pete wrote:

Any pointers on how to do the sums would be appreciated.

There are two ways to do this. One is a steady state approach. Wrap a small
(very small) heater in a duvet, wait for the temperature to stabilise, and
measure the temperature drop between the inside and the outside. You need to
measure the voltage drop and amperage of the heater, as well as the surface
area. All rather difficult to do, especially making sure there are no folds
in the duvet around the heater, etc. You'll need a fan to keep the room air
circulating briskly.

Marginally easier is just to wrap a known weight of warm water in a duvet
and plot the temperature drop.

I doubt that you'll get closer than around 50% without cutting the duvet up,
which rather defeats the purpose.





--
Electric cars are very healthy - when the battery runs out you have to
walk home.