The selection of a power inverter depends on the power in watts of the appliance(s) you want to run. You can find the power consumption of any electrical appliance by referring to the specifications nameplate of the appliance or refer to its user manual. Another very important thing you will need to know about your appliance is its continuous power rating and its peak/surge power rating.
Continuous Power
The continuous power of an inverter is the total power in Watts that it can handle indefinitely.
Peak/Surge Power
Peak/surge power is the amount of power an inverter is capable of providing for a specific period of time. Appliances like air conditioners, fridges, motors, etc can have a start-up peak/surge of up to 3-5 times their continuous power rating.
How To Calculate The Continous Power (W) Rating Of Your Appliances
After you have decided on which equipment you want to be powered by the inverter, you will need to determine the approximate power these appliances will consume individually. To do this, you will have to multiply the equipment/appliance Amps x 220 (AC voltage) = Watts (approximate).
Example:
To determine the continuous power rating of a Refridgerator with a current rating of 3.5A, we simply multiply the current rating (3.5A) by the AC voltage (220V) = 3.5A x 220V = 770W or 0.77kW (approximate). Do this for all the other loads and work out the total approximate continuous power rating.
How To Calculate Approximate Startup Load (peak/surge watts)
Multiply the equipment/appliance current rating by 3 = Peak/Surge Current (approximate) if it has not been given on the specifications nameplate or user manual. Only do this for appliances that have start-up surges like refrigerators and pumps. In this case, we are using the refrigerator as an example so it will be 3.5 x 3= 10.5A. The Peak/Surge power will be calculated as 10.5A x 220V = 2,310W or 2.31kW Peak/Surge Power.
Summary:
Step 1: List down all the appliances that will be powered by the inverter.
Step 2: Determine the Continous & Peak/Surge Power (if any) of each appliance and get the total estimated load.
Step 3: Find out the Peak/Surge Power the inverter can handle and for how long (get this from the seller or user manual).
From the results of our refrigerator example, you would need an inverter with a continuous rating of approximately 770 watts and a peak/surge rating of approximately 2,310 Watts. It is always advisable to build in a safety factor by overrating the continuous rating by 20 – 25 %.