# How to calculate your Inverter Power rating requirement

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Posted by Felix Okoli on Friday October 23, 2015 at 11:42:19:

There are many types of inverters in the market in terms of their brands and power rating. It can be really confusing for a non-tech person to decide which one of them to buy even though he already knows what a Power inverter can do.

How does a person decide on which inverter to buy in terms of their power rating? An inverter Power rating is expressed in Volt-ampere (VA).

We hear of inverters that are 900VA, 1KVA, 3KVA, 5KVA and so on. I've also come to know that the higher their power rating, the higher their market prices. Yes, some inverters sell for above N500,000 in Nigeria while some sell for as low as N20,000. It really depends on how much power you want and how much you can budget for it.

One of the first things to do before deciding on what type of inverter you need in terms of their power rating is to determine how much power you want it to generate.

Let's assume that you need at least 600 watts, then you'd need an inverter that can provide that 600 Watts while also considering it's efficiency using a discounting factor called Power factor. Since inverters are not considered perfect or 100% efficient in terms of power supply, we can consider a power factor of 0.8 or 80% efficiency.

The next step is to find an inverter than can provide that 600 Watts to your devices and also choosing a battery that can power it.

The calculation for that is => VA = Power/PF(Power factor) = 600/0.8 = 750 VA.

The above calculation shows that we'd need an Inverter with a Power rating of at least 750 VA if it's 80% efficient.

The next step is to find out what capacity of battery we need based on the given criteria which is to power devices with 600 Watts while also considering how long we need to keep supplying that power in hours.

Let's assume that we want to supply the 600 watts for up to 4 hours from a battery connected to a 750 VA inverter, then we can calculate that using the following formula;

Ah = Watt*hrs/Volts

i.e. Battery capacity(Ah) = Power(Watts) * Back up hours (hrs)/Battery Voltage (in volts)

So, if the battery Voltage is 12 volts and the hours need for backup is 4 hours while the power required is 600 Watts, then:

Battery capacity(Ah) = 600 * 4/12 = 200 Ah.

The above calculation shows that we need a 12 Volt battery that has a capacity of at least 200 Ah. You can always increase the Amperage-Hour(Ah) depending on your Power requirement by connecting more batteries in parallel.

So, a 750 VA(or 900 VA) inverter can work with a 200 Ah 12 Volt Deep cycle battery to supply 600 Watts for up to 4 hrs. The battery would need to be recharged afterwards. If you needed up to 1.2 KW of power, then you'd need to connect a second battery in parallel while also getting a 1.5KVA Power inverter.