How Long Will a Home Battery Actually Last During an Outage?

"This battery can back up your whole home" is one of the most overstated claims in solar and storage marketing — not because it's false, but because "whole home" during an outage almost never means running everything the way you normally would. Actual runtime depends entirely on which loads stay on and how much power they draw, which is a household-specific calculation, not a single number that applies to every home with the same battery.

Two numbers, not one, determine runtime

A battery's capacity (kWh) tells you total stored energy. Its continuous power rating (kW) tells you the maximum it can deliver at once. Runtime is capacity divided by your actual load — but your actual load changes constantly as appliances cycle on and off, which is why real-world runtime is usually longer than a worst-case constant-load estimate.

Typical household loads, for reference

| Appliance | Running watts (typical) | Notes | |---|---|---| | Refrigerator | 150–400 W running, briefly higher on startup | Cycles on and off — not continuous draw | | LED lighting (whole home) | 50–150 W | Modest total draw across a typical home | | Wi-Fi router/modem | 10–20 W | Negligible individually | | Sump pump | 800–1,300 W running, higher briefly on startup | Cycles based on water level, not continuous | | Well pump | 1,500–2,500 W running, higher briefly on startup | Can be a significant intermittent load | | Microwave | 1,000–1,500 W | Short-duration, high draw | | Central air conditioner (compressor) | 3,000–5,000 W running, notably higher briefly on startup | The single largest common household load | | Electric water heater | 3,000–4,500 W | Large but often not backed up in an essentials-only setup | | Portable heater | 1,500 W | Common gap-filler during a winter outage, adds up fast if several are used |

Essential-circuits backup vs. whole-home backup

| | Essential-circuits backup | Whole-home backup | |---|---|---| | What's included | A defined subset — fridge, lights, Wi-Fi, sump pump, select outlets, sometimes a mini-split | Everything on the panel, including central AC/heat, electric range, dryer | | Typical combined running load | 1–3 kW | 5–10+ kW, depending on what's running at once | | Requires a sub-panel? | Often yes — a dedicated critical-loads panel isolates backed-up circuits | Some systems (notably those with an integrated whole-panel gateway) manage the full panel without a separate sub-panel | | Typical runtime on a 13.5 kWh battery | Roughly 8–24+ hours, depending on load and solar recharge | A few hours to overnight, if AC or heat is running continuously |

A worked runtime example

Assume a 13.5 kWh battery (no solar recharge during the outage — a worst-case, nighttime scenario) and two different load profiles.

| Load profile | Combined running draw | Approx. runtime | |---|---|---| | Essentials only: fridge, lights, Wi-Fi, occasional microwave use | ~0.5–1 kW average | ~14–27 hours | | Essentials plus a well pump cycling regularly | ~1–1.5 kW average | ~9–14 hours | | Adding a mini-split or window AC running periodically | ~2–2.5 kW average | ~5–7 hours | | Central AC running continuously on a hot day | ~4–5 kW average | ~2.5–3.5 hours |

These are rough averages, since real households don't draw a perfectly constant load — a refrigerator that cycles on for 30% of each hour draws far less on average than its running wattage suggests. The practical takeaway is directional: adding continuous, high-draw equipment (central AC, electric heat, an electric water heater) shrinks runtime dramatically compared to an essentials-only setup, even on the exact same battery.

Why solar changes the math substantially

A battery paired with solar and experiencing a daytime outage can recharge from panel production even while discharging to the home — meaningfully extending effective runtime beyond what the stored capacity alone suggests, especially in the middle of a sunny day. This is one of the biggest practical differences between a standalone battery and a solar-paired one during a multi-day outage: a standalone battery only has what it started with, while a solar-paired one can partially refill itself day by day.

Getting a realistic number for your own home

1. List what you actually want backed up — not everything, just what matters during an outage (medical equipment, refrigeration, some lighting, communications, and anything else genuinely essential).
2. Add up realistic running watts, not nameplate maximums — most appliances don't run continuously at their peak rating; ask your installer for a load calculation based on realistic duty cycles rather than worst-case totals.
3. Confirm the battery's continuous power rating covers your combined peak load, not just your total capacity — a battery with plenty of stored energy can still trip if asked to deliver more instantaneous power than it's rated for, particularly at the moment a large compressor starts.
4. Ask specifically about your highest-draw item — central AC, a well pump, or an electric range are usually the deciding factor between "this backs up my home comfortably" and "this backs up my home for a few hours."

FAQ

Does a bigger battery always mean longer runtime? Only if the power rating also supports your actual loads — a large-capacity battery with a low continuous power rating can still struggle to run high-draw equipment simultaneously, even though it has plenty of total stored energy.

Can I add high-draw appliances to my backup circuits later? Sometimes, if your system has spare power capacity, but it may require electrical work to move circuits onto (or off of) a critical-loads sub-panel, and could reduce runtime for everything else on backup. Discuss this with your installer at the design stage rather than assuming it can be added later without any tradeoff.

Does my battery need to be at 100% charge for an outage to know how long it'll last? Runtime scales roughly proportionally with starting charge — a battery at 50% charge when an outage begins will last roughly half as long as one starting at 100%, all else equal. This is part of why VPP participation (which can lower your reserve level between events) is worth reviewing if outage preparedness is a top priority; see our VPP guide.

Is it worth backing up an electric water heater? Usually not in an essentials-only configuration — a water heater's high, sustained draw consumes a disproportionate share of available capacity relative to how critical hot water typically is during a short outage. Most essential-circuits setups exclude it deliberately.

How does cold weather affect backup runtime? Both the load side and the battery side are affected — heating loads (electric resistance backup heat especially) draw significantly more power than cooling loads typically do, and very cold ambient temperatures can also modestly reduce a battery's usable capacity and power output unless it has active heating, which most current LFP products include.

Fact-checked by Priya Nadar, P.E. Found an error? See our Corrections Policy.