How much your electricity bill will rise depends (rather obviously) on how much electrical heating you've got and how many hours you use it for.
A person just using an electric fan heater now and then to top up their gas central heating won't pay much. Someone with all-electric heating will pay far more.
Similarly, someone who's out of the house all day will pay less than somebody who's home all day.
The 'living style' of those in the house also has a large effect. Someone (like me) who never heats their bedrooms will use far less than a family where the kids use their bedrooms for doing their homework and playing.
�20 per month works out at about 67p per day. That's typically about 5� units of electricity. It would let you run a single fan heater on the 'low' setting for 5� hours per day. If you ran it on the 'high' setting, you'd only get half of that time
Looking at it another way round, let's suppose that don't leave any heating on overnight but you've got a 2kW fan heater (or central heating with the same power output) set to come on for an hour in the morning, while you get ready for work. You're out all day, but you return at 6pm and go to bed a midnight. During all of that time, you heat either the living room or a bedroom with a 2kW fan heater. (i.e. you don't heat both at the same time). At weekends you use that single fan heater (or it's equivalent) from 8am to midnight. That's roughly 100 units per week, at about 12p per unit, so you're looking at about �50 per month for your heating.
Chris