Multiply: AMPS X 120 (AC voltage) ＝WATTS This formula yields a close approximation of the continuous load of the appliance
⦿ To Calculate Approximate Startup Load:
Multiply: WATTS X 2 ＝ Starting Load This formula yields a close approximation of the starting load of the appliance, though some may require an even greater starting load.
NOTE: Induction motors such as air conditioners, refrigerators, freezers and pumps may have a start up surge of 3 to 7 times the continuous rating.Most often the start up load of the appliance or power tool determines whether an inverter has the capability to power it.
For example, you have a freezer with a continuous load of 4 amps, and a start up load of 12 amps: 4 amps x 120 volts ＝ 480 watts continuous 12 amps x 120 volts ＝ 1440 watts starting load You would need an inverter with peak-surge rating greater than 1440 watts.
⦿ To Convert AC Watts to DC Amps:
AC Watts divided by 12 x 1.1 ＝ DC Amps (this is the size vehicle alternator you would need to keep up with a specific load; for example, to keep up with a continuous draw of 1000 watts, you would need a 91 amp alternator)
⦿ To Calculate How Long Your Appliance Will Operate On Your Battery:
For a 12 Volt System: (10 x (Battery Capacity in Amp Hours) / (Load Power in Watts)) / 2 ＝ Run Time in Hours
For a 24 Volt System: (20 x (Battery Capacity in Amp Hours) / (Load Power in Watts)) / 2 ＝ Run Time in Hours
✓Tip1: Deep cycle (marine) batteries generally have the highest reserve ratings. They are also capable of withstanding repeated drains of power and recharging. ✓Tip2: Engine start batteries should not be discharged below 90% charged state, and marine deep cycle batteries should not be discharged below 50% charged state. Doing so will shorten the life of the battery based on most battery manufacturers recommendations.
Note: If you intend to use power tools for commercial use, or any load of 200W for more than 1 hour regularly (between battery recharging) we recommend installing an auxiliary battery to provide power to the inverter. This battery should be a deep cycle type and sized to meet your run time expectations with the engine off. The auxiliary battery should be connected to the alternator through an isolator module to prevent the inverter from discharging the engine start battery when the engine is off.