How to Calculate the Maximum Output Power of a Power Inverter

In this article, we go over how to calculate the maximum power output of a power inverter.

Power inverters are frequently used in off grid power systems in order to supply power to AC appliances.

Everything in a solar system from the solar panel voltage output to the DC battery works based on DC voltage (there are a few exceptions in which some solar panels have inverters attached to them, but that's the exception); therefore, to get AC power, we need a power inverter, which converts the DC power to AC power.

Power inverters contain transformers in order to step up the voltage.

Almost all home power systems will use either a 12V battery sytem, or a 24V battery system, or a 48V battery system.

12V is normally the lowest battery voltage used.

And 48V is normally the highest battery voltage used.

If you are in the United States, 120VAC is the norm for powering AC electrical appliances.

If you are in other parts of the world, it may be 110VAC or 230VAC.

Therefore, you can see that the transformer within the power inverter will need to be step up transformer, which means it increases the voltage. The opposite of this would be a step down transformer, which would decrease voltage.

When you're buying a power inverter for a power system, it will be specified what DC input voltage the inverter is rated for and what AC output voltage it gives out.

Just like batteries, power inverters are rated for voltages as batteries (which makes sense because the voltage from the battery is the input to the inverter). Therefore, you have power inverters rated for 12VDC, 24VDC, and 48VDC.

Some power inverters can work with multiple different voltage levels (eg., 12V/24V).

So we know now that a battery feeds into the input of a power inverter in the form of DC power. As output, we get AC power.

How do we calculate the power output from this power inverter?

So let's do a couple of examples.

Let's start with a 12V system.

So let's say that we have a 12V 30A battery.

And because it's 12V, we get a 12V inverter.

Let's say we're in the United States, and, thus, we need 120V. Therefore, we get a 12V to 120V power inverter.

Therefore, this power inverter increases the voltage tenfold.

We input 12VDC and as output, we get 120VAC. conver

Transformers don't create power though and simply convert it from one state to another, conserving power.

Therefore, if we increased voltage tenfold, we decrease current tenfold.

Therefore, though we get 120VAC as output, we get 10 times less the current.

So if the battery is rated for 30A, the maximum current that we will get out is 3A.

Again, a 12V 30A battery can produce a maximum power output of 120V and 3A.

The power of the battery is 360W (12V x 30A= 360W). The power output of the inverter is 360W (120V x 3A= 360W).

You can see that the transformer within a power inverter conserves power. Power isn't created but simply transformed (from a lower voltage higher current DC source to a higher voltage lower current AC source).

Now let's do another example with the same 12V battery system but let's say now we're in South Korea, which uses 220V. Therefore, we get a 12V to 220V power inverter.

Taking the output voltage and dividing it by the input voltage, we get 18.33 (220V/12V).

Therefore, current will by decreased by a factor of 18.33.

Since the current capacity of the battery is rated for 30A, the maximum current we can get at the output is 1.63A (30A/18.33).

So from a 12V 30A battery with a 12V to 220V power inverter, we get as maximum power 220V and 1.63A of power.

It will not exceed this current draw because a power inverter can only output the amount of power input.

If you want a higher current draw, you'll either have to buy a bigger battery.

So this is the type of math needed when you're working with inverters.

For example, with this battery and this type of inverter, you wouldn't be able to power a device that needs, for example, 3A of power. It would be insufficient power, since the maximum power draw with this battery would be 1.63A.

Now let's replace this 12V 30A battery with a 12V 100A battery.

The voltage will always be stepped up the rated voltage of the power inverter.

What we really want to calculate is what current draw can we get out and will this current draw be enough to power our given device?

So for a 12V 100A battery and a 12V to 120V inverter, we get 120V and 10A as the maximum power that can be drawn.

For a 12V 100A battery and a 12 to 220V inverter, we get 120V and 5.45A as the maximum power that can be drawn.

So all you have to do is find the ratio of the step up voltage by dividing the rated output voltage by the input (battery) DC voltage and then dividing the rated battery current by that ratio to find out the maximum current draw that can be achieved by the load connected to the power inverter.

Let's now do some examples with a 24V system.

For a 24V 50A battery with a 24V to 120V inverter, we can get 120V and 10A as the maximum power draw (50A/5 =10A). 120V/24V= 5, so the step up voltage is 5.

For a 24V 50A battery with a 24V to 220V inverter, we can get 220V and 5.45A as the maximum power draw (50A/9.16= 5.45A). 220V/24V= 9.16, so the step up voltage is 9.16.

For a 24V 100A battery with a 24V to 120V inverter, we can get 120V and 20A as the maximum power draw (100A/5= 20A). 120V/24V= 5, so the step up voltage is 5.

For a 24V 100A battery with a 24V to 220V inverter, we can get 220V and 10.9A as the maximum power draw (100A/9.16= 10.9A). 220V/24V= 9.16, so the step up voltage is 9.16.

Let's now do some examples with a 48V system.

For a 48V 50A battery with a 48V to 120V inverter, we can get 120V and 20A as the maximum power draw (50A/2.5 =20A). 120V/48V= 2.5, so the step up voltage is 2.5.

For a 48V 50A battery with a 48V to 220V inverter, we can get 220V and 10.9A as the maximum power draw (50A/4.58= 10.9A). 220V/24V= 4.58, so the step up voltage is 4.58.

For a 48V 100A battery with a 48V to 120V inverter, we can get 120V and 40A as the maximum power draw (100A/2.5= 40A). 120V/48V= 2.5, so the step up voltage is 2.5.

For a 48V 100A battery with a 48V to 220V inverter, we can get 220V and 21.8A as the maximum power draw (100A/4.58= 10.9A). 220V/48V= 4.58, so the step up voltage is 4.58.

Also important to note that in a home solar system, you should appropriately have wires rarted for the current flowing through the system.

With home systems from batteries from 12V to 48V, the power inverter will always step up the voltage; thus, the current will be lower at the output of the inverter.

With step up inverters, the wiring you use at the output of the inverter does not need to be as thick (or low of AWG) as the wires in the DC portion of the system.

Just make sure the power inverter is rated for the power (in watts) for the amount of power that you are looking to use.

So basically now you know the amount of power that can be drawn from a power inverter with a given battery.

It's basic math that can allow you to calculate the maximum current draw if a load is connected to an inverter.

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