Figuring out the Watt or Amp rating for an appliance or piece of equipment can be found somewhere on the equipment or you can use this estimator found online by clicking here.
Once you have your Watt or Amp rating, total all equipment and then follow these guidelines.
An inverter needs to supply two needs – Peak or surge power, and the typical or usual power.
- Surge is the maximum power that the inverter can supply, usually for only a short time (usually no longer than a second unless specified in the inverter’s specifications). Some appliances, particularly those with electric motors, need a much higher start up surge than they do when running. Pumps, compressors, air conditioners are the most common example-another common one is freezers and refrigerators (compressors). You want to select an inverter with a continuous rating that will handle the surge rating of your appliance so you don’t prematurely burn out the inverter. Don’t rely on the inverters surge to start your equipment because inverters don’t like to operate in their surge mode unless the manufacturer claims to have a longer surge time than normal.
- Typical is what the inverter has to supply on a steady basis. This is the continuous rating. This is usually much lower than the surge. For example, this would be what a refrigerator pulls after the first few seconds it takes for the motor to start up, or what it takes to run the microwave – or what all loads combined will total up to. (see our note about appliance power and/or name tag ratings at the end of this section).
You can use the following formula to determine the size:
Volts * Amps = watts
Watts / Volts = amps
1250 Watt example:
1250 / 120 Vac = 10.41 amps ac (typical number found on equipment)
1250 / 12 Vdc = 104.1 amps dc (battery drain per hour)
Here is an example:
First, you need to determine what items you need to power during a power failure and for how long. Here is a brief example (watt requirements vary):
- Lights – About 200 watts
- Refrigerator – About 1000 watts
- Radio – About 50 watts
- Heater – About 1000 watts
Total wattage needed is 2250 watts. The fridge and heater have a start up power requirement so let’s allow 2x the continuous wattage for start up requirements. 2250 * 2 = 4500 watts
***Remember to do your math based on surge rating of your equipment. Here are some guidelines:
- Air conditioners and compressors can surge up to 5x the continuous rating
- Microwaves can surge up to 3x the continuous rating
- Refrigerator, freezers, heaters can sure up to 3x the continuous rating
- Motors up to 3x the continuous rating
- Tools up to 2x the continuous rating