Inverter Size Calculator

How is an inverter sized?

A battery inverter has two ratings: the power it can deliver continuously and the surge power it can hold for a short time. Both must be checked.

Continuous rating

Add the power of the devices on the inverter, apply a demand factor for those that do not run together and add a spare margin:

P_inv = P × k_e × (1 + y)

Surge power

Motor driven devices such as fridges, pumps and compressors draw several times their rated power when they start. The worst moment is when such a device switches on while the other loads are running:

P_peak = running load − P_k + k_k × P_k

The surge rating in the inverter datasheet must exceed this and last for the whole start.

Example

Out of a 1500 W total, 500 W is a fridge with a starting multiple of 3. With a 25 percent margin the continuous rating is 1500 × 1.25 = 1875 W. At start 1500 − 500 + 3 × 500 = 2500 W is needed. On a 24 V battery at 90 percent efficiency the running load draws 1500 / (0.9 × 24) = 69 A.

Current on the battery side

At low voltage the same power means very high current. In a 12 V system 1000 W takes 83 A. That current sets the cable size, the fuse and the battery capacity, all of which need their own calculation.

Limits

The starting multiple varies a lot from device to device and the tool does not suggest a value. The result is a first estimate: confirm it with the manufacturer datasheet and the applicable standard.