The message
from Robert contains these words:

My house is 1700 sq ft total floor area and uses 2000 litres of heating
oil a year. At 11.53 KW/litre for light heating oil that amounts to
11530 KW a year.

I know it's nitpicking, but can get we this sorted...

kW (small k) is a measure of power - the amount of energy in Joules (big
J) that an appliance uses (or produces) in one second. One Joule is a
Watt Second - one Watt for one second, enough to lift a kilogram just
under 10cm in a second[1].

You can't say that a litre of fuel is the equivalent of 11.53 kW. This
is because you may be, for example, running an 11kW output heater, but
there's no indication of /how long/ it would run on a litre of fuel. In
this case, a little over an hour. There has to be something in an energy
measure that tells us over how long the number quoted can be sustained.

Thus - energy is paid for in kiloWatt hours - kWh. The energy density of
your fuel oil is 11.53kWh/litre.

Sorry - I'm sure you just slipped, but it really winds me up!

[1] In what passes for standard earth gravity round here.

--
Skipweasel
Pay no attention to that man behind the curtain.