Inverter Calculator
Add what you want to run — get the inverter, cable and battery you need.
Add something to run to see your result.
- Continuous
- — all items on
- Startup surge
- — largest motor starting
- DC current
- — at 12 V
- Cable
- — 5 ft · 3% drop
- Run time
- — 100 Ah lead-acid
- Battery / day
- — add hours per day
How these numbers are worked out
Continuous watts is the sum of everything you listed. If you entered amps, we multiply by the AC output voltage.
Starting surge is everything running at once, plus the extra draw of your largest motor as it starts — we allow 3× its running watts. Motors surge only for a second or two and two compressors rarely start at the same instant. For something genuinely hard to start — an air conditioner, a deep-well pump, an old compressor — allow more headroom, or choose an inverter quoting a high surge rating.
Inverter size is the smallest we sell that covers your continuous load and can handle the surge — most inverters tolerate roughly 2× their rating briefly, so the requirement is the greater of your continuous watts and half your surge watts. Pure sine wave is worth it for anything with a motor, a compressor or sensitive electronics; modified sine wave is fine for simple resistive loads like heaters and incandescent lights.
DC input current is the continuous watts divided by 10 at 12 V (20 at 24 V, 40 at 48 V). Dividing by 10 rather than 12 builds in the inverter’s efficiency loss, so the figure is realistic rather than theoretical.
Cable size uses the Voltage Drop Index method: VDI = (amps × one-way feet) ÷ (% drop × battery volts), matched against the standard copper AWG table — the same method used for marine and solar installs — and never smaller than the size rated to carry the current (ABYC figures for 105°C-rated cable outside engine spaces). Follow ABYC or local code for fusing and installation. Ready-made inverter installation kits pair cable with the matching fuse. For a quick lookup by amps and distance, see the 12 volt wire size chart.
Run time is the battery’s usable amp-hours divided by the DC current, as though everything runs at once. We count 50% of a lead-acid or AGM battery as usable and 80% of a lithium (LiFePO4) battery. Lead-acid gives less than its rated capacity when drawn hard, so at higher currents we apply Peukert’s law with an exponent of 1.2.
Energy per day is each item’s watts times its hours a day, plus about 20% for inverter losses; battery per day divides that by the battery voltage and the usable share. Leaving the inverter switched on adds its own idle draw — often 0.5 to 1.5 A at 12 V — so switch it off when nothing needs it.
Recharging: size a battery charger at 10–20 A per 100 Ah of battery — see our battery chargers compared.