Thursday, 7 July 2011

How to make the best use of the electricity generated from solar.

We're in July... I suspect that May, June, July & August are the 4 "peak" months for solar PV energy generation. My system has a 2.66 KWh rating and realistically the highest output I achieved in June was 2.56 KWh for a 10 minute period on the 30th June. If you view the spreadsheet for monthly information Click on link for data. you'll see I only averaged 1.1KWh between 10am and 2pm in June. This puts into context how many of your daily appliances are going to be totally energised by the sun.

I now realise that my original idea of heating all of my hot water using the PV panels was wildly optimistic. I've detailed my experiment with timing the heating of my immersion tank Hot water, I'm still collecting data ,but I suspect this is a blind alley. I WILL save 50% of my previous electricity usage by having the solar panels, but it’s going to be spread over a huge range of appliances and will be a portion of the power being used in most instances.

The chart below details the energy consumption of MY household appliances. I’m not sure how representative of what’s in most homes, I’m ashamed to say energy consumption wasn’t one of my main concerns when deciding which product to buy in the past, but it will be in future…

Added June 2012: There is a website where you can look at the energy usage of appliances to assess more accurately the kWh used. Link to website.

         Appliance                  Energy 
9.000 KWh
Tumble Dryer
3.000 KWh
Immersion Heater
3.000 KWh
2.800 KWh
2.200 KWh
Hair Dryer
2.000 KWh
Washing Machine
1.700 KWh
1.600 KWh
Vacuum Cleaner
1.600 KWh
1.200 KWh
PC , Monitor & Modem
0.25 KWh
Fridge Freezer
.880 KWh daily
.120 KWh
Micro Hi Fi
.050 KWh
.028 KWh
DVD Player
.010 KWh

I suppose the main thing I’ve learnt is ‘enjoy’ the free power, but don’t let it control your lifestyle.



  1. I think the same as you. Do I take it that you produced on average 6.03KWh in October?

    If that's correct you could switch the immersion load 100% to the PV direct during the winter, I think the Inverter will still run ok when not connected to the grid. In the summer switch it back. The immersion could be topped up in the evening with full paid for Electricity. Bonus if you have Solar Thermal as I think it could work. Any ideas?

  2. @Anonymous
    Hi. I think we all need a little more information on your comment.

    1:Yes, I generated an average of 6.03kWh daily in October 2011.
    2: my average hourly kWh between 11.00-1500 were below 1kWh.
    3: My immersion heater uses 3kW to heat the water.

    Could you provide some additional details on how inverter power can be switched between the grid and my house? You would have to bear in mind that I would be reluctant to forgo the FiT earnings (£83.80 in October) to heat my immersion.

    Best regards

  3. Excellent site!

    CLEANING - has anyone used the Professional Solar Panel Cleaning MKit sold by the Window Cleaning Warehouse costing £300 + VAT. It sounds appealing because it has a 10m water fed pole reach and so can be used from ground level ona typical bungalow installation (so the www tells us).


    1. From various sources I gather that there is really no need to clean panels, that's the beauty of them, they are self-cleaning, whether covered by leaves, Saharan sand or snow. Ultimately the rain washes them clean. There are companies with an eye to the main chance that offer a quarterly wash but it's money that could be better spent. IMO.

  4. The inverter must be connected to the grid to function. It must be synchronised to the grid or it will turn off. I have just fitted a great PV switch and heating tank via the immersion on the fly with spare electricity. I'm thinking about installing a pre heat tank and a suitable logic switch to divert between the two tanks. I'm running 4Kw of PV and hope to be running solar thermal soon into main tank only.

    1. This sounds really interesting. Could you put together some information and pictures about what's involved and the costs? I'm sure a lot of people would like to use their excess power to heat an immersion.
      I hope to hear from you soon.

  5. For those interested in this technology, it is worth trawling through this link ....

    Best to register first to stop being locked out as it is a very long thread. I am no techy but clearly there will shortly be commercial products to address this idea of converting PV energy to heat domestic HW.) There are some out now, but from my understanding there are improvements to come and soon so may be wise to research a little first before diving in!! Not an April Fool either!!

  6. Hi there,
    My 3.3kW Solar system was commissioned mid December, and I have been
    logging data from it (reading all my meters manually) once or twice a
    day. It is in a house I am renovating, not living in yet (no hot water
    system yet) so I am using PV power to drive an electric heater, to stop
    the place getting damp.

