How the conversion works
Power, voltage and current are tied together by one of the most useful relationships in electrical work.
Power factor only applies to alternating current. Resistive loads such as heaters and kettles have a power factor of 1.0, meaning voltage and current stay in step. Motors and compressors pull current slightly out of phase, so they draw more amps than the wattage alone suggests, typically at a power factor between 0.8 and 0.9.
Why this number matters off-grid
Current, not power, determines the physical hardware. Cable thickness, fuse rating, breaker size, terminal size and heat loss all follow amps. A 1,200 W inverter load on a 12 V bank pulls 100 A, which demands heavy cable, a large fuse and short runs. The same 1,200 W on a 48 V bank pulls 25 A and can be wired with something a fraction of the cross-section, at a fraction of the cost.
That is the entire practical argument for choosing 24 V or 48 V in any system beyond a small van build. The panels and appliances do not care; the copper between them does.
Worked example
A 1,200 W load on a 12 V system, with an inverter that is roughly 87% efficient:
Always size cable and fuses against the battery-side figure, not the nameplate. Inverter losses raise the current the battery must supply, and that is the current your wiring has to carry.
Common mistakes
- Using AC watts with DC voltage. An appliance rated 1,200 W at 230 V draws about 5 A on its own circuit, but the 12 V battery feeding the inverter still supplies over 100 A. Both figures are correct; they describe different sides of the inverter.
- Ignoring power factor on motor loads. A 1,000 W pump at 0.8 power factor draws the current of a 1,250 W resistive load.
- Sizing fuses to the load instead of the wire. A fuse protects the cable, not the appliance. It must be rated below the cable ampacity and above the expected continuous current, typically 1.25 times the load.
- Forgetting surge. The steady figure this calculator gives is not what a compressor pulls at start-up, which can be two to three times higher for a second.
Frequently asked questions
How many amps is 1000 watts?
At 12 V, about 83 A. At 24 V, about 42 A. At 120 V, about 8.3 A. At 230 V, about 4.3 A. The higher the voltage, the lower the current for the same power.
What size fuse do I need?
A common rule is 1.25 times the continuous current, rounded up to the next standard size, then checked against the ampacity of the cable you are using. The badge above applies that rule as a starting point, but the cable rating always has the final say.
Does this apply to three phase?
No. Three phase adds a factor of the square root of three into the formula. This calculator covers single phase AC and DC, which is what off-grid, RV, marine and small backup systems use.