Solar Grid Import figures lower than Smart Meter Readings

  • jacvictoria's Avatar
    Level 1
    The solar panel data readings are showing a lower grid import figures than what I am billed for on the smart reading consumption figures.
    My questions are:-
    Why and am I being over charged for electricity.
    Where is the problem, with smart meter or solar panel system.

    EON support are only interested in asking for current smart meter readings, and explaining how to read a liberty 101 smart meter.

    Which doesn't answer the questions.
    Last monthly bill was for a consumption of 489.5 kWh from smart meter readings
    From the solar panel data for the same period:-
    The grid import was 46.5
    Export to grid 1.1 kWh
    The Solar Panel Generation was 138.9 kWh
    Consumption was 184.3
    The different between the two consumption readings is a multiplication factor of 10.52688172

    Having Googled the problem I find that there are problems between solar panels and smart meter, but not any explanation of what to do about it.
    There are conversations about the clamp on the solar panel system side being in the wrong location, but not what to do about seeing if it is in the right location.

    So any ideas?
  • 4 Replies

  • rwh202's Avatar
    Level 5
    That's some discrepancy!

    So at least one must be incorrect, and for simplicity, assume it's just one of them.

    The solar generation looks ballpark right for an average 4kW array in Feb/Mar

    If the meter is correct, you import 490 and generate 140 for a total consumption of 630 kWh

    If the solar monitor is correct, you import 46 and generate 140 for a total consumption of 186 kWh

    The OFGEM 'average' electricity consumption for an 'average' household is 225 kWh a month for a dual fuel customer.

    I guess we need to know if you are below average or significantly above! Do you have gas or electric heating? Do you have an electric car? My own electricity consumption was 600 kWh last month with gas heating but 200 going into the car, so the high value isn't unreasonable, but obviously depends on your circumstances.

    Another point is the low export - are you set up to export and / or do you have a battery / diverter to stored heat? If not, then you are consuming everything generated even at peak generation so that suggests high consumption. Also, if a grid import of 46 kWh was having to just cover the pitch black night hours with 0 generation then that's under 100 W which is less than a fridge freezer and wifi router before you even put a light on or watch tv (or cook or put washing machine on...)
  • wizzo227's Avatar
    Level 21
    Be cautious. According to your solar controller, 46.5 kWh import and 138.9kWh generation ( sum = 185.4 kWh ) is barely more than "consumption" of 184.3 kWh by exactly the 1.1kWh exported to grid. That could only happen if you have a gadget on a prompt interruptible load such as a heater and that was able to use all of your solar generation. That is an almost perfect score for local use of nearly all of your generation; in my opinion too perfect. The room with the heater might get too hot.
    6.1 kWh per day of consumption seems plausible and is about what my parents use. I'd expected to see more than half of your consumption imported because the sun is rarely bright at the exact moment when you turn the oven on.

    The other reading; 489.5 kWh per month from the smart meter, is > 16kWh per day. If that much enters the house you'd be able to find where it went from a room being hot.
    Is it possible that the 489.5 number does include a very big use of electricity such as flats next-door or an electric car recharge, which is outside of the things looked at by the solar controller ?
    If so, pay an electrician to follow the wires if you don't know where they go and sketch what is connected where. You might get some genuine bill savings in the summer from getting some rewiring done so that the car recharge is under control of the thing which is presently inside the domain of your solar controller. Ask the garage though; not all home vehicle chargers can be interrupted by a solar diverter. You won't be able to usefully dump to heat in the daytime in summer, but might you be able to dump to car recharge in the daytime ?

    Regarding comments above, my hours of darkness usage total including a fridge/freezer and internet and raspberry pi computer and lighting plus kettle and radio is very marginally above 46 kWh per month. To get what you got at this time of year I'd need some help from solar to batteries.
  • jacvictoria's Avatar
    Level 1
    Thanks for the replies.
    I think from what has been said that us running 2 fans most of the day will push the consumption up.
    The only heating is a small oil filled radiator on min in an insulated log cabin in the garden.
    The max output of the panels is 3.3 kWh, yes it is all used with any battery charge each day. Cloudy days there is usually no export to the grid.
    So I think the smart meter could be right.