    I think you mean you Kettle 2.800 kW and Immersion Heater 3.000 kW, etc
    A 2.800 kW Kettle uses 2.800 kWh if on for one hour, so if it takes 2
    minutes to boil you would be using it 30 times in the day.
    A PV system that makes 6.03kWh in a day could be making 0.75kW for 8 hours.
    I plan to use my system to heat my water, in addition to having Solar
    Thermal. I designed and built a switch (about £180) that for now turns
    on a 1kW oilfilled radiator, when my PV is making over 1.2kW. I have an
    hours run meter on the circuit that tells me how long it has run for.
    There are similar circuits available on the www, that will drive a 1kW
    immersion heater. But a simple circuit like this will not turn off
    when, for example you turn on your 2.8kW electric kettle, but there is
    one circuit described on the electriciansforums quoted by Graham Apr 1,
    2012 that I think is not yet commercially available that measures the PV
    output, the house consumption and adjusts the power into a 3kW immersion
    heater like a light dimmer circuit, to use the difference.
    In about 130 days operation my PV has averaged 6.47kWh, ranging from
    0.23kWh to 17.76kWh per day, I have averaged 2.66h per day over 1kW with
    24 days never reaching 1.2kW, 49 days with less than 1h (including 0)
    reaching 1.2kW, 62 days with less than 2h reaching 1.2kW and 51 days
    with more than 3h reaching 1.2kW. I am about to add a second detector,
    so I can measure time over 2kW, or later 0.5kW.
    Recently, mid morning on a sunny day, I saw the power drop from 2.5 kW
    down to 500W in less than a second, when a cloud passed over. That is
    why you need a circuit that will react quickly, otherwise you could be
    drawing power from the grid, to heat your water.

    At present I am not sure how much gas it takes heating my water, but
    after I have read the meter a couple more times I will work it out, to
    compare. I have two cylinder stats with a time switch to select 1/3 or
    2/3 hot, as needed, on than system.

    The two circuits I have found are and this second one also does a 1+2kW
    immersion heater.

    Because of space limitations my water storage will be in two separate
    cylinders, so when the main cylinder gets to a reasonable temperature a
    cylinder stat will operate a diverter valve (ought to be a high
    temperature one, though the water at that point will have come out of
    the main cylinder) so that the solar thermal flows through the auxiliary
    cylinder after the main one to heat both. A similar cylinder stat will
    change over so that my PV will heat the auxiliary cylinder by energising
    an alternative contactor.

    CLEANING - A few weeks ago, after several foggy days I reckon my output
    dropped by about 500W (the panels looked dirty), until we had some rain,
    when it went back up again.

    Andy J

    1. Hi Anonymous!
      You've raised some very interesting points here. I'm sure we would all be interested in your systems output and how you manage it in the most efficient manner. Have you considered writing a piece for the "Guest Posting" page detailing your experiences?
      I hope to hear more from you. ( You can retain your anonymity if required.)

  7. Hi, I have been following with great interest your website articles especially the ones concerning using any excess generated solar power to heat hot water during the day.

    I have had a 4kW PV system fitted to our house in Bristol during November last year (just in time for the higher rate FIT!).

    I have wanted for a while to change our standard low pressure hot water system to a mains pressure system and during the change would like to incorporate some sort of power saving from the solar PV system.

    I have currently got a standard wall mounted gas boiler which heats the radiators and indirect hot water on demand via a three way motorized valve. This is an open vented system with tanks in the roof space for cold water and heating expansion. After some research it seems the best (and safest) system would be to change the hot water tank to a thermal storage hot water tank such as and leave the radiator system and boiler as they are.

    To use excess solar PV I am thinking to fit a 1kW immersion heater to the tank and automatically power it when the power output from the panels exceeds 1.5kW with a control box such as

    As the required temperature of the thermal storage cylinder is 75 to 80 degrees C, set the immersion heater thermostat to 80 degrees and the heating system to 75 degrees to try and ensure the temperature from the solar is sufficient on sunny days to stop the boiler from starting.

    From the data extracted from the Sunny Beam the length of time the power exceeds 1.5 kW (the control switching power level) varies anything up to a maximum of 8.7 hours a day

    Has any other followers of your site fitted or considered a system like this? and if so are there any benefits / pitfalls I should consider?
    I would very much appreciate any comments on this subject as I intend to fit this system later this year.

  8. The web addresses disappeared from the last post!
    Hot water tank:
    Solar switch:

  9. You need an intelligent switch unit that automatically diverts any excess unused electricity from your solar panels to the hot water tank, so you don't need to keep switching it over yourself. Our one is called Immersun but there are other types available now. Retail Cost to install c£500