How much electricty does the solar hot water panel pump use?

We have solar panels for heating our hot water. As well as using free energy from the sun, they use some electricity - this powers the pump that circulates heated water through the hot water tank. I have been assured that this pump does not use much electricity - finally I have been able to confirm this by measurement (more or less). In July, I estimate electricity for the pump is about 3.6% of the heat obtained. In winter the pump energy goes up to 6.4%. The reason the yield is not so good in winter is that the water from the panel is not so hot. The pump works just as hard but there is less heat being delivered. However even then, and allowing for the fact that the pump uses electricity whereas our backup water heating is from a gas boiler, we still get 80% savings on carbon emissions.

This is only a sample of one and your system may be different. Also, there are some uncertainties around my measurements. However, I believe these figures are at least plausible. Keep reading to find out how I made my calculations.

Measuring the heat delivered - using the Vaillant controller

We have a Vaillant heating controller and it purports to tell us how much heat is delivered from the panels. It calculates this based on the measured temperature of the fluid going into and returning from the hot water cylinder, and the configured flow rate. It knows when the pump is on and it relies on a setting for the flow rate. We have configured this based on a manual reading from the flow meter on the device. Our reading is probably only accurate to 10%.

The Vaillant controller also makes some assumptions as to the heat capacity of the fluid. This is not pure water - it also contains anti-freeze, probably glycol. At 25% by volume (enough to reduce the freezing point of water down to -10C or so, this could reduce the specific heat capacity, and hence our heating kWh value by 10% (Engineering toolbox). I have no idea what assumptions Vaillant makes regarding glycol concentration.

All in all, our measurement of heat delivered could be out by +20% to -10%.

(There was a period a few years ago when the pump was not working properly. We should have realised what was wrong because the controller reported huge amounts of delivered heat. This is because the flow rate was much lower than the configured value. We live and learn.)

Measuring the electricity used by the pump - using circuit level monitoring and some guesswork.

If the hot water panel pump plugged into a 3-pin socket, I could measure it directly with a plug monitor. Sadly it is wired in directly so this is not possible.

For almost a year we have been monitoring our electricity consumption at the circuit level, rather than the whole house. (There is a long story behind this which you do not need to know.) The circuit that runs the hot water panel also drives an MVHR unit (continuously), the battery charger for the burglar alarm (trickle charge all the time) and some valves for the central heating. However, the central heating mainly comes on during the morning and evening whereas the solar panel pumps only come on when the sun in shining during the day. The chart below shows power consumption on a typical winter day, with solar PV generation overlaid - this is a good indicator of solar power available to the hot water panel.

Power consumption on our bathroom circuit 16/Jan/2016 (blue area, left axis). As well as the solar panel pumps this circuit has a base load running an MVHR unit, and a burglar alarm. Also there is intermittent load from central heating valves. The solid line is the power generated by the PV panels at the same time (right hand axis). Power consumption is averaged over 15 minutes. Only current is measured - power factor is averaged across all circuits.

On the chart you can see that the overnight power consumption (MVHR plus burglar alarm) is about 70W and there are slight humps, in the morning from about 6pm and again in the evening, of about 10W. These are the central heating valves. The solar hot water pumps are about 40W and they coincide more or less with generation on the PV panels, as you would expect.

During the summer we turned the MVHR down to low as we were opening windows a lot, so this is down to about 40W. Also there are no other humps in the morning and evening because we do not use the central heating.

Power consumption on our bathroom circuit 4/Aug/2015. Otherwise this chart is similar to the one above.

Based on these charts, I have derived the power consumed by the panel pump as follows. For each day:

• Calculate the mean power consumption midnight to 4am. This is the MVHR load.
• Look at periods when the power consumption is 20W more than this - so ignoring periods when the solar pump panel is not working.
• Subtract the overnight power consumption and assume the difference is the solar panel pump.

This method overestimates solar panel pump power because if the central heating valves are on at the same time as the solar panel pump it will be counted as well. However, the central heating valves take much less power and also when it is sunny enough for the panels to function it is likely that the heating is not needed. I estimate this is unlikely to overestimate by more than 10%.

The circuit level monitoring does not actually monitor the power consumed - it only measures the current. Both voltage and current are measured separately at the supply level but not per circuit. In calculating the circuit level power consumption I have assumed that each circuit has the same power factor, which is in fact unlikely. The main loads on this circuit are pumps and fans: the MVHR fan and the solar panel pump; so it probably has a higher power factor than average and the real power consumption could be lower than this. Therefore my estimate of the solar panel power consumption may be too high.

Overall results
The solar panel heat yield varies through the year and from year to year. I calculated the heat yield and power used by month. The  table below shows the best and worst months for the period I have monitored  (not quite a year).

Month	Pump kWh/day	Heat kWh/day	Pump kWh/ Heat kWh
Jul 2015	0.119	3.30	3.6%
Jan 2016	0.043	0.674	6.4%

Your system may or may give similar results to this. The amount of heat from the panels depends not just on your panel and its location but also on the pump and your water tank and use - how big is your tank, how well insulated it is, the maximum temperature you have set and how much hot water you use each day. The system will not heat the tank any more if it is hot already. See also Living with solar hot water panels.

These measurements are only approximate but they are the best I have available.

There are uncertainties in both sets of measurement. Fortunately I believe both are more likely to overestimate rather than underestimate and hopefully the inaccuracies cancel out.

To get more accurate measurement for the heat delivered I would have to install a proper heat meter on it (cutting into the pipe to install a flow meter) and measure the heat capacity of a sample of the fluid.

To get more accurate measurement for the solar pump I could get an electrician to rewire the pump with some kind of socket - but this is probably against the rules given this is in a bathroom. Alternatively, if you monitor the whole house consumption at much higher resolution you can use sophisticated algorithms to distinguish power going to different devices. This is called disaggregation and it is both difficult and hard to verify. People having been working on it for years. There are some open source algorithms such as LIDS from MIT. There are also some commercial services available - such as Informetis. I am looking forward to an affordable domestic solution real soon now (but I'm not holding my breath).