    I think the problem might be with the location of the CT clamp on the live line from the isolator switch. It is around 1 live wire and the back of the clamp is touching another live wire.
    Would that throw out the readings on the solar system meter?
    (Funny they call it a clamp when it hangs loosely around the cable.)
  • wizzo227's Avatar
    Level 21
    I always put my "Current Transformer "CT" clamp around the "N" Neutral but it should work exactly the same on the corresponding "L" Live because it detects current passing in and out through whatever heavy wire goes through the hole. I don't know your system but it sounds like that CT from "the isolator switch" (!they add several extra ones with some types of solar and battery) could be monitoring net house switchboard+solar+battery and is the point through which 1.1kW import passed, with the inverter and the battery passing their internal numbers to the smart house controller to minimize import through that CT clamp. Other nearby wires even touching should make little or no noticeable change to the current through the fat wire through the hole.

    About units. 3.3 kiloWatts(peak) might describe a moderate sized typical rooftop solar. Last month you write that yours provided 138.9 kWh/month, meaning about 4.6 kWh/day on average, and you'll get more kWh in the summer. kW(p) defining the datasheet performance of the sum of the solar panels under ideal test conditions is great for estimating what to expect, and not to be confused with kWh metered energy.

    Your house definitely has at least one large load outside of the solar-battery-import-export zone and it sounds as though that could be the electrically heated shed. Fans use 2 to 40 Watts ( 0 to 1 kWh per day; 1-30 kWh per month) so fans definitely won't account for the extra. Fan heaters on 'I' use 950 to 1100 Watts. With the thermostat, they always use less than 24kWh per day to avoid overheating. 489.5-189.4 = 300 kWh/month = 10 kWh / day billed by the smart meter but not seen by the solar battery controller. In February, that was about half enough heat to keep my whole 2 bedroom house sufficiently warm, but 10 kWh/day is plausible for a moderate sized shed, especially if it has too many patio doors. It sounds too much for a properly insulated shed as that much heat would notice in my 2 bedroom house. Same as for fan heaters, 10kWh/day through an oil filled heater is plausible. You could get that down to about 3kWh/day or possibly less with the heat pump capability of an air conditioner, so here I'll write down enough for you to decide the economics. There are other sorts of machine also called air conditioner. The heat pump type which I mean is similar to

    Summer : capability to cool the shed with air conditioner might be desirable and might be a requirement of workplace health and safety.
    Late Spring / early autumn : better to just open some doors while it is > 16C outside
    Early spring / late autumn : best time to run the heat pump
    Winter : you can always buy imported electricity when more heat is necessary than 1-2kW from an air conditioner provides.
    Winter coldest day : -6C outside and the heat pump has a sneezy-fit in its cycle which disables it for ten minutes in every hour ? keep the oil filled heater and expect to use both on the coldest day of the year.

    Now economics; refine these figures when you know. 10 kWh/day of heating x 182 days per year. Presently provided by oil filled plain old electric heating outside the solar-battery-smart house. Keep the fat power line to the non-solar part of your power supply; is that a second switchboard in the garage near to the smart meter? Consider hiring an electrician to fit an Elster 100C or similar to sub-meter from the garage to your shed. That could be used to show a work expense if there is a company name or a boss. An air conditioner should save about 7kWh/day x 182 days/year = 1274 kWh/year out of 1820 kWh/year present use by the shed. At 25p/kWh, savings are around 318 pounds per year. So to break even in eight years you have a budget of £2.5k
    That might be plenty enough to get an air conditioner for the shed, to be used as its main heating (as well as occasional cooling).

    Consider changing the wiring to power the shed from inside the smart house solar-battery-controller zone. It might be not worth changing this; you do your sums. I'd have done that from the start, with an Elster 100C showing kWh which your boss pays to you for electricity from your smart house to her shed, at the Ofgem cap price p/kWh, and partly supplied from your rooftop solar electricity.

    Now for practical steps to make things better next winter. If you are going to be working from home from that shed long term then it is a workplace and health and safety requirements might define sufficient heating and cooling for the type of work, which might be less active than hacksawing and filing steel parts these days. Since you own the place, it is your job to make it suitable for your requirements.
    Last edited by wizzo227; 23-03-24 at 11:07.