GOING SOLAR: Understanding The Equipment – The Inverter (Part 2)

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In last week’s instalment of GOING SOLAR we looked at the various types of the POWER INVERTER available. This week we are going to look at how we select the right inverter to meet our needs.

Before selecting an inverter there are three main inverter ratings that we MUST understand about the inverter we want.

  1. Continuous Rating: This is the amount of power you could expect to use continuously without the inverter overheating and shutting down.To operate “comfortably” an inverters continuous rating will be around 70% of the inverters maximum rating. A 1000 watt inverter would power a 700 watt load comfortably for hours on end.
  2. Half Hour Rating: This is very useful as the continuous rating may be too low to run, say a large power tool or appliance but if the appliance was only to be used intermittently then the half hour rating may well be high enough to cover this.This is normally at 100% of the inverters capacity, and then it will start to warm up and then trip.
  3. Surge Rating: A high surge is required to start some appliances that once up and running may only need considerably less power to keep functioning. The inverter must be able to hold this surge rating value for at least 5 seconds. Pumps and refrigerators are two such items that require only relatively low power once running but require a high surge to start.This rating is usually 200% the inverters rating, thus a 1000 watt inverter should be able to deliver 2000 watts of power over 5 seconds.

Selecting an Inverter

As with selecting the right sized solar panel or solar battery, we need to understand our energy demand as well as the electrical behaviour of the loads. We have already looked at an example when we looked at energy efficiency. Let’s look at that example one more time:

AppliancePower rating (watts)Hours UsedConsumption
Total Power Used(watt/hours)330

Taking the example above,and looking at the three rating categories, we can describe our load above as having the following characteristics.

  • The demand will not fluctuate much below or above the 165 watts, and remains constant over a long period
  • The appliances do not demand a large amount of power to start and therefore there is no large surge demand.
  • The TV is the most sensitive of the appliances and requires clean power.

The inverter size – Using the “continuous output rating” ratio, our load above requires an inverter of approximately 250 watts in size.

The Inverter type – Our inverter does not have to have a large surge rating so we can select a high frequency inverter.

The type of wave form – As we have a sensitive piece of equipment in our schedule, the TV, we shall select a pure sine wave inverter. If our load was only made up of the lights we could select a modified sine wave inverter.

Let us now look at the same load, but now let us include a motor based appliance like a fridge.


AppliancePower rating (watts)Hours UsedConsumption
Total Power Used (watt/hours)1,010

Looking at this particular schedule, we can now describe the load as follows:

  • The demand will fluctuate between 165 watts and 335 watts, and is thus not constant. This means our inverter must be able to handle the higher 335 watts over possible longer periods.Of all the appliances the “fridge” is the one that will demand a large amount of power to start. Our inverter therefore needs to be able to carry the constant load of 165 watts above the fridge surge power requirement.
  • The TV is still the most sensitive of the appliances and requires clean power.

Now sizing and selecting the inverter for this load:

The inverter size–In this example we need to look at two major issues, the first being the constant power required and the second the surge rating. Note that that to avoid any inverter failures, the surge rating will take precedence over the constant rating.

In this case and based on the continuous rating and a total load of 335 watts our inverter size will be a minimum 500 watts. This inverter will be capable of a surge of 1000 watts.

Fridges/freezer motors have a surge rating of between 4 to 10 times their running ratings. For our purposes let’s average this at 7 times. This means our fridge above will demand 1,190 watts for at least 5 seconds to start up.

This means that our selected 500 watt inverter is too small. The minimum size inverter we need must already be providing 165 watts of continuous power to the other appliances, and then accommodate a surge of 1190 watts. The total being 1,355 watts required for 5 seconds. As we do not want the inverter to trip, we add a 30% margin for the inverter to be “comfortable”. The total will be 1,760 watts. Round this number up, we can work with a total 2,000 watts.

Assuming this to be double our inverters size, the appropriate inverter size will be 1000 watts.

The Inverter type – As our inverter will need to handle a large surge demand several times per day, we need a more robust design, that being a transformer based inverter.

The type of wave form – To efficiently handle both the sensitive equipment and large surge demand a pure sine wave inverter will work best for us. Although a larger modified sine wave inverter may also do, it may not be the best for our fridge or TV, the unclean power will shorten the life of our appliances.

There are several more considerations to be made when selecting an inverter, and we will touch on these slightly more complicated options as we learn more about GOING SOLAR in the near future.

Next week we will take all we have learnt and size a practical solution for a real home. If you have been following us so far, please fill in the form HERE and you will receive a free energy assessment that will include recommendations for a solar power solution. We will then select one of these to use as an example next week for a complete OFF-GRID solar power solution.

. . . . . . . And remember this:-

Choose a cheap modified sine wave inverter if your needs are modest and occasional.
Choose a high quality modified sine wave inverter if you want value for money and a 99% usable product.
Choose a high quality sine wave inverter if you want the best available. It also to note that if you want to power “anything” then this is the range to go for.

After the inverter series we look at other solutions that will help you reduce or even eliminate your need for grid power. Please keep sending your questions to us at tech@clamorepower.com or visit our FAQ page for more information. Please also follow us on Twitter  and FACEBOOK

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