A 1kW heater will use 1 unit of electricity for every hour that it is on.
Economy 7 charges you more for your daytime electricity in return for charging you less at night. Unless your panel heaters are going to be used only at night they will be very expensive to run. Of course, if they are storage heaters this is the right way to do it!
TBH, I think 2kW may be a bit cool for a space 18x10ft during the winter, but I don't know what you are using it for or how well insulated the area is. Building a boat will keep you far warmer than soldering PCBs!
Electrical heaters are virtually 100% efficient So 1kW in - 1kW out. running such a heater for an hour will consume 1 unit of electricity - also known as 1kWh - 1 kilowatt hour. on economy 7, this will be cheaper at night for about 8 hours BUT will be very expensive during the day. Generally speaking only storage heaters will be at all economic when used on economy 7 tariffs.
As to whether 1kW is enough, you need to do some sums and possibly measurements.
All your 1kw will leak out of the workshop but how quickly? this depends on the difference between inside and outside temperatures and how good the insulation is. It is vital that all draughts are eliminated as well.
My workshop is 30sq metres and average 2.7 metres high and well insulated and draughtproof. I have a 750 watt heater on a thermostat. measuring over a month after Christmas I kept the inside at around 14 degrees with the heater on for 1/3 of the time ie average 250 watts continuously. external temperature was possibly +6 degrees over that period.
14 degrees is not warm but once I get working in there it is about right. If I'm not very active, or trying to get paint/varnish to dry, I use a 1kW fan heater for an hour or so to raise the temperature.
If the space is very well insulated (i.e. minimum of 50mm Celotex or
75mm polystyrene in the walls and roof *and* you have effectively draftproofed it, then 1kW would be just about enough to take off the chill in the winter - i.e. get the temperature to between 12 and 15 degrees.
If it isn't - i.e. is a wooden shed or a single brick building with no insulation and a typical up and over door, then frankly you would be wasting your money. For a size of 18sqm, you would need in the order of 6kW to achieve anything comfortable.
sites (and most others you may find I suspect) major on home heating calcs but the principles are the same but you might have to search elsewhere for the U values.
I assume these heaters you are using are some form of storage heaters? Otherwise all the heat you are going to need in the day needs to be input during the off-peak hours and last at least till the end of your working day say up to 12 hours after then end of off peak!! If the heaters don't store the heat and allowed a controlled release you will end up being toasty hot in the morning and a lot cooler at 6pm.
You might need to think what effect these wide temperature swings will have on your wood products as well as your personal comfort.
What are the heaters for? If they're to prevent condensation, then a dehumidifier used with or instead of a heater might be more cost effective, although you might need an accurate humidity controller to avoid damaging your timber stock.
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