kWh · 12 |
Find your own daily kWh with the appliance energy calculator, or from your utility's hourly data: the lowest-usage day in spring or fall is roughly your baseline load.
Battery vs. generator for backup
Batteries are silent, need no fuel and switch over in milliseconds, but a multi-day outage without solar outlasts them. A generator runs as long as you have fuel. Many homes pair a battery for short outages with a portable generator for long ones. Compare fuel needs with the fuel consumption calculator.
Safety. Home batteries connect to your service equipment and must be installed by a qualified installer under permit, following NEC Article 706 and the manufacturer’s instructions. Your utility usually needs to approve it too.
Common questions
How many kWh do I need to power my house during an outage?
For essentials only (fridge, lights, internet, phones) plan on 3–5 kWh per day. A whole home without central AC typically uses 15–25 kWh per day; with central AC in summer, 40–60.
Is one 13.5 kWh battery enough?
For essentials, yes, for about two to three days. For a whole home it covers most of a night but not a full day, and its 5 kW output limit (on many units) will not start central AC.
Does solar change the math?
Yes. With panels, the battery only needs to cover the night and cloudy stretches; a day of good sun can refill it. Without solar, this calculator's result is the full requirement.
Guides that use this calculator
- How Long Will a 13.5 kWh Home Battery Last in an Outage?
- How Long Will a 1,000 Wh Power Station Actually Last?