How long will a home battery run your house? Honest runtime math
A 13.5 kWh battery runs essential loads for 12–24 hours, most of a home for 6–10 hours, or about 3 hours with central air conditioning running. Runtime tables for ten appliance mixes, the wattage numbers behind them, and the two mistakes that wreck every estimate.
In 50 words: A 13.5 kWh battery runs essentials (fridge, lights, wifi, outlets) for 12–24 hours, most of a home for 6–10 hours, or roughly 3 hours if central air conditioning runs. Runtime equals usable kWh divided by average load in kW — the honest number depends entirely on which appliances you keep.
"How long will it last?" is the question every battery salesperson answers with a number and every homeowner should answer with a spreadsheet. The honest reply is that a battery does not have a runtime — your appliance mix has a runtime, and the battery just supplies it.
Here is the arithmetic, the wattage table behind it, and the two errors that make most published estimates wrong.
Table of contents
- The formula
- What your appliances actually draw
- Runtime by scenario
- The air conditioning problem
- Two mistakes that wreck estimates
- How solar changes everything
- FAQ
- What to watch next
1. The formula
Runtime (hours) = usable capacity (kWh) ÷ average load (kW)
Three refinements make it accurate:
- Usable ≠ nameplate. Most batteries reserve a small buffer; a "13.5 kWh" unit typically delivers 12.5–13.5 kWh usable. Check the spec sheet, not the marketing.
- Round-trip efficiency of roughly 90% applies to energy that went in via charging.
- Average load, not peak. A fridge rated 150 W runs a compressor maybe a third of the time, so it averages closer to 50 W. Using nameplate ratings is the single most common estimating error, and it makes runtimes look far worse than reality.
2. What your appliances actually draw
| Appliance | Running watts | Realistic average |
|---|---|---|
| Refrigerator (modern) | 150–250 W | 40–70 W (duty-cycled) |
| Chest freezer | 100–200 W | 30–50 W |
| LED lighting, whole house | 100–200 W | 60–120 W |
| Wifi router + modem | 15–30 W | 20 W |
| TV + streaming device | 80–150 W | as used |
| Laptop / phone charging | 30–80 W | 40 W |
| Gas furnace blower | 400–800 W | 150–300 W in winter |
| Well pump (1/2 HP) | 800–1,200 W | 60–150 W (short cycles) |
| Sump pump (1/3 HP) | 700–1,000 W | 50–300 W (storm-dependent) |
| Microwave | 1,000–1,500 W | as used |
| Electric water heater | 4,500 W | 300–500 W |
| Central AC (3-ton) | 3,000–4,000 W | 1,500–2,500 W in heat |
| Electric resistance heat | 10,000–20,000 W | enormous |
| Electric dryer | 3,000–5,000 W | as used |
| Electric range/oven | 2,000–5,000 W | as used |
The pattern that determines your runtime: everything that makes heat or moves air is an order of magnitude larger than everything else. Lights, electronics and refrigeration are rounding errors by comparison.
3. Runtime by scenario
Using 13.5 kWh usable (one typical battery):
| Scenario | Average load | Runtime |
|---|---|---|
| Bare essentials: fridge, wifi, LED lights, phones | 0.25 kW | ~54 hours |
| Essentials + TV + occasional microwave | 0.4 kW | ~34 hours |
| Above + gas furnace blower (winter) | 0.6 kW | ~22 hours |
| Above + well pump + sump pump (storm) | 0.8 kW | ~17 hours |
| Most of the house, no AC/electric heat | 1.3 kW | ~10 hours |
| Whole house, summer, central AC running | 3.5–4.5 kW | ~3–4 hours |
| Whole house, electric resistance heat | 8+ kW | under 2 hours |
And how that scales with capacity, at the "most of the house, no AC" load of 1.3 kW:
| Battery bank | Usable | Runtime |
|---|---|---|
| 1 unit | 13.5 kWh | ~10 h |
| 2 units | 27 kWh | ~21 h |
| 3 units | 40 kWh | ~31 h |
This is why the buyer's guide sizing table puts whole-home-with-AC backup at three or more units: it is not about capacity for its own sake, it is about surviving a hot afternoon.
4. The air conditioning problem
Central air is where honest runtime math gets uncomfortable. A 3-ton system pulls 3–4 kW while the compressor runs, and on a 95°F day it runs 50–70% of the time — an average of 1.5–2.5 kW all by itself, which is five to ten times your entire essentials load.
