As an Amazon Associate, this site earns commissions from qualifying purchases. For more details, click here.
There are a lot of factors to consider when buying an inverter for your solar system. But the most important are its capacity and efficiency as they determine what you can do with it. Is bigger always better? Well, size does matter, but there is more to it.
An inverter uses 10% more power than its appliance load due to inefficiency and standby mode requirements. Inverter efficiency increases with a higher load, so they should always run close to full capacity.
How to Calculate Inverter Output Load Efficiency
There are two figures you need to calculate, the inverter efficiency rating and its load output efficiency.
A large inverter with a small load wastes more power than a small inverter carrying a similar capacity. But if you increase the inverter load, the efficiency level goes up.
The formula is watts in / watts out = inverter load efficiency
Inverters use power when in standby mode, that is, even without any load. With a 200W portable inverter, this could be 5 watts.
If you load a 35W fan, the system would use a total of 40 watts. 35 watts for the fan and 5 watts by the inverter. Divide watts in (40) by watts out (35):
40 watts / 35 watts = .87
The inverter has an 87% efficiency rating.
Now take a 4000 watt inverter like the Energizer 4000 which uses about 25 watts in standby mode. Plug the same 35 watt fan into it and the inverter consumes 55 watts. Use the same steps as above.
35 / 55 = .63
The efficiency rating is only 63%.
But if you increase the load, efficiency goes up. Suppose you place a 3600 watt load on the 4000 watt inverter. It still uses 25 watts when idle so let us factor that in.
3600 / 4025 = .89
The inverter is now running at an 89% efficiency rate.
The bottom line:
Inverters consume power even without load, as we explain here . You have to account for this apart from the inverter efficiency rating, but what is obvious is bigger is not always better And also, a bigger load leads to greater efficiency.
How to Calculate Inverter Efficiency
You will see inverter efficiency ratings like 85%, 90%, 93% etc. This rating determines how much energy is lost when the system converts DC to AC. Solar panels produce DC power so an inverter is needed to run any AC powered electronic device.
An inverter with a 93% efficiency rating means it uses 7% more power than what its load consumes. An 85% rated inverter uses 15% more power and so on.
Inverter watt capacity x energy loss = additional watts used
Take a 3000 watt inverter with a 93% efficiency rating. If you load 3000 watts it will use 3210 watts.
3000 x 7% = 210
3000 + 210 = 3210
If that 3000 watt inverter is 85% efficient, the total watts used will be 3450 watts.
3000 x 15% = 450
3000 + 450 = 3450
Compare this with the Bestek 300 watt portable inverter.
300 x 15% = 45
300 + 45 = 345
Obviously a 300W inverter with an 85% efficiency will consume less power than a 3000W unit with the same efficiency rating.
Just like with load efficiency, you get more value for the inverter when the load increases. If you take advantage of the system capacity you can run more appliances and compensate for its inefficiencies.
Can an Inverter be Too Big?
You should buy the biggest inverter that you will actually use. If the inverter is too big you not only wasted money but you also waste power every time you run it.
If you consume 2500 watts for example, you can buy a 3000 watt inverter. That gives you room for inefficiency, power surges and extra watts in case you run more appliances in the near future.
But if you only load 500 watts do not buy a 3000 watt inverter. It is a waste of time and money. As our calculations show, running a small load on a large inverter also wastes a lot of energy.
if you are not sure what inverter size to get, here is a guide. In addition here are some suggestions.
Calculate how much power you need. Add at least 10% to the total watts to make up for inverter inefficiency. If your total watt consumption is 2800 watts, round it off to 3000 watts.
Decide if you need a modified or pure sine wave inverter. Pure sine wave inverters are more efficient but cost more. Some appliances run better on pure sine but others are compatible with modified. Contact the inverter manufacturer if you are not certain what appliances the device will run.
If you are not sure what to buy, always go with the next largest size available. If you consume 3300 watts, get a 3500 or 4000 watt inverter. And if you use 4600 watts, buy a 5000 watt inverter.
You will lose some watts here but that is all right. You get a few extra hundred watts in reserve in case you need to run a higher load than normal.
What Affects Inverter Efficiency?
The efficiency is determined by how much power is being used. The more an inverter is used to full capacity, the higher the efficiency goes up.
Here is an example. You place a 2500W load on a 3000W inverter. The system consumes 20W without any load. The numbers look like this:
2500 / 3020 = .82
The inverter is running at a 82% efficiency rate. Now what if you increase the load to 2800 watts?
2800 / 3020 = .92
The efficiency goes up to 92%.
Of course you should buy the most efficient inverter you can afford. But the inverter size and its load determine how effective it will be.
Most people focus on the energy loss stemming from efficiency. But you should also consider the load and its effect. Contrary to what some may believe, a small load does not save money. As our calculations indicate, you actually waste more when you do that.
Another factor are the appliances. Most homes -even not on solar power – use energy efficient appliances. But with a solar inverter, energy efficient appliances are a must.
No long winded explanation is needed here. The more efficient the appliance, the less power it consumes. The less power used, the longer the inverter runs and the more you can load. This is especially true with large inverters.
How to Increase Inverter Efficiency
There are several things you can do to improve inverter performance. It all begins with a high efficiency system, but there are other ways to improve performance.
- Use the inverter as indicated in the manual. This is simple enough but bears repeating. Do not use the inverter in any way the user guide does not recommend. If you follow the instructions, the system will run fine.
- Use an efficient battery bank. In an off grid system, the inverter depends on the battery bank to supply power to the load. The battery bank must be large enough to meet the demand and be of high quality. Combine this with energy efficient appliances and your system will get the job done.
- Use energy efficient appliances. Energy efficiency is crucial for inverters and solar power in general. If you plan to go full solar power, invest in energy efficient appliances first.
- Take care of the inverter. Do the same with your entire solar power system and it will take care of your appliances.
There are many other factors, but it boils down to the following. Buy an inverter that is the right size for our power needs, not too large and not too small. Investing in the right system and appliances will provide a healthy return on your investment in just a few years.
Do Inverters Lose Efficiency Over Time?
Do you notice your inverter not running as efficiently as before? Could it be a sign of efficiency problems?
A well designed inverter should keep running at the same level for years. This assumes there are no issues with the system and it is being used properly.
This is exactly the reason why you should buy the highest quality inverter. If the system is no longer running at the same efficiency as before, do some troubleshooting to see what the problem is.
The inverter is one of the most important parts of any solar system. If you want to get the most out of your system, get a large inverter. But do make sure it is appropriate for your power needs.