Inverter Size Calculator
Sized by the surge, and the DC current will surprise you.
Work out Inverter Size. Sized by the surge, and the DC current will surprise you. Answers in the units the job actually uses.
Recommended inverter
2,500 W
1,500 W continuous, 2,500 W surge
The surge, not the running load, sizes the inverter. A fridge or pump compressor draws several times its running wattage for a fraction of a second at start, and an inverter sized to the running figure shuts down every time the fridge kicks in. DC current is the figure that governs cable and fusing, and it is much larger than people expect: 1,500 W at 12 V is over 135 A, which needs seriously heavy cable over any distance. Moving to 24 V halves it and 48 V quarters it, which is the main reason larger systems use higher voltages. Idle draw runs whenever the inverter is on, whether anything is using it or not. Twenty watts sounds trivial and is nearly half a kilowatt-hour a day — often a significant share of a small off-grid system's entire budget.
How the Inverter Size Calculator works
Inverter sizing from continuous load and motor surge, with the DC-side current that governs cable and fusing. The surge sizes the inverter — a unit rated to the running load shuts down every time the fridge starts.
Also known as: what size inverter for my system · inverter surge rating explained · dc amps from an inverter · 12v vs 48v system
Frequently asked questions
What size inverter do I need?
Large enough for the surge, not just the continuous load. Add the starting demand of your largest motor to everything else running, which is usually a much larger figure than the running total.
Why do motors need surge capacity?
A compressor or pump draws several times its running wattage for a fraction of a second at start. An inverter sized to the running figure hits its limit and shuts down every time.
Why is the DC current so high?
Because power is voltage times current, and the DC side runs at 12, 24 or 48 V rather than mains voltage. 1,500 W at 12 V is over 135 A, which needs seriously heavy cable over any distance.
Should I use 12, 24 or 48 volts?
Higher voltage for higher power. Doubling the system voltage halves the current for the same power, which allows thinner cable and cuts resistive losses fourfold. Anything above about 2 kW belongs at 48 V.
What is idle draw?
What the inverter consumes just being switched on, typically 10 to 30 W. That sounds trivial and is up to 0.7 kWh a day — often a significant share of a small off-grid system's entire budget.
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