Battery tools

How long will a 10 kWh battery last?

It depends entirely on the load — and on the fact that a 10 kWh label does not mean 10 kWh reaches your appliances. Here is the arithmetic at realistic household loads.

How long does a 10 kWh battery last?

A 10 kWh nominal battery holds 9.00 kWh of usable DC energy at 90% depth of discharge, and delivers 8.28 kWh to AC appliances after 92% inverter conversion. Carrying a 250 W average load — a fridge, freezer, router and some lighting — that is 33 h 7 min. At a 690 W whole-home essentials load it falls to 12 h 0 min. Load, not capacity, is the variable that moves this answer most.

Runtime at common loads

10 kWh nominal, 90% usable depth of discharge, 92% inverter efficiency, no additional reserve held back. Generated by batteryEngine-1.2.0. Runtime assumes a constant average load.
Load	Average draw	Runtime	Runtime in hours
Router, phone charging and a few LED lights	90 W	3 d 20 h	92 h
Refrigerator only (cycling)	55 W	6 d 7 h	150.5 h
Fridge, freezer, comms and lighting	250 W	33 h 7 min	33.1 h
Essential circuits, typical home	690 W	12 h 0 min	12 h
Essentials plus a window air conditioner	1200 W	6 h 54 min	6.9 h
Small ducted heat pump running hard	1800 W	4 h 36 min	4.6 h

Change the assumptions

The capacity is pre-filled at 10 kWh. Adjust the load, depth of discharge, inverter efficiency and reserve to match your own system.

Your inputs

How do you know your battery size?

Nominal battery capacity

Average AC load

The average draw across the outage, not the sum of nameplate ratings.

Largest startup surge

Motor loads such as a fridge, well pump or air conditioner. Used for the power check, not for runtime.

Usable depth of discharge

Inverter efficiency

Reserve held back

Additional reserve kept above the battery's usable-depth limit. This is separate from depth of discharge, and is applied after it.

Inverter continuous rating

Optional. 0 leaves the continuous-power check off.

Inverter surge rating

Optional. 0 leaves the startup-surge check off.

Scenario: average load

33 h 7 min

at 250 W average load

10.0 kWh nominal becomes 8.28 kWh delivered to AC appliances

Calculated from your inputs

Nominal energy: 10.0 kWh
Usable at DC: 9.00 kWh
Delivered to AC: 8.28 kWh
Delivered fraction of nominal: 83%

Runtime in hours

33.1 h

Exact quotient before rounding to hours and minutes.

Energy not delivered to AC

1.72 kWh

Depth-of-discharge limit (1.00 kWh), additional reserve (0.000 kWh) and inverter loss (0.720 kWh).

Startup surge entered

None entered

No inverter surge rating entered, so no check is performed — compare it with your inverter manufacturer's surge rating.

Power check

Continuous-power check off — enter an inverter continuous rating to run it.
Startup-surge check off — enter both a startup surge and an inverter surge rating to run it.

These checks compare power ratings only. They say nothing about how long the battery lasts, and a continuous rating does not by itself define what the inverter can supply for a few seconds.

Method

10 kWh × 0.9 usable depth of discharge = 9 kWh available at DC
9 kWh × 0.92 inverter efficiency = 8.28 kWh delivered to AC loads
8.28 kWh ÷ 0.25 kW = 33.12 hours

Energy capacity and power capability are separate limits. Passing the energy check does not guarantee the inverter can start the load.

Assumptions

Usable depth of discharge 90% of nominal capacity.
Inverter/conversion efficiency 92% on the DC→AC path.
No reserve held back.
Load is treated as constant at the average value; real loads vary.
Temperature effects, ageing and self-consumption of the system are not modelled.

What can change the result?

The average load, which dominates the answer
Usable depth of discharge set by the battery management system
Inverter efficiency, lower at very light loads
Cold temperatures reducing usable capacity
Any solar recharging during daylight hours

Where the missing kilowatt-hours go

Of the 10 kWh on the label, 1.00 kWh is held back by the depth-of-discharge limit that protects cell life, and roughly 0.720 kWh is lost as heat in the inverter converting DC to AC. That leaves 8.28 kWh, about 83% of the headline figure. Any runtime estimate that divides the nameplate capacity straight by your load will overstate reality by about 21%.

At 10 kWh the constraint moves from energy to power

This is the capacity at which the interesting question flips. A pack this size delivers enough energy that a normal essentials list is no longer the problem: it will sustain roughly 1035 W for a full eight-hour night, which is comfortably above what refrigeration, communications and lighting actually draw. So instead of asking how long the battery lasts, the useful calculation at 10 kWh is the inverse — given the window you want to cover, how many watts may you average?

Sustainable average load solved backwards from 8.28 kWh of delivered AC energy on a 10 kWh nominal pack, using the site default depth of discharge and inverter efficiency. Generated by batteryEngine-1.2.0. These are averages across the whole window, not instantaneous limits.
Window you want to cover	Duration	Sustainable average load	In kilowatts	Note
Evening peak, dinner to bedtime	5 h	1656 W	1.66 kW	Cooking and lighting included
Overnight, lights out to morning	8 h	1035 W	1.04 kW	The most common planning window
Evening plus overnight	12 h	690 W	0.69 kW	Outage starting at dusk
A full day with no recharge	24 h	345 W	0.35 kW	Requires deliberate load shedding

Read the third column carefully, because it is an average, and averages are exactly what a 10 kWh system fails on. The energy budget permits well over a kilowatt across an evening, but the inverter has a separate, unrelated rating: a continuous power limit, and a short-term surge limit for motor starting. A well pump, a submersible sump pump, an air-conditioning compressor or a deep-well booster can demand several times its running wattage for the first second or two. When a 10 kWh installation fails during an outage, it is usually the inverter tripping on that surge with eight kilowatt-hours still sitting in the battery — not the battery running flat.

That changes what you should verify before buying. At 5 kWh the binding question is stored energy; at 10 kWh you have bought your way past that, and the specification that decides whether the system actually works is the inverter’s continuous and surge rating against your largest motor load. Two systems with identical 10 kWh batteries and different inverters are not equivalent products, and the battery datasheet will not tell you which one keeps your well running.

The second thing that becomes worth modelling at this capacity is daily cycling rather than outage survival. Ten kilowatt-hours is a meaningful fraction of a typical household’s daily electricity use, which makes self-consumption and rate arbitrage a real question here in a way it is not for a smaller pack. Whether that pays depends on the spread between your import and export rates, not on the battery specification.