How Long Does a Home Battery Last During an Outage?
A 13.5 kWh home battery runs essentials for about a day, half a house for 4–5 hours, and a whole home with A/C for under 2. See the runtime math, load-by-load table, and what shortens it.
The honest answer is a formula, not a number: runtime = usable battery capacity ÷ what you keep running. A 13.5 kWh battery that powers essentials for a day will run a whole home with air conditioning for under two hours. Same battery — the load is the variable.
The runtime table
| Critical load | What it typically covers | Runtime on 13.5 kWh |
|---|---|---|
| 0.5 kW | Fridge cycling, LED lights, router, phone charging | ~27 hours |
| 1 kW | Essentials plus a TV, fans or a small appliance | ~13 hours |
| 3 kW | Half the house — lights, fridge, some plug loads | ~4.5 hours |
| 7.5 kW | Whole home without A/C, or with intermittent large loads | ~1.8 hours |
| 3–5 kW continuous | Central A/C alone (compressor cycling) | ~3–4.5 hours of A/C |
| 1.5 kW | One space heater | ~9 hours |
Ideal-case math: capacity ÷ load. Real-world runtime is shorter — see the factors below.
What actually shortens runtime
- Usable vs nameplate capacity. Some systems reserve capacity or limit depth of discharge; a “13.5 kWh” battery may deliver less than that.
- Round-trip efficiency. Roughly 90% of the energy you put in comes back out; the rest is lost as heat.
- Surge loads. Motors (fridge compressors, well pumps, A/C) draw several times their running power at startup.
- Temperature. Cold garages and extreme heat reduce usable capacity.
- Battery age. Capacity degrades over time — most warranties guarantee about 70% at year 10.
What a battery can and cannot carry
| Load | Typical running draw | Can a 13.5 kWh battery run it? |
|---|---|---|
| Fridge / freezer | 0.1–0.2 kW average (cycling) | Yes — most of a day or more |
| Lights, router, phones | 0.1–0.3 kW | Yes — days |
| Sump pump / well pump | 0.8–2 kW when running | Yes, but check surge and total energy |
| Space heater | 1.5 kW | Hours, not days — a big battery drain |
| Central A/C | 3–5 kW | Hours at most; surge matters |
| Heat pump (winter) | 3–6 kW | Hours at most; aux heat can exceed output |
Extending runtime: solar and good habits
- Solar + battery can recharge during the day, turning a one-day reserve into multi-day resilience in sunny outages.
- Drop non-essentials. Turning off A/C, space heaters and the oven can multiply runtime.
- Stagger big loads. Run the well pump or microwave one at a time to avoid surges and overloads.
- Sizing matters. If your goal is whole-home overnight coverage, one 13.5 kWh unit is rarely enough — use the battery size calculator to work backward from your loads.
Pick a preset (essentials, half house, whole home) or enter your own critical load — the calculator shows runtime for a battery, a generator and a portable station, plus 10-year cost.
Wondering whether the battery is worth it beyond outages? The ROI calculator adds up bill savings, incentives and degradation, and the three-way backup guide compares it with a generator and a portable station.
Quick answers
Will a 13.5 kWh battery run my whole house?
For a short time, yes — but not for a day. Whole-home loads of 7.5 kW exhaust 13.5 kWh in under two hours. Most homes back up essential circuits instead, which stretches the same battery to a day or more.
How long can a home battery run a fridge?
Modern fridges average roughly 0.1–0.2 kW when cycling, so a 13.5 kWh battery can keep one running for several days in theory. In practice, the fridge shares the battery with lights, router and other essentials, so plan for one to two days.
Can a home battery run central air conditioning?
It can start and run A/C briefly if the battery’s continuous and surge output are high enough (many units deliver 10–11.5 kW). Runtime will be short — hours at most — and a second battery is usually needed for meaningful cooling during an outage.
Sources
- Home Battery Report — home battery runtime reference (13.5 kWh class)
- EnergySage — battery capacity and cost benchmarks (2026)
- Aurora Solar — battery backup vs generator (2026)
- NREL ATB — representative residential battery capacity and efficiency
All figures checked November 2026. Runtime depends on your equipment and conditions; treat tables as estimates.