Mike Buckley wrote:

To work out the watts needed, multiply the temp uplift by the heat loss,
so:

104.4 x 5 522w Frost Protection
104.4 x 10 1044w Ideal

So that gives me the size of heater I need - I was counting on something
like a 2Kw convector which we have in our conservatory and it really
does a great job, so this should easily be enough to heat the garage to
these levels, even allowing for underestimating the heat loss through
the door.

Assuming all that looks ok, and even if I'm quite a way off I've still
got another Kw+ before I need another heater, how do I calculate the
cost of the heater? I can easily do it per hour, but that assumes it's
on full blast all the time which it won't be - so how do you calculate
the cost based on partial use - is it even possible?

You've already done it. The heater is presumably controlled by a thermostat
which you cat set to give the desired 5 or 10 degree inside temperature.
This thermostat will in effect run the heater at quarter blast (522W) or
half blast (1044W) by switching it onto full blast for a quarter or half
of the time. The actual on and off times will depend on the hysteresis of
your feedback loop, but as an example for 5 degrees it might be on for 2
minutes and then off for 6. In this case the cycle length would be 8 mins,
but for cost purposes it doesn't matter what the cycle length is, since if
the heater is on for a quarter of every 8 minute period, it is in effect
on for a quarter of any period, such as a day. So if on full blast it
would use 48kWh per day, on quarter blast it would use 12kWh per day.