OK lets assume you are paying a mid level amount for your leccy at 11p per unit (kwh).
This means that if you leave an appliance of 1000w on for one hour then you will pay 11p.
So if you have an 80 watt tube heater on for 10 hours (Say 9pm until 7am) then you will use 800w and at 11p per kwh hour it will cost you about 9 pence.
If you use an oil filled radiator rated at 400w with a thermostat and 2 heat settings (200 and 400w) and you set your thermostat it will heat above that, switch off and when the temperature cools it will click on again.
Let's say you set it at 200w and it runs for 5 hours out of ten then it willuse 1000w and cost about 11p to run.
Don't forget to put the costs on your business spend.
As for me, my RO is in my car port in an insulated box and I have a 40w tube heater in there. If the temperature is forecast to be 1 degree celsius or less then I switch on the tube heater when I go to bed.
If the temperature is forecast at 2 degrees C or less then I put an oil filled rad in the van with a thermostat as I like pliable hoses and a working pump first thing in the a.m.!
I have been known to dip a 300w pond heater in my tank too, but only until I bought the radiator.