Inverter Size Calculator
The inverter or UPS size for a list of appliances: their running watts, the surge when a motor starts, the VA rating at your power factor and a safety margin.
Result…
How it's calculated
Running load = Σ watts × quantity Peak (one at a time) = running load + max[watts × (surge multiplier − 1)] Peak (all together) = Σ watts × quantity × surge multiplier Rating = load × (1 + margin) · VA = W ÷ power factor
An inverter has two ratings that both matter: the continuous power it can deliver for hours and a short surge rating for the moment a motor or compressor starts, which can draw several times its running power. The running load is every appliance's watts times how many there are. For the surge, if loads start one at a time the worst moment is everything running plus the biggest single start-up extra; if they can all switch on together after a power cut, every surge counts at once. The safety margin is headroom on top. Many UPS and inverter labels give VA as well as watts: VA is watts divided by the power factor, so check that the load fits both.
Your numbers, step by step
About the Inverter Size calculator
This inverter size calculator finds the continuous and surge rating an inverter or UPS needs for the appliances you want to run. Type each load on its own line with its name, watts, how many there are and, for anything with a motor or compressor, a surge multiplier: the starting power divided by the running power. Choose whether surge loads start one at a time or all together, enter the power factor from the inverter's datasheet and a safety margin. You get the running load, the peak at start-up, both ratings with the margin added, and the same figures in VA. It suits anyone planning backup power, an off-grid cabin, a van or a solar system, and each step shows your own numbers.
How to use it
- In “Loads, one per line: name, watts, quantity, surge multiplier”, type lines such as Fridge, 150, 1, 3. Leave out the quantity and multiplier if they are 1.
- Choose how “Surge loads start”: one at a time, or all together after a power cut.
- Enter the “Power factor” from the inverter or UPS datasheet (its W rating ÷ its VA rating).
- Set a “Safety margin” for headroom.
- Pick an inverter whose continuous W and VA ratings and surge rating are at least the results.
Worked example
A fridge of 150 W with a surge multiplier of 3, a 400 W water pump that also surges 3 times, 8 lights of 10 W and a 12 W router run 150 + 400 + 80 + 12 = 642 W. Starting one at a time, the worst moment is the pump starting: 642 + 400 × (3 − 1) = 1,442 W. With a 25% margin the inverter needs 642 × 1.25 = 802.5 W continuous and 1,442 × 1.25 = 1,802.5 W surge. At a power factor of 0.8 that is 802.5 ÷ 0.8 = 1,003 VA.
Updated
Questions people ask
How do I work out what size inverter I need?
Add up the running watts of everything that will be on at once, then add the extra power the largest motor draws as it starts. Add a safety margin to both, and check the inverter's continuous and surge ratings against them.
What is the difference between watts and VA on an inverter or UPS?
Watts are the real power the load uses; VA is the apparent power, watts divided by the power factor. A unit labelled 1,000 VA at a power factor of 0.8 gives at most 800 W, so the load must fit both numbers.
What is a surge multiplier?
It is how many times its running power an appliance draws for a moment as it starts. Motors, pumps and compressors surge; lights and electronics usually do not. The appliance's label or maker gives the starting current or power.
Is it a mistake to size an inverter on running watts only?
Yes, for anything with a motor. A fridge or pump can trip an inverter that handles its running power easily, because the start-up peak is several times higher. Always check the surge rating and how long the inverter can hold it.
Does this tell me how long my batteries will last?
No, it sizes the inverter only. Use the battery runtime calculator with the running load to see how long a battery bank lasts. Have a qualified electrician install anything wired into a building.