Batteries

Battery capacity vs inverter power

Energy and power are separate limits. A battery can hold plenty of energy and still fail to start a well pump.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

A large industrial pump and motor in a plant room, supplied from a battery and inverter on the wall behind

Quick answer

Energy and power are separate specifications, and they fail in different ways. A battery with plenty of kilowatt-hours cannot start a well pump if the inverter cannot supply the pump’s inrush current — the system trips within a second, with the battery still nearly full.

Check three numbers before trusting a backup design: stored energy, continuous power, and surge power for a few seconds. Runtime arithmetic only answers the first.

How to read the numbersSourced valueIllustrative modelCalculated result

Energy limit: kWh — how long; Sourced value
Power limit: kW — what at all; Sourced value
Surge limit: kW for seconds; Sourced value

Three limits, three questions

energy limit : usable kWh ÷ average load (kW) = runtime (h)
power limit : sum of simultaneous running watts ≤ inverter continuous rating
surge limit : largest starting surge + other running loads ≤ inverter surge rating
battery limit : continuous power ≤ pack C-rate × nominal voltage × capacity

All four must pass. The one that fails first determines what the system can actually do, and it is rarely the energy limit.

The fourth line catches a case people miss: the battery itself has a discharge current limit. A 5 kWh pack rated 0.5C can supply only about 2.5 kW continuously, no matter how large an inverter you attach. Pairing a 10 kW inverter with a small, low-C-rate pack buys nothing.

Starting surge is the usual failure

Anything with an induction motor — compressors, pumps, older air conditioners, some power tools — draws a locked-rotor current several times its running current for a fraction of a second while the rotor accelerates. Three to eight times running power is normal.

Inverters advertise a surge rating, typically 1.5–2× continuous for a few seconds. That headroom is what absorbs motor starts, and it is consumed by whatever else is already running when the motor starts.

Typical running power and starting surge by appliance
Appliance	Running	Typical starting surge
Refrigerator compressor	150 W	600 – 1,200 W
Freezer compressor	120 W	700 – 1,400 W
Circulation pump	80 W	240 – 500 W
Well pump, 1 hp	750 W	2,500 – 6,000 W
Submersible sump pump	500 W	1,500 – 3,500 W
Air conditioner, non-inverter	1,200 W	3,600 – 7,000 W
Microwave (as drawn, not cooking power)	1,400 W	1,400 – 1,800 W
Resistive space heater	1,500 W	1,500 W (no surge)

Worked example: a 3 kW inverter and a well pump

Already running: fridge, freezer, lighting160.5 W average, 270 W when both compressors runIllustrative model
Well pump running power750 WIllustrative model
Well pump starting surge≈ 4,000 WIllustrative model
Peak demand at the moment of start270 + 4,000 = 4,270 WCalculated result
Inverter continuous rating3,000 WYour input
Inverter surge rating (2× for 5 s)6,000 WYour input

VerdictPasses surge, but only just

What reduces the power problem

Soft starters. Fitted to a compressor or pump, they cut inrush by roughly half and are far cheaper than a larger inverter.

Inverter-driven appliances. Modern inverter fridges, heat pumps and air conditioners ramp up instead of switching on hard, and have almost no surge.

Load shedding. A managed backup panel prevents the biggest loads from running together.

Honest scoping. Deciding that the well pump is not a backup load is a legitimate engineering answer, and usually the cheapest one.

Check running, average and surge power together

Select the loads you want to keep running and see all three figures at once — running power, average power for runtime, and worst-case surge — against four common battery sizes.

Open the backup builder

Assumptions and limitations

Surge figures are typical ranges for the appliance class, not measurements of your equipment. Motor inrush varies with motor design, load and supply voltage.
Inverter surge ratings are specified for a limited duration and at a stated temperature. A hot inverter in a warm cupboard may not deliver its rated overload.
Life-safety equipment — medical devices, sump pumps protecting a property, heating in freezing conditions — should be sized by a qualified installer, not from a web calculator.

Sources

IEC 62040 — Uninterruptible power systems, performance and test requirements — International Electrotechnical Commission
Supports: Separate specification of stored energy, continuous output rating and overload capability.
NEMA MG 1 — Motors and generators — National Electrical Manufacturers Association
Supports: Locked-rotor starting current of induction motors, several times rated running current.