Is my house suitable for Solar PV?

There are, as I'm sure you've noticed, a lot of folks advertising solar PV at the moment, despite all the fun with Her Majesty's Government, the courts and the feed-in tariff (of which more in a later post). Setting aside for that later post the question of 'can we afford it?', this post looks at the practicalities of solar PV.

The ideal roof for solar PV is big enough to hold a 4kW system (about 30 sq m), slopes at a bit over 30º, faces due south, and has no shade. However, if you can't manage that, you're probably still OK. Provided you have a roof, or more than one in fact, with not over much shade, that faces at least a little bit south, you will be able to generate a reasonable amount of electricity. Surprisingly, all other things being equal, a roof facing east or west potentially generates 86% of the power of a south-facing roof.

There are a number of handy online sites that you can plug a postcode and a rough idea of your roof configuration into, and in return it'll spit out an equally rough idea of how many kWh you can expect to generate in a year. They're generally much of a muchness, since they all tend to use the SAP 2009 based calculations, which in a nutshell multiply the output rating of the panels by a factor that takes into account an element for facing, for angle of roof and for shade. If you have half a brain and a pocket calculator or a spreadsheet you can do it yourself, pretty much!

Consensus is that the independent estimator at PVGIS is better (SAP 2009 is apparently a bit pessimistic), but suppliers are, as I understand it, required to give you estimates of ROI based on SAP 2009. Of course, it's all entirely weather dependent, on top of that. This past week we've had a 16+ kWh day on April 1st, and a miserably wet day on the 4th that generated a stupendously uninspiring 1.24 kWh.

Most suppliers will be reasonably straight with you, although when we got our quotes we did find that the SAP 2009 factor varied from 690 to 820 for the same roof! It's worth plugging everyone's quotes for the power output of the system and the predicted annual generation in kWh into a spreadsheet and comparing them, just to see who might be over-egging the pudding a bit.

To summarise, though, as long as you have a decent roof that isn't entirely shaded by trees or next door, and faces at least a bit south, it's worth asking for a quote. Most if not all companies don't charge for quotes, and as long as you don't let them pull the wool over your eyes as regards ROI, you'll be fine. Of which, more next time.

Solar PV - the basics

As I mentioned in the original post, as well as solar water heating we also have (as of last week) solar electricity, or solar PV as it's more commonly known.

I thought I'd start out with a brief description of how a solar PV system goes together. First, a diagram!

A generic domestic solar PV system

Ok, So...

The two main components of the system are the PV cells themselves, and the inverter.

PV is short for "photo-voltaic' from the Greek photos, light, and Alessandro Volta, who invented the first battery - basically PV cells turn light into DC electricity, and the more light, the higher the voltage they generate.

Now, DC is not much use for running household equipment, as that requires 240V AC. Enter the inverter, whose job, pretty simply, is to turn DC into AC. This is where things get a little less simple, since inverters have a minimum voltage at which they will 'kick in', so if it's not bright enough to generate that minimum voltage? No electricity. Once it does kick in, the higher the voltage off the PV cells, the more power (and thus the more units of electricity) the inverter kicks out.

In our configuration[*], the inverter feeds into a spare breaker on our fuse box, via a meter that measures the total units of electricity generated (very important, as this is how you get your Feed-In Tariff, of which more in a later article). If there's more being generated than the house is using, then (by the magic of electricity) the excess is fed back to the grid, otherwise the grid supplies the balance.

And that's pretty much it. Other than to note that, if you were dreaming of being able to go 'off-grid' when the power goes out? Most systems don't, and more awkwardly, they actually shut down when the power goes off. Annoying and somewhat paradoxical - the reason behind it is that in order not to cause disruption to the grid, it's essential that the inverter syncs up to the frequency of the grid supply, and it's a requirement that if the grid goes off, it shuts down until it comes back.

Next in this series of posts, I'll look at how to calculate what kind of power you can get from a PV system.

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[*] In some systems, the inverter is connected up in the meter cabinet instead of to the fuse box: this makes little or no difference to how things work.