Six years on solar

On 18 May, my 3.5kW-peak solar panel installation passed its sixth anniversary so I'm taking the opportunity to get a blog post out. The installation is going strong with a fairly stable output over the long piece. First, some totals:

2010-2011 - 2612.9 kWh

2011-2012 - 2403.2 kWh

2012-2013 - 2414.2 kWh

2013-2014 - 2433.4 kWh

2014-2015 - 2573.9 kWh

2015-2016 - 2514.8 kWh

That's pretty consistent with no obvious sign of any overall fall-off in performance, either due to aging or accumulated dirt. In the first couple of years, the panels were cleaned on two occasions but never since then. Yet both the best and the worst years were the first two. I suspect the regular Scottish rain is sufficient to them reasonably clean. As far as aging is concerned, the inherent variation in sunshine is too great to show up any trends.

I can see from watching a meter in front of me that, just like when they were new, the very maximum power output I ever see is between 3.5 and 3.6kW. It gives me a good sense that they are not deteriorating to any worrying extent. It bodes well for the lifetime of the feed-in tariff (FiT) arrangement I have which extends over 25 years.

Here are the graphs that illustrate generation.

Six years worth of daily generation measurements, along with a line that represents an average taken across 30 days. This average smooths out the curve somewhat.

The maximum, minimum and average generation that was seen for each calendar day across the six years, beginning 18 May.

The cumulative power generation across each of the six years, beginning 18 May.

Financial position

A question of interest to folk is whether the financial aspect is working out. The original cost to me for the system was £14,300. (I'm led to believe that a similar system nowadays would be less than £5,000.) I got a 10-year £4,000 interest-free loan from the government and the rest was funded from our own resources. My expectation was that the payback time would be 10 years and we've had 60% of that time pass already.

The feed-in tariff pays me an amount for every unit of electricity the panels generate. It doesn't matter whether I use that electricity or if it goes into the grid for others to use. Currently this is 48.84 pence. (I'm led to believe that new installations attract about 15 pence per unit.) Additionally, it is deemed that half of my units go into the grid and I receive an additional small payment for them, currently 3.44 pence. (Apparently it is about 1.5 pence for new installations.) These are index-linked to RPI and are tax-free. Over the six years, I have received £6,856 in FiT payments, which represents 48% of the original cost. That doesn't look as good as I had hoped. However, there is another aspect to this; I don't have to buy as much power as I would otherwise.

We're a relatively power-hungry household. Various set-top boxes stay on 24/7, we're fond of our tumble dryer and the kettle tends to be over-filled for the cuppas being made. Plus there is rather a lot of computer kit that is powered, much of it 24/7. So while the average UK household uses 9 units a day, we're a profigate bunch at 19 units a day! Happily, while light is shining on the panels, some of the house's power is being supplied by the Sun and I don't have to buy that power. If the available solar power is less that the house's consumption, then all of its output goes to the house. It is only when the house's needs are exceeded by the panels' output that power goes to the grid. Indeed, for much of the summer, the house spends hours totally powered by the Sun. Therefore, a substantial amount of our daytime consumption is free; we don't have to buy in that power. Unfortunately, I have no direct way to measure how much that is.

But as regular readers will know, I'm the type that keeps track of these things and I have enough data to take a decent stab at this. Across the last year before the panels were installed, our average daily consumption was 24 units. I know I can't account for how our pattern of consumption might have changed but it seems fair to suggest that, thanks to the panels, we are not having to purchase an average of 5 units a day.

Ignoring the standing daily charge, which we have to pay regardless, our current price per unit of electricity (one of the cheapest available - I've checked) is 11.76 pence. That makes 58.8 pence saving per day or £215 over a year. If that figure were valid for all six years, it would represent a total saving of £1,288 which, when added to the FiT payments so far, gives a total benefit from the panels of £8,144. This represents 57% of the initial outlay. That seems to be reasonably near to the 60% that would have matched expectation. Not bad for a set of panels that are far from being aligned due south. (Actually west-southwest.)

Solar Motion

Eight months ago, I leased an electric car which somewhat changes things. I figured that rather than sending power into the grid, I could use spare power to charge the car. Therefore, if I know the car could do with a top-up and I see that a sunny afternoon is generating lots of excess power, I'll run outside and get the car plugged in to mop it up. This not only saves me from having to buy that electricity had I plugged the car in at night, it also saves the cost of petrol that I would have had to buy to run my car for the miles that the car will do.