Battery tools

How long will a 5 kWh battery last?

It depends entirely on the load — and on the fact that a 5 kWh label does not mean 5 kWh reaches your appliances. Here is the arithmetic at realistic household loads.

How long does a 5 kWh battery last?

A 5 kWh nominal battery holds 4.50 kWh of usable DC energy at 90% depth of discharge, and delivers 4.14 kWh to AC appliances after 92% inverter conversion. Carrying a 250 W average load — a fridge, freezer, router and some lighting — that is 16 h 34 min. At a 690 W whole-home essentials load it falls to 6 h 0 min. Load, not capacity, is the variable that moves this answer most.

Runtime at common loads

5 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	46 h 0 min	46 h
Refrigerator only (cycling)	55 W	3 d 3 h	75.3 h
Fridge, freezer, comms and lighting	250 W	16 h 34 min	16.6 h
Essential circuits, typical home	690 W	6 h 0 min	6 h
Essentials plus a window air conditioner	1200 W	3 h 27 min	3.5 h
Small ducted heat pump running hard	1800 W	2 h 18 min	2.3 h

Change the assumptions

The capacity is pre-filled at 5 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

16 h 34 min

at 250 W average load

5.00 kWh nominal becomes 4.14 kWh delivered to AC appliances

Calculated from your inputs

Nominal energy: 5.00 kWh
Usable at DC: 4.50 kWh
Delivered to AC: 4.14 kWh
Delivered fraction of nominal: 83%

Runtime in hours

16.6 h

Exact quotient before rounding to hours and minutes.

Energy not delivered to AC

0.860 kWh

Depth-of-discharge limit (0.500 kWh), additional reserve (0.000 kWh) and inverter loss (0.360 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

5 kWh × 0.9 usable depth of discharge = 4.5 kWh available at DC
4.5 kWh × 0.92 inverter efficiency = 4.14 kWh delivered to AC loads
4.14 kWh ÷ 0.25 kW = 16.56 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 5 kWh on the label, 0.500 kWh is held back by the depth-of-discharge limit that protects cell life, and roughly 0.360 kWh is lost as heat in the inverter converting DC to AC. That leaves 4.14 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 5 kWh the decision is which appliance you exclude

A 5 kWh pack is small enough that the sizing question stops being “how many hours do I get” and becomes “what am I willing to leave switched off”. The reason is that the essentials list is a low, cycling load, while the appliances people most want during an outage are resistive and constant. A single 1,500 W space heater draws 86% of the combined load in the table below, more than the fridge, freezer, router and lighting put together.

That is why the arithmetic below is not a smooth curve. Adding one heater to the essentials list costs you 14 h 12 min of runtime, collapsing 16 h 34 min into 2 h 22 min. There is no depth-of-discharge setting or inverter upgrade that recovers it — the energy simply is not in the box.

Each row adds one appliance to the same 250 W essentials baseline on a 5 kWh nominal pack, using the site default depth of discharge and inverter efficiency. Generated by batteryEngine-1.2.0. Continuous operation is assumed, which is pessimistic for the microwave and the air conditioner and realistic for the heater.
Added to the essentials list	Total average load	Runtime	Runtime lost	Why
Nothing added — essentials only	250 W	16 h 34 min	—	Fridge, freezer, router, lighting
One 900 W microwave, running continuously	1150 W	3 h 36 min	−12 h 58 min	Resistive; only used in short bursts in practice
One 1,200 W window air conditioner	1450 W	2 h 51 min	−13 h 42 min	Compressor load, cycles rather than runs flat out
One 1,500 W portable space heater	1750 W	2 h 22 min	−14 h 12 min	Purely resistive; no cycling if the thermostat never satisfies

The practical conclusion for this capacity: treat a 5 kWh battery as a device that keeps refrigeration, communications and light alive through an outage of roughly overnight length, and plan to heat or cool by some other means. If space heating during an outage is the actual requirement, the honest answer is that no realistic battery size solves it and a fuel-burning appliance or a generator is the right tool. A 5 kWh pack is also commonly a single module, so “buy a bit more later” usually means matching the same product and firmware rather than adding an arbitrary second battery.