Inverter / UPS Calculator
Home & ConstructionSize a home inverter and battery for the load you want to run and the backup hours you need.
Runs entirely in your browser — nothing is uploaded
Features
- Inverter VA rating from your actual connected load.
- Battery capacity in Ah for the backup hours you want.
- Accounts for power factor, inverter efficiency and depth of discharge.
- 12 V, 24 V and 48 V system options.
- Runs in your browser.
How to use the Inverter / UPS Calculator
- 1Add the appliances you want running during a cut.
- 2Set how many hours of backup you need.
- 3Read the inverter VA and the battery Ah to buy.
Frequently asked questions
What is the difference between VA and watts?
Watts is real power; VA is apparent power. Inverters are rated in VA, and the ratio between them is the power factor — around 0.8 for a home inverter. So a 1,000 VA inverter runs roughly 800 W of load, which is why a unit that looks big enough often is not.
What battery size do I need for 4 hours of backup?
Work out the watt-hours — load in watts times hours — then divide by the battery voltage, and again by the losses. For 500 W over 4 hours on a 12 V system that is around 275 Ah once inverter efficiency and a 60% depth of discharge are allowed for, so a 200 Ah battery will not do it.
Can I run a fridge or an air conditioner on an inverter?
Not on an ordinary home inverter. Motors and compressors draw several times their rated power at startup, and heating elements — geysers, irons, induction hobs — pull their full rating continuously. Sizing for those needs a much larger system, usually with a different battery chemistry.