There should be a plaque on every appliance. For example my printer here uses 0.4A maximum. If it was printing away using full power for an hour, that would be 240Volts x 0.4Amps x 1 hour = 96 Wh or 0.096 kWh (one kWh = 1 unit) which would cost me about 1p. Of course, when it is printing, it wont be using full power all the time. It will be using much less when it is just thinking but not moving and even less than that when on standby. Generally speaking, appliances that create heat (heaters, kettle, fridge, etc.) are the ones that use most power and other appliances use very little. You should certainly fit energy saving bulbs and turn most things off at the mains at night but you shouldn't worry about the usage for PCs and printers if you are sitting next to an electric heater.
You need to be careful with your business supply contract because many suppliers will charge silly rates if you accidentally go out of contract by, for example, forgetting to renew it.