Inverters are used to transform a low voltage DC source into a high voltage AC output, letting you run household devices from a low voltage DC electrical system. These are ideal for running appliances such as laptops, televisions, and coffee machines where an equivalent 12V variant might be considerably more expensive, or not available at all.
Modified Sine wave
Modified (also known as Quasi) sine wave inverters produce a wave format that is square in shape, loosely resembling a sine wave. This inverter variant is ideal for non-sensitive equipment, such as laptop chargers and TVs, and is priced more affordably than pure sine wave inverters.
Pure sine wave inverters are more advanced, producing a much smoother waveform that closely matches the waveform from grid AC power. While more expensive than modified sine wave inverters, pure sine wave inverters are necessary for sensitive devices that may not work correctly with modified sine wave inverters. Devices that may require a pure sine wave inverter include power tools that have AC motors, battery chargers and certain fluorescent lights.
When choosing an inverter, the primary factors to consider are the format (modified or pure) and the maximum continuous output capacity.
To calculate the minimum output power required by the inverter, the total power usage of all devices that will be connected should be measured. This is often printed on the device label, either in a power rating (in Watts) or a current usage rating (in Amps).
A current rating can be converted to a power rating by multiplying the current by the voltage (e.g. 2.5A X 230V = 575W).
The inverter chosen should have at least 50% higher capacity than the total power rating of all devices connected to it.
Surge currents should also be considered, certain appliances might draw a large current at start-up to power a motor for example, potentially damaging to an underpowered inverter. Therefore, if the continuous load is just 0.5A but the start-up current is 4A then the inverter should have a surge rating of no less than 920W (4A X 230V = 920W).
The cable size depends on the power and voltage of the inverter, as well as the length of the cable. The higher the power and voltage, and the longer the cable, the thicker the wire gauge should be to avoid voltage drop and overheating.
You can use this formula to calculate the maximum current that will flow through your circuit: watts ÷ volts = amps. For example, if you have a 1000 watt inverter with a 12V input, the maximum current will be 1000W ÷ 12V = 83.3 amps. Inverters can surge to double this amount for example if you are running equipment with a motor will require more energy on start up. With this in mind its best to double the fuse rating but be careful to check your cables maximum rating.
1000W Inverter Example: 1000 ÷ 12 = 83.33 x 2 = 166.66. The recommended cable size for under 3 metres would be 35mm² which is capable of 240A. Based on this a good fuse rating would be 150-200A. This allows for surges but also doesn’t get too close to the maximum cable rating. We recommend that you use a high-quality fuse such as Midi, Mega, ANL or even T-Class. We also recommend you keep the wire lengths as short as possible by keeping the inverter as close to the battery as you can.
Inverter Size (W)
(> 3 metres)
Fuse Size (A)
The battery capacity depends on the power and duration of the appliances that need to be run by the inverter.
To calculate the battery capacity for an inverter, we need to know the total load (in watts) and the usage time (in hours) of the appliances.
Battery capacity (Ah) = Total load (W) x Usage time (h) / Input voltage (V) x Efficiency
The input voltage is the nominal voltage of the battery, usually 12V or 24V. The efficiency is a factor that accounts for the losses in the inverter and the battery, usually around 0.8 to 0.9.
For example, if we want to use a 1000W inverter at 12V to power a 400W TV and a 200W laptop for 3 hours, the battery capacity would be:
Battery capacity (Ah) = (400 + 200) x 3 / 12 x 0.8 = 75 Ah
This means we need a battery with at least 75 Ah capacity to run these appliances for 3 hours with a 1000W inverter at 12V.
However, this is only a minimum requirement and does not account for other factors such as battery aging, temperature, depth of discharge, and safety margin. Therefore, it is advisable to choose a battery with higher capacity than the calculated value or use multiple batteries in parallel to increase the total capacity.
A general rule of thumb is to have at least 20% of the inverter capacity as battery capacity for a 12V system, and 10% for a 24V system. For example, for a 1000W inverter at 12V, we should have at least 200 Ah of battery capacity.