Inverter Efficiency: Complete Guide and Calculator

By:Michael Johns

Inverter efficiency

Inverter efficiency can be a real head-scratcher…

You think you think you’ve finally worked out the best size inverter to run your appliances and then someone mentions inverter efficiency.

It’s annoying as it can throw all your calculations out the window if it’s not included.

That’s why I include it in all my guides such as How Long Will a Battery Power an Inverter.

If you’re wondering what inverter efficiency is all about and how to calculate it, then keep reading.

What is Inverter Efficiency?

Inverter efficiency is how much Direct Current (DC) is converted into Alternating Current (AC)

This is the primary function of an inverter, unfortunately, it is not 100% efficient.

It means that energy is lost during the conversions. So less energy is output than is input.

In fact, inverter efficiency can vary dramatically between products, on average it is between 85% and 95%. For example, if you have an inverter with 85% efficiency it means only 85% of your battery power is being sent to your appliances. The other 15% is lost/used up in the inverter.

There are 2 real reasons that you lose energy in an inverter:

  1. Heat loss – During the conversion of DC to AC some of the energy is lost as heat.
  2. Internal systems – Inverters need a little power for run systems like cooling, safety protections, LEDs, and digital screens. (That’s why an inverter will use energy even when no appliances are drawing power).

What Affects Inverter Efficiency?

No brand or model of the inverter is the same, and they all have a different efficiency rating. It’s a bit like cars and their fuel efficiency! 

So what affects inverter efficiency and how can you spot the most efficient products?

Inverter Type – Pure vs Modified Sine Wave

The big thing to consider when looking for an efficient inverter is pure and modified sine wave.

Pure sine wave inverters are the most modern type of inverter which deliver superior performance. Pure sine wave inverters offer between 90% and 95% efficiency. 

The other option is a modified sine wave inverter which is generally cheaper but is less efficient. These lower-performance products usually have an efficiency of below 90%.

How to Calculate Inverter Efficiency

It is possible to calculate the efficiency of a power inverter although it can be a little complicated.

The easiest way to find an efficiency rating is to check the manufacturer’s technical information.

Inverter Efficiency Calculation Formula

There are 2 different formulas used to calculate inverter efficiency:

European: This is a weighted number that accounts for the inverter operating at different levels of power output. It is more accurate than simple peak efficiency as the efficiency varies depending on the power output. This is the formula:

ηEu = 0.03η5% + 0.06η10% + 0.13η20% + 0.10η30% + 0.48η50% + 0.20η100%

California Energy Commission: This is very similar to the European formula above but uses a slightly different weighting factors. Here is the formula:

ηCES = 0.04η10% + 0.05η20% + 0.12η30% + 0.21η50% + 0.53η75% + 0.05η100%

Peak Efficiency: When a manufacturer gives you an efficiency rating, this will be the peak efficiency. But an inverter does not always maintain peak efficiency, hence the power weightings in the above formula. The following graph shows you have it changes based on changes in power.

Above you will see that efficiency is reduced when you a drawing a small load from an inverter. It is not until you reach around 30% load that you get the best performance.

Inverter Efficiency and What You Can Run

Now you understand that inverters are not 100% efficient we can discuss when that means for you. It plays a part in dictating what you can run with your inverter.

It is best practice to work on the basis that your inverter has 80% efficiency so that you don’t accidentally overload it. Let me explain what I mean.

calculate inverter efficiency

When adding up the total wattage of all your devices you should add another 20% to get your minimum inverter size. 

For example, if your appliances use a total of 1000 watts, you should add another 20%. You can do this by multiplying the number by 1.2. So 1000 x 1.2 = 1200 watts. 

Now you know to draw 1000 watts the minimum inverter size you need is 1200 watts with efficiency included.

Is Inverter Efficiency Important?

Yes, by now you should understand that inverter efficiency is important as it directly affects the performance of your equipment. When drawing high loads, it could be the difference between your inverter overloading or not. Adding to this, it is important to take into account when working out how long your battery energy will last.

Are Larger Inverters More Efficient?

No, in most cases bigger is not better for efficiency. In fact, as you can see on the graph above, trying to draw a small load from a big inverter will massively reduce efficiency.

Some larger inverters won’t even work when you try to draw 50 watts or less as they have an eco-mode to save power. This means you might not be able to charge and phone or run a small fan from a big inverter.

Final Thoughts: Inverter Efficiency

Inverter efficiency can be frustrating in many ways, not least because it is not simple to calculate an exact answer.

For everyday use, it is easiest to use a safe figure such as 80% efficiency when working out what you can power. This way you should give yourself some leeway in power consumption.

To be honest, no electronics are 100% efficient as energy will always be lost through heat.

Of course, the better quality product you buy, such as the best pure sine wave inverter, the higher performance you will get.