Five years and counting

Across five years of operation, I've never tired of gathering readings from my solar panels and watching the daily variations in power generation. The long term changes are as fascinating as those that occur across days or weeks. I now grasp in a way that I never had before why nature shuts down in winter. Though the eye perceives daylight in mid-winter when the Sun sits low in the sky, there really is very little energy remaining for plant life to photosynthesise. An overcast day in January can yield so little power that the power meter doesn't even register a tenth of a unit and remains at the same figure as the previous day.

So I have had these panels for five years. Here are the results thus far. First, how this last year compares to the first four:

2010 to 2011 - 2613.7 kWh
2011 to 2012 - 2398.0 kWh
2012 to 2013 - 2414.2 kWh
2013 to 2014 - 2433.4 kWh
2014 to 2015 - 2573.9 kWh

Second best year thus far, and higher than my original estimate of 2,500 annually. However, the average over the five years is a little low. I certainly don't get a sense of a slow decrease in the panels' efficiency. There's too much variability in the totals to perceive that. Here's how each year rose to its final value:

One interesting point to make is that we had a very sunny April this year. As a result, this year's generation had actually overtaken the first year on a single day less than a month ago before the high pressure system collapsed and the rain returned. Also, with just a week to go before the fifth anniversary was reached, the system appeared to have died. Except for the fact that the Sunny Boy inverter is guaranteed for five years, I felt a pit in the stomach. In the event, I need not have worried. The generation meter was also dead, a sure sign that the system wasn't receiving mains power. It needs a feed of mains to know how to synchronise the alternating current it generates with the grid cycles. In the event, I located a circuit breaker in the mains feed to the inverter and found that it had tripped. At this moment, it was a sunny day and the panels were surely generating significant power. I took the precaution of phoning the engineers at the installation company and checking there was no reason not to reset the breaker before doing so. All is well.

This graph shows the daily generation over these first five years:

As if I need proof, it shows just how variable Scotland's weather is, yet how consistently variable it is from one year to the next.

An important question is whether the finances of the system are performing as expected. At the start, I had estimated that it would take ten years to pay for itself. After five years, I would hope to be halfway to that goal, and so it seems. The installation cost me £14,300. Payments from the Feed-in Tariff (FiT) system has brought in £5,600 so far and I reckon to save a further £300 annually in power that does not have to be purchased - about £1,500 thus far. That makes a shade over £7k. Since the FiT payments rise with the retail prices index and the cost of electricity is set only to rise, I'd expect gains from the next five years will be more than equal. My ten-year target seems spot on. Everything beyond that is pure gain.