During this time of year, my GH is a big coldframe. I would like to heat it, but both my wife and I work all day and I don't like leaving a heater on without anyone being there.
Does any anyone have any thoughts?
An electric heater with a thermostat will only come on when it reaches the set temperature. The warmest part of the day in winter tends to be during the day so it may not come on at all during the day. I would not worry about those being left on.
My GH has no electricity so I use paraffin heaters and if it is forecast to be low temps all day I just leave it on - fuel lasts for 5 days if needs be.
In my experience over years the only easy way is to use a GH fan heater set to whatever temperature you need to grow whatever you want. Remember, heating in winter will be expensive - I limit the time in my 6 foot x 6 foot GH to heating for the autumn rooted cuttings (Pelargonium, Cosmos, Penstemum, etc. from early March to mid may including my tomatoes.
Having a large heated conservatory, I grow all the cuttings and early sown Toms (around January 20th each year) in there until the rooted cuttings are growing new shoots and have been separated into individual 4 inch pots in late February and the Toms are in 5 inch pots in early March. They all then go into the GH at minimum temperature 55-60 degrees. If there are not too many plants I build a mini GH with polythene sheets inside the GH to reduce cost of electric - I did this with great results early this year.