Inverter Efficiency Calculator
Inverter efficiency is the ratio of usable AC power out to DC power in: efficiency (%) = (AC output power ÷ DC input power) × 100.
Enter Values
How to use this calculator
- Enter any two of the three fields: DC input power (from the battery/PV side), AC output power (the load), and efficiency (%).
- Leave the third field blank — the calculator solves for it and also shows the power lost as heat.
- To size a battery draw, enter your AC load and the inverter's rated efficiency to get the true DC watts pulled.
How it works
Inverter efficiency is the ratio of usable AC power out to DC power in: efficiency (%) = (AC output power ÷ DC input power) × 100. Because every inverter dissipates some energy in switching devices, transformers and control circuitry, output is always less than input, so efficiency is below 100%. The shortfall, input minus output, leaves the unit almost entirely as heat.
Given any two of the three quantities the third follows by rearranging that one formula: input = output ÷ efficiency, or output = input × efficiency. This is a snapshot at one operating point — real inverters show an efficiency curve that peaks near their rated load and drops at very light loads, where fixed no-load (standby) consumption dominates.
Worked example
A 500 W load drawing 550 W of DC. An off-grid inverter delivers 500 W to an AC load while pulling 550 W from the battery bank. Enter DC input power = 550 W and AC output power = 500 W. Efficiency = 500 ÷ 550 = 0.9091 = 90.91%. The inverter wastes 550 − 500 = 50.00 W as heat. If instead you knew only the 500 W load and a rated 92% efficiency, the tool solves the other way: input = 500 ÷ 0.92 = 543.48 W, with 43.48 W lost.
Common mistakes
- Swapping input and output — the DC input is always the larger number. If you enter output greater than input the tool rejects it, because an inverter cannot output more than it draws.
- Entering efficiency as a fraction (0.92) instead of a percentage (92). This field expects a percentage from 0 to 100.
- Assuming one efficiency figure applies at every load. Peak efficiency is quoted near rated power; at light loads standby losses pull real-world efficiency down.
Frequently asked questions
How do I calculate inverter efficiency?
Divide the AC power delivered to the load by the DC power drawn from the source, then multiply by 100. For example, 500 W out from 550 W in gives 500 ÷ 550 × 100 = 90.91% efficiency, with 50 W lost as heat.
What is a good inverter efficiency?
Modern pure sine wave and solar inverters typically peak around 90–97%, with high-frequency and grid-tie models at the top of that range. Below about 85% peak, or a steep drop at partial load, suggests an older, cheap, or oversized unit.
Why does my inverter draw more DC watts than the AC load?
The extra draw is the inverter's own losses — switching, magnetic and standby consumption — that turn into heat. If a 500 W load pulls 550 W of DC, those 50 W are the efficiency loss, which is exactly what this calculator reports.
Does inverter efficiency change with load?
Yes. Efficiency is low at very light loads because fixed no-load consumption is a big share of the total, rises to a peak near the rated load, and can taper slightly at maximum output. This tool gives the figure for the single operating point you enter.
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