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7 min read
The export rate is the whole argument
A kilowatt hour you use is worth the import rate. The same kilowatt hour sold back is worth about a third of that. Everything else in a solar quote is downstream of which pile it lands in.
Written by Nell Fairhurst
There is one number in a solar quote that decides most of the outcome, and it is almost never on the first page. It is the fraction of your generation you use yourself.
Two piles
Every kilowatt hour your roof makes lands in one of two piles. Either you use it as it is made, in which case it is worth whatever you would otherwise have paid to import it, or it goes out to the grid, in which case it is worth the export rate.
On current tariffs those two numbers are roughly sixteen pence and five pence. That is not a rounding difference. It is a factor of three, applied to the majority of what most domestic arrays produce.
Why quotes avoid it
Because it is the number that argues against a bigger array. Self-consumption falls as system size rises: the first four kilowatts on a house get used, the next four mostly do not. A quote that models self-consumption honestly is a quote that recommends fewer panels.
We publish it on every job we have done, next to the payback it produces.
Next step
Run the numbers this argues about
The calculator carries every effect described here. Change one input at a time and see which of them your payback is actually sensitive to.