Three ways households actually handle it:
- Accept it. Run essentials on battery, sweat, and treat AC as a grid luxury. Cheapest and most common.
- Swap to a mini-split or high-efficiency heat pump. A single-zone inverter mini-split cooling one room draws 400–800 W — a tenth of central AC — and turns "3 hours" into "most of a day."
- Buy enough battery. Three units and solar recharge. Expensive, and only sensible where outages are long and heat is genuinely dangerous.
Option 2 is the underrated one: for households in hot climates, the cheapest path to comfortable backup is usually reducing the load, not enlarging the battery.
Note also the continuous-output limit: a battery rated 5 kW continuous cannot run a 4 kW AC plus a 1.5 kW well pump simultaneously, regardless of how much energy it stores. Motor surge on startup is a second, separate constraint — many compressors draw 3–5× running watts for a fraction of a second, and a soft-start kit ($300–$500) is often what makes AC-on-battery possible at all.
5. Two mistakes that wreck estimates
Mistake 1: using nameplate watts instead of duty-cycled averages. Adding up the labels on a fridge, freezer, furnace and pump gives 2,500 W and suggests 5 hours of runtime. In reality those loads average nearer 400 W and deliver 30+. Nameplate ratings tell you what the circuit must support, not what the battery will supply.
Mistake 2: forgetting recharge. Runtime assumes the battery starts full and never refills. If you have solar, it refills every day, and total outage endurance is limited only by your daily solar surplus. Without solar, a battery is a one-shot device — full at hour zero, empty when it is empty, no matter how long the outage lasts.
6. How solar changes everything
With solar, the question shifts from "how long does the battery last" to "does daily solar production exceed daily consumption?" If yes, you can ride out an outage indefinitely; if no, the battery buys you the difference.
| Solar array | Typical daily production | Covers |
|---|---|---|
| 5 kW | 18–25 kWh (varies by season/latitude) | Essentials plus much of a normal home |
| 8 kW | 30–40 kWh | Most homes without heavy AC |
| 12 kW | 45–60 kWh | Whole home including AC in many climates |
One critical caveat that surprises people every hurricane season: solar panels alone do not work during an outage. A grid-tied inverter shuts down without the grid, for line-worker safety. You need a battery with islanding capability — or a hybrid inverter with a backup port — for solar to produce anything while the grid is down. That interaction is the core of what we cover in choosing battery storage.
7. FAQ
How long will a 13.5 kWh battery run a house?
About 54 hours on bare essentials, 17–22 hours with a furnace blower and pumps, roughly 10 hours running most of the house without AC, and only 3–4 hours with central air conditioning.
How long can a Powerwall run a refrigerator?
Effectively for days. A modern fridge averages 40–70 W after duty cycling, so a 13.5 kWh battery would run one alone for over a week — refrigeration is never the constraint.
Can a home battery run central air conditioning?
Only briefly, and only if the battery's continuous output rating exceeds the AC's draw. Expect 3–4 hours from a single battery. A soft-start kit helps with compressor surge; a mini-split is the far cheaper path to cooling on battery.
How many batteries do I need for whole-home backup?
Two to three for a home without electric heat, more where central AC or resistance heating must run. Solar substantially reduces the requirement by recharging daily.
Do solar panels work during a power outage?
Not by themselves — grid-tied inverters shut off for safety. You need a battery with islanding capability or a hybrid inverter with a backup port for solar to keep producing during an outage.
8. What to watch next
Three things worth tracking. Continuous-output ratings are rising across new battery models, which matters more than headline capacity for anyone who wants AC or a well pump on backup. Soft-start technology is becoming standard in new HVAC equipment, quietly making battery-backed cooling feasible at smaller system sizes. And load-shedding intelligence in backup gateways keeps improving — the ability to automatically drop the dryer, water heater and AC as the battery drains is what turns an optimistic runtime estimate into a reliable one, and it is now the feature that separates good backup systems from expensive disappointments.
This guide was researched and drafted with AI assistance and edited by a named member of the Earth Energy Log editorial team. Wattage figures are typical ranges for US residential appliances; your actual runtime depends on specific equipment, climate, duty cycles and battery specifications. See our editorial standards and AI disclosure. Related reading: home battery backup buyer's guide, how to choose battery storage, bidirectional EV charging. Explore BESS and the United States hub.