Using a simple heat loss calculation which is meant for horizontal pipes but will be close enough for our purposes since our block heater is really just a short piece of un-insulated horizontal pipe when it is switched off.
Heat loss Q = h A ∆T in BTU/hr
Where
h is the coefficient of heat transfer
A is the surface area in ft2
∆T is the difference between the air temperature and the surface temperature in deg F
I have stuck with 18 as the coefficient of heat transfer as this is what is used for horizontal pipes in free air but it could be lower as the under bonnet air is somewhat enclosed but will be higher if there is a flow of air due to the car moving. (The possible range is 2-25 btw)
I have worked out the surface area of my heater is 0.8 ft²
I will use 0°C (32°F) for the under bonnet air temperature and 80°C (167°F) for the water temperature leaving the heater matrix as these are what I would consider the extremes with an engine running and the heater fan on
Therefore
Q = 18 x 0.8 x 144
=2073.6 btu/hr
If you consider that the maximum heat output (fan on full and vents open) of an average heater matrix is about 60,000 btu/hr then the block heater would loose an additional 3.5% more heat than the heater matrix.
But if you look at it while the block heater is heating the engine or defrosting the windows then if you are still loosing 2073 btu/hr that is about 600 watts, 20% of the heat you are putting in so it would be a good idea to insulate the heater.
Slide rule not required!