Today I ran both my geysers. The main geyser is heated by a heat pump, the guest geyser has a 2kW element. Both geysers are the same capacity.

Some variables:

1) Main geyser has a "blanket" (isotherm) and has hot water pipes insulated to the point at which they enter the walls.

2) Main geyser last got heat from the pump at 13:00 yesterday, so had not been heated for 17 hours when it turned on at 6:00 this morning.

3) Guest geyser has not been heated for at least a week.

But still the figures are woth considering. Main geyser ran for 45 minutes, using about 1.7kW. So about 1.2 kW

Guest geyser ran almost exactly 2 hours, so 4kWh.

So the heat pump used about 30% of the power and took about 37% of the time.

Just now, Bobster. said:

So the heat pump used about 30% of the power and took about 37% of the time.

If it's sunny tomorrow I can repeat the test (if I remember) and see if the element heated geyser does better with some residual heat.

3 hours ago, Bobster. said:

Today I ran both my geysers. The main geyser is heated by a heat pump, the guest geyser has a 2kW element. Both geysers are the same capacity.

Some variables:

1) Main geyser has a "blanket" (isotherm) and has hot water pipes insulated to the point at which they enter the walls.

2) Main geyser last got heat from the pump at 13:00 yesterday, so had not been heated for 17 hours when it turned on at 6:00 this morning.

3) Guest geyser has not been heated for at least a week.

But still the figures are woth considering. Main geyser ran for 45 minutes, using about 1.7kW. So about 1.2 kW

Guest geyser ran almost exactly 2 hours, so 4kWh.

So the heat pump used about 30% of the power and took about 37% of the time.

Did you perhaps check what the temp was on the heat pump controller when you switched on the heat pump?

I suppose one could guess the guest geyser was around the 26 deg mark?

4 hours ago, Scorp007 said:

Did you perhaps check what the temp was on the heat pump controller when you switched on the heat pump?

No. I was in bed, playing Wordle.

You are right that my test was not very scientific.

My intent this morning was to see how long it took the guest geyser to heat up. As a bonus I found out that on a not great day for PV I could heat both geysers.

But if I can repeat that test tomorrow, the guest geyser will start off warmer, so I will have a better comparison.

I'm getting more smart breakers installed. These will track consumption on specific circuits.

13 hours ago, Scorp007 said:

Did you perhaps check what the temp was on the heat pump controller when you switched on the heat pump?

This morning, just before the compressor turned on at 06:00, the pump's control unit was showing 49 degrees. So just an 8 degree drop over night. Water from the guest geyser was warm, and one might have had a tolerable shower, but I can't put a number to it.

I know from experience that generally the reported temperature in the guest geyser drops a degree, sometimes two, straight after the compressor starts running. I guess this is because as the water starts circulating, colder water surrounds the thermostat.

Anyway, I have an electrician working here today, and it's overcast, so no repeat testing. But the electrician is fitting more CBI smart controllers. One will control the guest geyser, and the other will just gather data for the heat pump, so I will start gathering better data after this work is done.

15 minutes ago, Bobster. said:

This morning, just before the compressor turned on at 06:00, the pump's control unit was showing 49 degrees. So just an 8 degree drop over night. Water from the guest geyser was warm, and one might have had a tolerable shower, but I can't put a number to it.

I know from experience that generally the reported temperature in the guest geyser drops a degree, sometimes two, straight after the compressor starts running. I guess this is because as the water starts circulating, colder water surrounds the thermostat.

Anyway, I have an electrician working here today, and it's overcast, so no repeat testing. But the electrician is fitting more CBI smart controllers. One will control the guest geyser, and the other will just gather data for the heat pump, so I will start gathering better data after this work is done.

Thanks for the info. Yes the drop in temp is normal. The water temp in the whole geyser would be the same after heating with a heat pump. With the element it would measure around the element. This is also the reason why a element system would heat and switch off and after say 20min the geyser switches on again for a few minutes without hot water used. There can be up to 8 deg difference between the bottom and top layer of water. That is what I found with my "geyser" in the caravan where I have a thermometer stuck at the bottom of my big Jim fitted with an old whistle kettle element. I use a setting for shower of about 42 deg on the thermostat.

I have found my geyser can have a drop in temp during the night as low as 5 deg if no hot water is used

This is turning out to be a difficult test. Today the electrical score is 1.45 for the pump V 2.1 for the element. Both were turned on twice, they didn't necessarily draw any power on the 2nd run, but they were able to.

Differences...

The heat pump first ran at 6 this morning. I showered just before 8 and so some cold water was drawn in to the geyser and it did run a second time (otherwise it would have been 0.99 V 2.1).

The guest geyser was heated but other than me turning on a tap to feel the water (after the second & final run) no hot water was used.

As regards my manual test, the water didn't feel that hot. The timer turned on for the second run, but no power was drawn. That test was after the second run, so that geyser is not going to run again today anyway.

I need to have the thermostat settings checked. Maybe it is turned down a bit low.

I can't do it myself. I am not allowed up ladders (medical reasons & promise to wife). There is also a good chance that any activity involving me in the roof will result in foot through boards.

A plain old element is a pretty crude device. On the heat pump I can not only program a maximum temperature, I can also program the number of degrees the temperature drops before it runs again. I have left it on the factory default, but I can change it if I want to. Devices like a geyserwise that give finer control start making sense.

#tooManyVariables.

Time wise it is no contest. The pump heats much faster.

Edited December 20, 2025Dec 20 by Bobster.

18 hours ago, Bobster. said:

I need to have the thermostat settings checked. Maybe it is turned down a bit low.

Thermostat was all the way down. It's now turned up to 60.

Edited December 21, 2025Dec 21 by Bobster.