Compared to the calculations required to size a solar thermal hot water system, space heating is complicated. There are so many more variables to consider – building size, heat loss, insulation, air flow and everything else – so the best place to start is with the current heating load. From there, you can calculate how big your solar thermal installation needs to be, just like you would with a hot water system.

Before you do that, it is important to realize that a solar thermal space heating installation will not heat your entire home on the coldest day of the year. That would be impractical because you would be wasting tons of energy on every other day, so you want to size the system to cover the average daily heating across the entire cold season.

For example, if you turn on your heating in the middle of October and turn it off in the middle of April, you will want to take your heating energy use for that period and divide it by the number of days to find your average use, and design a system to provide that amount. If you don't want to be so specific, count the months in which the heating is always on, every day – in our example, from November to March.

Here's a sneaky time-saving tip for you. If you have Excel or a similar spreadsheet program on your computer, you can get the number of days between two dates like this (the number in C1 will be the number of days, though you may need to change the format to a plain number as Excel will probably show it as a date):

So, gather all your utility bills for the heating period and start working out each month's average, or just total them all up and divide by the number of days in the entire period. You will probably find that the average over the entire heating part of the year is about a third less than the amount you spend per day in the coldest month.

The problem in the solar thermal sizing calculation comes from the fact that we need the figure in Btus and heating systems don't work like that – you pay for your home's heating in electricity, gas or fossil fuel volume. You will need to convert the figure you worked out from your bills into Btus before you can go any further, which is where the table on this page comes in. Simply multiply the amount of fuel used by the figure shown to get a Btu value.

You may have noticed another problem: electric heating. Every other heating fuel is easy to calculate: natural gas is almost exclusively used for heating in winter (unless you have a gas cooking range or pool) and fossil fuels are bought by volume. But how can you tell how much of your electricity was used on heating, with so many other loads in the house? The only