I use this electric radiator -- you can get them cheaply from Argos, amazon ... Although it has a built-in thermostat (a 'thermostatic cut-off'?), that never worked very well and the temperature would fluctuate, so I bought a separate thermostatic plug. This is plugged into the wall, with the radiator plugged into that. For the sake of this photo I took last year, I have placed the white unit indicating the temperature (actual temperature given at the top, intended temperature below) in front of the radiator. (Usually it is at a lower level and nearer the hutch to give a more accurate reading of the temperature that the guinea pigs are experiencing.) You set the electric radiator to its maximum setting and then set the desired minimum and maximum temperatures on the white sensor. This will switch the radiator on and off within 1-1.5 degrees of the upper and lower temperatures that have been set -- so a setting of between 18-20 degrees would result in a shed temperature of roughly 17-21 degrees. That kept the temperature constant and worked really well. The shed is insulated though, so retains the heat quite well. (I don't think these thermostatic plugs have great longevity, however, as the one I bought from amazon eventually stopped working and so I will have to buy another this year.)
The black unit next to the white thermostatic plug sensor allowed me to keep a double eye on the temperatures. A sensor inside the hutch gives the reading to the right of the black unit, together with the minimum and maximum temperatures it has reached over a 24-hour period, with the reading on the left giving the current, minimum and maximum temperatures inside the shed as a whole. The temperature readings on the two units corresponded pretty well.
I will see if I can find a couple of links for the radiator and the thermostatic plug.
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