What does it cost? 

Cost of panels:?Solar panels are usually the biggest single cost. Make sure you include costs such as VAT (usually 5% on installed systems) and delivery.  

Cost of inverter:?The cost of the grid connect inverter (or inverters for a large array) is another significant fraction of the total cost.  

Mounting system cost:?This will depend partly on the type of roof the system is to be installed on.  

Balance of system cost:?Wiring, meters, disconnects.... and all the other little components needed to make the system work!  

Labour cost of installation:?Your installer will quote for this. Again, include any VAT applicable, and any other costs such as getting scaffolding erected, or getting your electricity company to install an export meter (this can't usually be done by the installer themselves).  

TOTAL:

How well does it work? 

Peak power of array (W):?To get this, simply multiply the rated power of each panel by the number of solar panels in your array.  

Ideal output (kWh/kWp/yr):?This is how much power an ideal 1kW array can be expected to produce in your location each year, allowing for average cloud cover and latitude etc.

You can get this from our map which shows the expected output from a shade-free array on a south-facing roof.  

Orientation correction:?If your array doesn't point south at the optimum angle, it won't produce quite as much power as the ideal array. Use the orientation correction table to select the appropriate factor.  

Shading correction:?If shaded at any point during the day, your array will produce less electricity. This can be hard to measure exactly, but the following values are the values used in the Governments Standard Assessment Procedure for energy rating:
1.0 - 0-20% shading
0.8 - 20-60% shading
0.65 - 60-80% shading
0.5 - 80-100% shading  

Electricity Prices 

Initial feed-in tariff (p per kWh):?This depends on the size of the array, and where it is installed. The range is from 29.3 to 43.3p per kWh. Best to look up what the correct values are for you.  

Price of bought electricity (p per kWh):?This is what you pay for electricity at the moment - have a look at your electricity bill!  

Price of sold electricity (p per kWh):?If you don't need the electricity as you generate it, you can sell it to the grid. This rate has been set at 3p per kWh  

Proportion exported:?This is the proportion of electricity that you expect to sell, rather than use yourself. For most small installations, this won't be measured with an export meter - it will be assumed to be 50%  

How much things change 

Initial Depreciation (%):?The percentage loss in value of the main components of the system - the solar panels and inverter - at the point at which they are installed. We assume the mounts etc lose all their value immediately - and of course you get nothing back for labour costs.  

Degradation of array output (% / year):?Solar panel outputs very slowly degrade over time - in 20 years time your panels might only have say 90% of their original rated output. The rate we've set here is typical of good-quality crystalline panels.  

Increase in electricity price (% / year):?It's highly likely that electricity prices will rise significantly over the next 20 years. This is the annual rate you expect the prices to rise at.  

Equipment depreciation (% / year):?This is how much you expect the solar panels to depreciate in value eah year (beyond the initial depreciation which we have already allowed for)  

All ready?