Guide to buying a power inverter

Power Inverter Buyers Guide

All the information you need to choose and buy your power inverter


What is a Power Inverter?

A power inverter is a piece of equipment that converts low-voltage DC (direct current) power to standard household AC (alternating current) power. An Inverter lets you to operate electrical appliances and electrical equipment using the power produced by a car, truck or boat battery.


How are Inverters typically used?

Inverters provide a source of household power for electrical equipment in Campers, Caravans, Cars, Vans, Trucks and Boats. They can also used to protect your home appliances during a power cut by supplying emergency backup power to essential equipment such as phones, freezers, pumps, or medical devices.

What size of inverter do I need?

One of the main differences between inverters is how much AC power they produce. Before choosing your inverter add up the wattage of each device you intend to operate simultaneously then simply ensure the inverter you select will supply sufficient AC power.

How long will my inverter operate when in use ?

Inverters Powered In-vehicle can run equipment indefinitely while the vehicle's engine is running and battery power is available.

Inverters Powered By Stand Alone Batteries require you to know the size of the battery bank and the load to be supported by the inverter. Usually, this question is phrased as "How long do you want your load to run?", and then specific calculations can be used to calculate the proper size of battery bank for your requirements.

Formulas and Rules for Estimation

1. Watts = Volts multiplied by Amps
2. Battery capacity is expressed by how many Amps for how many hours a battery will last in Amp-Hour (A.H.) capacity. This measurement will normally be visable on your battery.
3. In a 12-Volt power inverter, each 100 Watts of the inverter load requires around 10 DC Amps from the battery
4. In a 24-Volt power inverter, each 200 Watts of the inverter load requires around 10 DC Amps from the battery

The first step is to estimate the total Watts (or Amps) of load, and how long the load needs to operate. This can be determined by looking at the electrical nameplate for each electrical appliance you intend to use simultaneously then adding up the total requirement.

Some loads are not constant, so estimations must be made. For example, a full-sized refrigerator with a 750-Watt compressor running 1/3 of the time would be estimated at 250 Watts-per-hour.

After the load and running time is established, the battery bank size can be calculated. The first calculation is to divide the load (in Watts) by 10 for a 12-Volt system or by 20 for a 24-Volt system giving you the number of Amps required from the battery bank.

Example of Inverter Calculations

1. Total Output Required in Watts = 1000 W
2. Amps drawn from 12-Volt battery = 1000 ÷ 10= 100 Amps DC
3. Amps drawn from 24-Volt battery = 1000 ÷ 20= 50 Amps DC
Next, the number of DC Amps must be multiplied by the time in hours that the load is to operate.

Example of Amp Hour (A.H.) Calculations

If the electrical load is to operate for 3 hours:
For a 12-Volt battery: 100 Amps DC x 3 hours = 300 A.H.
For a 24-Volt battery: 50 Amps DC x 3 hours = 150 A.H.
Now, the proper type and amount of batteries must be selected. Traction batteries, (also called deep cycle or golf cart type), should be used in order to be able to handle the repeated discharge/charge cycles that are required.

Advantages of Pure Sine Wave inverters over Modified Sine Wave inverters:

a) Output voltage wave form is pure sine wave with very low harmonic distortion and clean power like utility-supplied electricity.
b) Inductive loads like microwave ovens and motors run faster, quieter and cooler.
c) Reduces audible and electrical noise in fans, fluorescent lights, audio amplifiers, TV, Game consoles, Fax, and answering machines.
d) Prevents crashes in computers, weird print out, and glitches and noise in monitors.
e)Reliably powers the following devices that will normally not work with modified sine wave inverters:

  • •Laser printers, photocopiers, magneto-optical hard drives
  • •Certain laptop computers (you should check with your manufacturer)
  • •Some fluorescent lights with electronic ballasts
  • •Power tools employing "solid state" power or variable speed control
  • •Some battery chargers for cordless tools
  • •Some new furnaces and pellet stoves with microprocessor control
  • •Digital clocks with radios
  • •Sewing machines with speed/microprocessor control
  • •Medical equipment such as oxygen concentrators