Confused about how our solar panels work, and our electricity costs

Autopilot

Distinguished Member
Joined
Feb 5, 2005
Messages
9,889
Reaction score
3,842
Points
2,266
Location
Region 2
Hi. We live in a new build and have Solar and ASHP power DHW and under floor heating. It’s larg-ish 5 bed house with annex, occupied 24/7. We moved in in sept 2019

Our bills are extremely high, or at least they are going to be judging by our meter readings. We seem to be using £200-£250 a month in the winter, but assume this will drop down considerably in summer months. Anyway, this got me looking in more detail into how much our solar helps.

Now I was under the impression that the way it worked was that we draw power from the solar panels whenever they are active. When that isn’t enough (when the ASHP kicks in, night time etc) it takes for the grid also. Sometimes we are using solar only, but in reality we take some from the grid most of the time when the house is occupied. Obviously at night it’s all of the grid.

Now... I’v been told that actually we never directly use any of our solar power. The PV inverter puts it all on the grid. We take 100% of our power from the grid all the time. But we get paid for what we generate and export to the grid by the power company.

The thing is that we are still, after months of moving in, in dispute with British Gas about the feed in tariff and still don’t get anything. It’s long story, but down to the installer not sending the required documents to them.

So if we are currently not on the feed-in tariff, just taking power and paying for it like normal (non-solar) house, then that surely means we are not benefiting from our solar panels at all. Basically we are dumping all the power they generate onto the grid, with zero pay back. A complete waste of time and money having them installed, so far.

Or am I getting this all wrong?
 
Last edited:
£250 a month!

We consume what ours generate in the first instance, only using grid electricity when our demand exceeds the supply from the panels. We do the majority of things during the day where possible (washer/drier, vacuuming, electric showers etc.).

Our electricity bills over winter have been c. £30 a month inc. standing charge.

This can be most easily verified by turning on a full kettle - does your grid consumption meter start totting up as quickly in bright sunshine as it does at night? You should see a notable difference in rate of consumption if it is being offset.

When the FIT dispute gets resolved you should be able to claim for the full amount generated/exported from the point of the previously accepted reading. We missed a reading once and it just increased the subsequent payment.
 
£210 in the last month yes - if simply multiplying the increase on our meter by the unit cost. I think much of the cost is due to badly configured underfloor heating system that make the ASHP too often.

But this is a 5 bed house, with large annex, so it’s basically two houses really. Busy family (three adults. No gas or oil, 100% electric.

Is anyone else around here paying anything like that? Or is there something seriously wrong?

What I need to know to start with; are we actually using the electricity the PV array generates, or not. Do some people have systems that only feed back to the grid (seems inefficient) that we may have inadvertently been configured with?

I will try the kettle idea when I next get chance.
 
The first thing you need to do is ensure you are on the best tariff for your usage pattern. I’ve just moved company and am around 10p/kWh again for the first time in a while.

Like you, I have a large house, all electric (with ground source heat pump for underfloor heating and hot water), detached double garage with bedroom suite above (electric UFH, electric shower and electric point-of-use hot water), a 4kW solar PV setup, and currently two electric cars. Following my tariff change I’m expecting to be paying around £1800-£2000/year for electric, before taking into account my solar PV FIT payments.
 
Actually that doesn't sound too bad for a large property over winter. Do you have any of the documentation for the solar PV installation, or any meters that will tell you how much power it's generating? Most systems are up to about 4KW and of course over the winter months will not get anywhere near that kind of figure. You're most likely generating <10kWh a day whilst consuming >30kWh in just heating alone. For every degree it gets colder, add roughly another 10%.

I'd expect the situation to sort of reverse in summer, but only that you should generate more than you consume, but not by anything like the same magnitude.
 
And in my case I know we use a decent amount of PV directly ie it reduces the power being taken in through the main billing meter. I also have an intelligent EV car charger that allows me to utilise looser that otherwise would be exported to instead charge one of my cars. So I can be using 3.5kWish of power when the solar PV is in full swing yet my incoming meter shows no power draw.
 
£210 in the last month yes - if simply multiplying the increase on our meter by the unit cost. I think much of the cost is due to badly configured underfloor heating system that make the ASHP too often.

But this is a 5 bed house, with large annex, so it’s basically two houses really. Busy family (three adults. No gas or oil, 100% electric.

Is anyone else around here paying anything like that? Or is there something seriously wrong?

What I need to know to start with; are we actually using the electricity the PV array generates, or not. Do some people have systems that only feed back to the grid (seems inefficient) that we may have inadvertently been configured with?

