I’ve checked the kWhr readings of my geyser for about 4 weeks, trying to interpret the results. It wasn’t as easy as I thought it would be.
The thermal resistance was by far the most difficult to nail down. The trouble was that when the geyser is on standby (during the night) it would come on only once during the night, giving me the problem of determining the time interval over which to average the consumed power (because the thermal conductivity equals the average power required to maintain the temperature of the geyser). I overcame this problem by leaving the geyser on standby for an entire weekend while we were away. This way I got a far more accurate number, ie:
Best estimate = ±0.46 °C/Watt
The water usage was mainly derived from the power consumption, which has its own little problem, because some of the power to heat the cold intake water also has be to used to compensate the natural cooling. I now know that this factor equates to a loss of 125 Watt to the geyser heater element. So instead of 2kWatt, it must be derated to 1.875kWatt
BTW: The average power consumption of the geyser over a period of about 4 weeks was equivalent to an element of 315Watt burning 24/7!