500N
In the late 70's, I think around 78 - 80 when the UK had a really cold winter,
we left out heating on in Hereford but it still froze the pipes so I agree, the damage can be way more than any saving.
That may have been the year London ordered the thermostats to be lowered to save money, which may be ok when you're that close to the equator. In our (REME) workshops the pen recorders showed the temperature didn't rise above -10 for two weeks. Normally enhanced maintenance would kick in but we weren't allowed to spend the money. Repairing the damage took weeks, just to save a few grand. But they would be different budgets, so that's ok.