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Understanding how to design a PV system is not rocket science, but it is more complex than many people consider. Here’s a very quick overview of the important points.

Solar panels produce direct current (DC). This means you need an inverter to turn that electricity into mains frequency alternating current (AC).  Inverters come in a range of power ratings. The more solar panels you have, the more power the inverter has to deal with, so the size and cost increases. It’s very important to match the size of the inverter to the number of solar panels.

If the inverter is too small, you will lose out on some of the energy that your system produces. If it is too large, the inverter may not perform at its optimum efficiency, and you will have paid for more than is necessary. In the UK, the optimum situation is to have an inverter that is rated at 80% of the power rating of your PV system, since it is rare you will be producing at 100% power.

More critically than getting the power right, you need to ensure the voltage and current of your solar panel system remains within the input range of the chosen inverter. To re-cap, solar panels on your roof are generally connected together in series, in a ‘string’. This increases the system voltage, but does not increase the current. Once a certain number of solar panels have been connected in series, the voltage will become too high and the system needs to be arranged in two strings, each of the same number of panels, connected in parallel. This generally occurs after a string exceeds 8 – 11 solar panels. When strings are connected in parallel, the currents add-up, but the voltage remains constant.

By adding more and more strings in parallel, the current and voltage can be controlled to remain in the inverter limits. For large solar installations, inverters can used that that have a very high power capacity, or alternatively it is possible to use many small inverters connected in parallel.

It is important to remember certain constraints. Inverters come in several sizes, but there may be some numbers of solar panels for which no inverter is ideal. For instance, because it is necessary for all stings to be equal in size, you can only use an even number of solar panels when using multiple strings. In addition, all solar panels must receive the same amount of sunlight when connected to the same inverter. It is no good to have some solar panels facing different directions on different parts of the roof. New technologies, soon to become widely avaialable that will make this process much easier. Namely micro-inverters, which convert DC to AC at every solar panel, will mean that solar panels can face different directions, however these are not yet widely available.

If you have a sales visit from a solar company, make sure the salesman understands these points as he’s designing your system.

3 comments

Rachel

July 1, 2010

For more information about domestic solar installations, please contact http://www.solcentric.co.uk or call us on 01872 865662. Thanks!

Trev

September 12, 2010

There are many inverters on the market, but none seem to risk much more than a 5 yr guarantee. Panel selection is a nightmare. What’s more confusing is that I’m told mono crystaline is best then told Mitsibushi panels are the best, but Mitsibushi are infact polycrstaline! Chinese panel suppliers are saying they are better than anything other than Mitsibushi. It goes on and on. The best solution would be to have the same equipment as the free system suppliers as they have already done the homework I suppose. Does anyone know what they fit?

I notice that some fixing systems rely on actually drilling through the roof tiles, such as N-Power and some hook underneath, which is less destructive to the roof? Which is best?

Some companies charge up front fees of up to 83% before any thing is even done. What if they go bankrupt or to busy to fit? It would be fair if the deposit was in a government scheme similar to rental deposits on property lettings. Or pay on a credit card for protection and get around £15,000′s worth of airmiles.

Cleopatras Mother

December 2, 2010

EON are supplying Mitsubishi not free but via them.

We are in the throws of decisions. It seems, as with all new technology, there will be a better one along in a minute. I am trying to find a uk supplier of panels with built in inverters – anyone know?

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