OK, found the link
Solar PV (Photovoltaic) Feed-In Tariff Calculator - Discover how much a Solar PV System costs to install and the payback profits from the Feed In Tariff
Enter your post code, roof direction and KW size that your installer has said will fit.
Next click on assumptions because some of the ones they use are widely out,
My daytime electricity rate: £ per kWh (inc VAT) -
Enter your KWh price here. Their default is slightly more then most people pay I expect.
Use Standard Export Tariff (4.64p/kWh in July 2013) ... OR ...My energy supplier has agreed to buy back at £ per kWh -
Leave the default unless you have a buy back price from your supplier
The install type is: -
Pick which ever one applies. I assume it will be first or second option if you already own the house.
% of the energy generated will be used in my home -
If you have a lot of stuff on during the day, fridge, freezer, washing machine, computers, etc or people in the house all the time then this will probably be > 50%. Or <50% if you are pretty energy efficient. I would suggest 50% though as a starting point.
I will have an export meter:
Depends on your install. I didn't want one as I knew I would be using most of my output. My supplier British Gas assumes 50% if you don't have a meter.
I want to assess the benefits over a year period -
Put in whatever period you are interested in, goverment pays FIT for 25 years I think.
Assume RPI of % and annual energy price inflation of % -
There default is 10% which I think is crazy high, I change it to 1%. The higher the increase the more benefit you will see.
The roof angle is ° (or panel mounting angle if standalone) -
Enter your roof angle if you know it. For the latitude the UK is, 35% is optimal but is quite shallow. Older houses are often over 45%.
Assume panel degradation to % of year 1 output after 30 years.
You can get data from the panel vendors but I think most quote around 80% over 20 years so 70% over 30 years would be more realistic.
Assume inverter lifetime of years -
Loads of research (and my installer) said to expect 10 to 12 years is more realistic. At the moment inverters cost around £800 though who knows in 10 to 12 years time.
Assume cable/inverter system losses of % of output -
This will depend on your install but unless you have really long cable runs or a crap inverter then leave it at 18%
So plug all that information in and see what it pays back per year. The first year figure is in the right ball park for all the years as loss in panel efficency is offset by increase in FIT payment, energy price increases. Don't forget to budget for a new inverter after 10 to 12 years.
So I plugged the data I suggested, 3KW system on a east-south-east facing roof and it gives me a figure of around £515 a year. If I change that to a 4KW system on a south facing roof with all optimal values, that gives me £750 a year so no way is it going to cover a £120 a month loan.
Also if you go directly to an installer, that kind of system is going to cost around £7k plus say another grand in 12 years if you pay all up front. So that would be 8K over the 20 years (though government will pay for 25 years).
Calculate that against the loan the company is offering, you says its £120 a month but how many months is it for ?
If you are getting £750 a year payback (remember, that would be an optimal install), in theory that is 10 years to payback and then £750 profit a year, though of course you need the money up front and you could have used that for investing elsewhere.
Phew, hopefully that makes sense.
