bugs said:
This evening I recorded the heater lamp for 1 hour (having turned OFF the heater cable...).
Known:
Heater wattage: 150w.
Tank capacity: 75 litres.
Room temp (start): 19c
Room temp (end): 20c
Heater on periods:
1. 2mins, 50secs
2. 2mins, 35secs
3. 2mins, 29secs
So, based on these variables, the heater would run for 3.16 hours per day. Total cost: £20 to £38 per annum (secondary rate / primary rate) running cost.
So... working on a principle of 2 watts heating per litre, a 350 litre tank would dictate a 700 watt heater to apply these findings. Total cost: £94 to £181 per annum running cost.
Anyone want to buy a second hand 350 litre tank?
If only it were that simple!
Were these test done during lights on or lights off?
The temp gradient between the water and the air is ~6C in these tests how does it compare for greater or lesser gradients? (Different times of the year)
What about the surface area to volume ratio, this will change as tank size changes and therefore the heat lost to the surroundings per litre will differ between tank sizes and dimensions.
I think the only way to know for sure how much energy your heater uses is to get an energy monitor and measure it. You can plot weekly consumption on a graph and then see how it changes throughout the year. But remember the results you get will be specific to your tank/room set up and will probably be completely different from someone else's results. (Unless of course you have identical set-ups in similar rooms)