"Martin Brown" wrote in message
...

[snip]

A quick and dirty rule of thumb is that the cost to
maintain the temperature an extra 5C above ambient roughly doubles.

That does not make sense. What would it, according to that rule, cost
to keep the temperature just 5 deg C above ambient? Nothing?
In fact, the cost does not vary exponentially, but is is roughly
proportional to the desired temperature differential.

Franz