The Joy of Solar 3: Making Electricity

Solar Array – South Facing (Image: T. Larkum)
Solar Array – South Facing (Image: T. Larkum)

I previously described the installation and setup of the solar system on the back (south facing) roof of our house. It was installed by Greenday Renewables who I highly recommend – we have had no trouble with it and it works very well.

The most important information, however, is of course how successful it has been at generating electricity. The system is a 3.7 kWp array, consisting of 10 panels each of 185 Wp (plus an inverter in the loft), where Wp indicates ‘Watts peak’. What this means is that in an ideal (‘peak’) situation the 185 Wp panels could each produce 185 Watts of electrical power, so all ten together could generate 3.7 kilowatts. Of course, they are never used in perfect conditions so an estimate is generally provided of the likely total amount of power generated for a particular system arranged at a given angle, direction and latitude. For our system this was 3333 kWh per year, where 1 kWh of energy is a kilowatt of power provided for one hour.

The size of the system was determined by the roof space – it was the largest system that could fit. It is eligible for the government feed in tariff (FIT) which applies to any domestic system up to 4 kWp. We would have gone for the full 4 kWp if there had been the space.

The system was installed in September 2010 so we now have two full calendar years of data on electricity generation, covering 2011 and 2012. Up until the start of 2012 I recorded the meter reading virtually every day but since then I’ve been doing it once per week. The data is recorded manually and then transcribed into an Excel spread sheet (soon I hope to replace that process with an automated system, but more of that in a later post). From the spread sheet I have been able to graphically chart the data – see below (and click to enlarge).

Electricity Generated by Solar Array in 2011 and 2012 (Image: T. Larkum)
Electricity Generated by Our Solar Array in 2011 and 2012 (Image: T. Larkum)

Considered in broad terms the chart shows pretty much what you would expect – low generation in the early part of each year, building up through the summer and dropping again as winter returns. However, looking in more detail it is perhaps surprising just how much the rate of generation varies week by week as well as year by year. It is possible for one week to generate twice as much energy as another week in the same month. Similarly each year can have very different weather, so for example 2011 had generation peaks in April and May, while 2012 had peaks in May, August and September. In fact 2012 was significantly more variable than 2011.

Fortunately over a whole year the peaks and troughs average out pretty well, and the system has performed well. It generated 3650 kWh (3.65 megawatt hours) in 2011 and 3500 kWh (3.5 MWh) in 2012, and these numbers compare very favourably with the 3333 kWh that was predicted. Over these two years it was eligible for FIT payments of £3130 which is about one-fifth of its installation cost (£15750) so it is on course to pay for itself in about 10 years without even counting the cost of the electricity saved.

After it’s paid off the benefits don’t stop, of course – it is eligible to get FIT payments for a further 15 years – index linked – and after that we will have free electricity for life.

Overall it has been a great investment, so much so that we are looking to add another system on the front of the house with a view to using it to power our ZOE – more to follow on that in a future post.

[The Joy of Solar 4: A Second Installation]

[See also Grid Electricity Usage]