We had some power issues last week - the line out to the garage that also powers our tub was tripping the RCD, so the tub was without power for six days before we had it fixed. Total temperature loss was 15c (27f), which seems a little high for a modern tub.
Over the week I only lifted the cover twice, for under a minute each time, so shouldn't have made much difference.
The weather here hasn't been great - ranging from 4 to 14c (40 to 58f) while the heater wasn't running, but the loss is more than I expected.
My cover has clips on the corners which I generally don't fasten. Would it make much difference if I did use them?
Cheers,
Mr Lobster