Your electricity supplier will quote a price per unit of electricity which take the form of price per kilowatt hour. This means the cost of running a 1 kilowatt appliance for one hour e.g. a 2kilowatt fire would consume two units of electricity in one hour (2 x 1kw/hr) - if your supplier charges 10p per kw/hr then you would consume 20p worth of electricity.
A 1w appliance would take 1000 hours to consume 1kw/hr so the cost would be (using 10p unit as above) 10/1000 i.e. 0.01 of a penny each hour (or Â£0.0001 in pounds if you like).
Nuff said - if it's a 1w appliance forget about it, you probably use more power on a long cable run to an extension lead due to resistance in the copper wire. :?
Tanks are a nightmare to cost as some elements are switched according to ambient conditions, e.g. solenoid on co2 (if you still switch by pH) and heater in a tank. There are devices out there that can calculate usage by plugging the appliance in. In the case of a tank I would suggest plugging an extension lead into the "calculator" and plugging everything else into the extension lead so that you can see total usage. If you want to determine usage of a single element fo the system simply remove it from the extension and calculate the difference over a set period of time.
Trouble with that answer is that if you apply it to a heater it tells you that a 100w heater uses 2.4kw/hrs per day - which is blatantly untrue (unless you keep your tank in a fridge :? ) So unless you're looking at worst case it would be difficult to form an accurate answer.
Incidentally the fridge would have to cool the water at a faster rate than the heater warmed it for the above statement to be true - I don't know how you would define rate of cooling in coparison to heating wattage.
PM me your email address if you're interested in a couple of Open Office spreadsheets - one to calculate heat loss of your tank (and therefore the running time of your heater) and the second to calculate component and total costs of running a tank based on your electricity costs.