Free electrical planning tool
Inverter Size Calculator
Estimate a continuous inverter rating from the 230V appliances you expect to run together, and see the corresponding leisure battery demand.
Your system
Preset input powers are illustrations, not appliance specifications. Edit to match your equipment.
Your results
- Total simultaneous running load
- 1,000 W
- Estimated minimum continuous inverter capacity
- 1,200 W
- Estimated DC battery current at running load
- 92.59 A
- Estimated battery energy usage
- 555.56 Wh
- Battery energy at nominal voltage
- 46.3 Ah
Common nominal capacities for comparison: 1500 W · 2000 W · 3000 W Not a product or suitability recommendation.
Startup/surge requirement: undetermined
Manufacturer startup data is missing for one or more selected appliances. The surge requirement is incomplete.
Verify surge duration and overlapping starts separately. No common surge multiplier is assumed.
Planning estimates, not installation approval.
How to use this calculator
Add appliances using input power rather than output power (particularly for microwaves). Tick those operating together. Add manufacturer-stated startup power where available, then set voltage, efficiency, headroom and runtime.
Calculation method
Running load = sum of selected running watts × quantity. Continuous rating = running load × (1 + capacity margin). DC current = running watts ÷ efficiency ÷ nominal battery voltage. Battery Wh = running watts ÷ efficiency × runtime.
What the results mean
The margin applies to continuous inverter capacity, not consumption. Surge estimates assume one unit starts while all other selected units run. Multiple simultaneous starts and surge duration are not modelled. Missing surge data remains unknown, not zero.
Worked example
A 1,000W simultaneous load with 20% headroom suggests at least 1,200W continuous capacity. At 12V and 90% efficiency it draws approximately 92.59A, consuming 555.56Wh over 30 minutes. At 24V the current is approximately 46.30A.
Common mistakes
Do not treat a microwave’s cooking watts as its electrical input. A continuous watt rating does not establish surge performance. Leaving an appliance unticked excludes it from both running load and the surge scenario.
Frequently asked questions
Why can a large inverter overload a battery?
The DC input current can be very high at 12V. Battery discharge limits, the BMS, cables and fuses must all support the load.
Should all appliances use the same surge multiplier?
No. Startup behaviour varies. Use actual manufacturer data, and check peak duration and simultaneous starts.
Does a 2,000W inverter use 2,000W continuously?
No. Consumption depends on connected loads and losses, plus standby consumption. Its rating is a capacity limit, not a constant load.
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