Originally Posted by
peterb
The rating of the fish tank heater will be at a given voltage - nominally 230. If the actual mains voltage is higher, the consumption of the heater will increase, if the voltage drops, the consumption will be less. Mains voltage does vary during the day.
A plug in power meter may monitor voltage and current, giving a true power consumption - others that clip round the incoming power cable may not monitor voltage and are less accurate.
As for the readings.
your 48W heater is consuming power at 48 watts. In an hour it will have consumed .048 KWh (0.048 of a Kilowatt in one hour) of energy. In ten hours it will have consumed .48KWh (0.048 for ten hours) and will have cost you about 6.5p.
Is it on a thermostat? If so, the power consumption will drop to zero. So, if it was heating the water for 7 hours in a ten hour period, the energy consumption would be 7*0.048 = .336KWh.
The absolute accuracy of these meters is not that good - especially at low power levels, but they are useful for monitoring trends. If you want to accurately monitor your energy use, just read your electricity meter every day or week (at the same time of the day) and build up a long term picture. If you put the readings into a spreadsheet, you can turn them into a graph showing daily or weekly trends over time.