Example:
You want to power a computer, some lights and a radio with power as below.
Computer:
300W
2 x 60W lights:
120W
Radio:
10W
Total Power Needed:
430W
For this application, an inverter with minimum power of 600W is recommended. As 600W is twice the
power of the highest power device and the total power of 430W is still within the maximum power
rating of the inverter. Of course it will be more ideal to move one step up and choose a 1000W inverter
which would allow for additional devices to be powered or added on in the future.
Helpful formula:
Many specifications on the product only quote current (amps) instead of power, to convert current to
power, below formula can be used:
To convert Amps to Watts:
Current (Amps) × 230 (AC voltage) = Power (Watts)
This formula yields a close approximation of the continuous load of the appliance.
To calculate approximate start-up load:
Power (Watts) × 2 = Start-up Load
This formula yields a close approximation of the starting load required by the appliance, although some
appliances 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 up to seven times their continuous power 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 2 Amps, and a start-up load of 6 Amps:
2 Amps x 240 Volts = 480 Watts continuous
6 Amps x 240 Volts = 1440 Watts starting load
You would need an inverter with peak-surge rating greater than 1440W.
To work out current/battery capacity requirement (for 12V system):
Power (Watts) / 12V = Current hour (Ah)
This is the rough calculation for the size of a vehicle alternator you would need to keep up with a specific
load, as an inverters efficiency is only rated around 90%, the current requirement will need to be 10%
higher; for example, to keep up with a continuous draw of 1000W, you would need:
(1000 ÷ 12) × 1.1 = 91.67Ah
Therefore an alternator with minimum output current of 91.67A at 12V is required to run continuously.
Alternatively a fully charged 12V battery with capacity of 91.67Ah can run continuously for around an
hour.
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