What will you run?
Enter watts from the appliance label. If it only lists amps, use the amps box instead. Start typing a name for common wattages. Tick Motor for anything with a compressor or motor — fridges, freezers, pumps, power tools, air conditioners — which draw several times their running watts for a moment at startup.
Your system
The battery voltage and cable run decide your DC current and the cable size you need. Cable length is the one-way distance from battery to inverter.
Enter a load above
- Continuous load
- 0 W
- Starting surge
- 0 W
- DC input current
- 0 A at 12 VDC
- Minimum cable
- — copper, one way
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, so sizing as though every motor surged together would push you into a far bigger inverter than you need. For something genuinely hard to start — an air conditioner, a deep-well pump, an old compressor — allow more headroom than this tool suggests, 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.
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. This is the same method used for marine and solar installs. Cable that is too thin wastes power as heat and can stop an inverter starting under load — when in doubt, go one size thicker. Follow ABYC or local code for fusing and installation.