Thursday, 6 June 2013

Two Year Anniversary of Solar Power

If You're a parent you will know the feeling. 

One moment they've just arrived and the next thing you know, they're going to university and getting married! 

It's the second anniversary of our system today and the end of year report is a little like mine when I was at school. "Could do better."

Saving the details.
Because I'm a deeply sad "geek", I recorded data about my energy use even before I had PV panels installed on our roof. 

The figures below show that before installing PV panels I had an annual electricity usage of 3,618 kWh's @ a cost of £405.22. As you can see, my usage has dropped by an average of 33%, which means that based on my 2010-11 usage multiplied by the relevant tariff rates I'm saving an ever increasing amount.

Influence of the weather.
The figures below illustrate how the poor weather in 2012-13 reduced the amount of power generated by my roof. My output dropped by 44 kWh over the year (1.75%). 
The good news is that because the FiT rate is indexed linked and was raised by 3.1% on April 1st 2013 my income, even with 1.78% reduced output, rose by 2.7%.

Was I happy?
Adding together my FiT income and the savings I made from electricity usage, £1,164 + £158 gives me a total revenue in 2012-13 of £1,322. 
Based on my initial investment of £10,000 in June 2011 that's a return on investment of 13.22%. 

With the £1254 last year and the £1322 this, I'm a quarter of the way to paying off the system. The prediction was 7.5 years and that's still looking good.

One factor always makes compiling these figures difficult. I use my Sunny-Beam monitor to provide all of the figures I generate each month.Link to data.

The monitor records daily output in various forms and gives a graphical "real-time" snapshot of the energy coming from your roof in terms of current kWh, income and totals. What it doesn't do is accurately record the energy generated by mine (or your) system. 
The official meter reading which I send off to NPower each quarter is ALWAYS higher than the figure displayed on my monitor. If you take a look at figures which I record each month from the monitor it has a grand total of 2367 kWh, yet the figures above indicate 2459, that's a difference over 12 months of 92 kWh's (3.9%).

I suppose it's better to have a pleasant surprise with the higher output, but it once again demonstrates what a flawed piece of equipment the monitor is. 

Twitter: (@solaricarus)


  1. My estimate (based on one completed year of generation) is it will take 8 years and 4 months from my installation date of October 2011 to have my initial investment returned from FITS payments. It's only THEN that I start getting 'interest' on the investment. However I have discounted the approx 40% savings in electricity consumption that I have been gaining.

    I also use a Sunny Beam monitor which tells me at any second of the day how much I'm generating. I use Sunny Boy inverter bluetooth readings downloaded each week to get accurate readings of the generation in five minute 'average' slots. Hence my 3.92kW system recently showed a 4.27kW peak output on the Sunny Beam that was not evidently sustained for a whole five minute period using the Sunny Boy readings.

    I also get a slightly higher reading from the generation meter than I get from the Sunny Boy inverter data but the figure does not differ as much as your Sunny Beam apparently does from your generation meter.

    The disparity between my 3.92 capacity and occasional flashes of 4.27 generation, I am told, is because there can be an excess generated but that this can then have a 'damping' effect so the figure then drops - it's not sustained. I'm also told that hot weather is not the same as bright weather and heat on the panels creates panel resistance and reduced generation. I don't pretend to understand the science ...

  2. Just a postscript: my income figues do include the payment for kWh sold to the grid at approx 3.1p, but exclude the savings I've made from reduced electricity usage.

  3. Hi Chris.
    Interesting points. I'd never considered that the output indicated by the monitor was in any way influenced by the type of inverter. Mine is an SMA Sunny Boy type which you would assume would match the monitor perfectly as they're the same manufacturer. At least the difference is in my favour...
    In my opinion you should be including the difference between the cost of electricity used prior to the installation. You're using 40% less electricity as a result of investing in PV panels, it's a saving on your previous costs and should be factored into the pay-back period.
    One other "difficult to get my head round" point is based on the FiT rate increasing with RPI each year. In theory this will materially affect the pay-back period as potentially your income is rising by 3% each year.
    Countering the increase in your output payments argument is the FALL in output from ageing PV panels. All manufacturers admit that this happens, we don't have the data to confirm to what degree this will affect our systems. YET.