Inverter Size Calculator
Add up the watts of everything that will run at once, enter the biggest start-up surge, the battery voltage and how long you need power. The calculator suggests an inverter size and the battery to match.
- Running and surge watts
- 12, 24 or 48 V
- Battery for runtime
Find watts on each appliance's label. For motors, surge is about 3× running watts if not listed.
An inverter must handle both the steady load and the start-up kick. Motors in fridges, pumps and power tools draw two to three times their running power for a moment when they start. Inverters can supply a short surge, usually about twice their rating, so they must be big enough for both.
Appliance watts chart
Typical running and surge. Search for an appliance.
| Appliance | Running (W) | Surge (W) |
|---|---|---|
| LED lights (10) | 100 | 100 |
| Laptop | 65 | 65 |
| TV | 120 | 120 |
| Fridge | 150 | 600 |
| Microwave | 1200 | 1200 |
| Kettle | 2000 | 2000 |
| Water pump | 750 | 2250 |
| Drill | 600 | 1200 |
| Air conditioner (small) | 900 | 2700 |
Labels show the maximum. Real use can be lower, such as a fridge compressor cycling on and off.
Load and inverter size
With surge at twice the load. Slide the load.
Battery current is the hidden cost. 3,000 W on 12 V draws about 280 A, which needs very heavy cable.
How to size an inverter
Watts, surge, voltage. Step through it.
List appliances
Those used together.
Keep battery cables short and thick. Long thin cables lose voltage and can overheat.
Pure or modified sine wave
Output types. Compare them.
Clean mains-like power. Runs everything.
Stepped waveform. Fine for simple loads.
Pure sine with a built-in battery charger.
Some medical equipment, chargers and variable-speed tools only work properly on pure sine.
Before installing
Safety first. Tick each item.
Tick each item as you go.
Fit a fuse or DC breaker close to the battery on the positive cable. It protects the cable if anything shorts.
What you get
The whole system. Each tile explains one result.
Standard size in watts.
Peak covered.
DC amps.
Ah for runtime.
Suggested system.
Battery Ah assumes 90% inverter efficiency and using 80% of the battery, which suits lithium. Lead-acid should only use 50%.
Frequently Asked Questions
No question matches that search.
What size inverter do I need?
Add running watts, add 25%, and check the surge is under twice the rating.
What size inverter to run a fridge?
A 300 to 600 W pure sine inverter. A typical fridge runs at 150 W but surges to about 600 W when the compressor starts.
What size inverter for a microwave?
About 1,500 to 2,000 W, since a 1,200 W microwave can draw more from the supply.
How many batteries for a 2,000 W inverter?
At full load for 3 hours on 12 V, about 690 Ah of lithium.
Should I use 12 V or 24 V?
Above about 1,200 W, 24 V or 48 V keeps current and cable size manageable.
Is pure sine wave worth it?
Yes for motors, fridges and electronics.
What is inverter surge?
Short-term extra power, usually about twice the rating for a few seconds.
An inverter that copes
Add your running watts, allow for the biggest surge and pick a standard size. Match the battery and cables to the current, and you will have reliable power wherever you need it.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.