I will try the kettle idea when I next get chance.
Ah, if there is no gas/oil then that's not wildly excessive, but I would still query whether you're benefitting from offset consumption through your PV inverter (as of course you are doing!).

Our gas heating and cooking this winter has cost c. £120pm. This is in a 175sq.m. 5 bed.
 
I've been looking at the running cost of ASHPs for a while, but it's hard to get any real idea as most websites/installers don't give out much real life data.

£200+/month is ALOT to spend on the fuel bill, our last month gas bill was £120 for a 5 bed detached bungalow with not fab insulation.

For electricity costs with solar PV, in theory if you are getting paid the same £/kWh for exporting the electricity as you use than when your FIT payment comes through you should get a half decent chunk of money.

The last few weeks have been fab for solar in the UK. We have home battery storage for our 4KW solar PV setup, yesterday we used just 0.1kWh of grid electricity, and for the whole week we are at sub 20kWh despite also charging an electric car (am a key worker so still doing 10-15 mile daily commute). So about £3 bill last week including standing charge as we are also on E7!!!

49766671708_1bc508d973_c_d.jpg


49766696763_75c2e7bc5a_c_d.jpg
 
@Autopilot Regardless of solar PV FIT payment issues you are having, your electricity bill has put an end to any idea I had about getting an ASHP.

If I spent a few £££ on better insulating our house our gas bill could drop by 10-20% easily which than makes your electricity bill look quite crazy!! In winter our solar PV setup barely generates 10kWh for a whole week, versus 15kWh a day at present.

So getting an ASHP would INCREASE our heating bill in winter, so I see little point in getting one installed:(.

49762159297_7088bd69f6_c_d.jpg
 
@Autopilot Regardless of solar PV FIT payment issues you are having, your electricity bill has put an end to any idea I had about getting an ASHP.

If I spent a few £££ on better insulating our house our gas bill could drop by 10-20% easily which than makes your electricity bill look quite crazy!! In winter our solar PV setup barely generates 10kWh for a whole week, versus 15kWh a day at present.

So getting an ASHP would INCREASE our heating bill in winter, so I see little point in getting one installed:(.

49762159297_7088bd69f6_c_d.jpg

We wouldn’t get a ASHP if doing it again, for a number of reasons. The noise they make is another factor. And no one, even the so called qualified installer seem to fully understand them.

Back to my original question, can anyone answer my solar if is powering my house directly, or exporting 100% to the grid, as I have been told some people’s systems do.
 
Impossible for us to be sure about your specific circumstances, but to me it doesn't sound like you are using the energy your panels are generating, based on the very high leccy bills.
For comparison the weather in the last few weeks of lockdown has been so good that we haven't registered any Grid consumption during daylight hours.

What does your installation documentation and FIT paperwork state? I see you've mentioned it is an issue...

Our system doesn't measure the export rate, so uses the blanket 50% of generation assumption to calculate the export payments. Yours may be 100% if you're not using your generated power.
 
How much power a day are you using?

When it’s sunny take an hourly meter read and see if you are using any power.

Does your meter show “red” on its display (reverse energy detected?

Take a photo of you consumer unit and PV meter and main meter and post them here so we can see what you have currently installed.

If you electrical supply is single phase then unless the PV has been wired in to the wrong side of the billing meter (which would be illegal) then it cannot just export and not allow you to use the electricity generated to offset the power drawn from the grid. If you have 3-phase power but only a single phase inverter then you may have PV generation only offsetting one of your phases so power on the other two is unaffected by PV generation.

Regarding the heat pump, what make and model? Is it independent to your heating controls? ie does it run in heating mode even when your heating controls are off, or is it demand driven by a call for heat?

What flow temperature does it call for when heating? What about hot water temperature? Having heating flow or hot water setpoints too high could be an issue. Or your heatpump may run all the time using weather compensation in which case you need to look at those settings to see if they are appropriate.

Is there a buffer tank and is there any immersion heater supplementary heat?

There are lot
 
Isn't the first question- how big is your solar array? How many panels do you have on the roof is a good indicator as each will be roughly 250-300W. Then answer neilball's questions above.

I'm about to move to a place with oil/ electric only available and considering going whole hog with solar and GSHP, but there are varying degrees of horror from installers when I discuss the plans.

@Autopilot do you have another form of heating available? If so what would you choose over ASHP?

@gangzoom Although gas prices are currently low, if it's not available you don't have any choice and it is a finite resource so doesn't solar and a ASHP or GSHP work out in most cases? Depends on how long you intend to stay I guess?
 
Can’t remember exactly how big the solar array is, I’ll get back to you.

But I would definitely go GSHP over ASHP if you can.

Either way, presuming you go for under floor heating downstairs (it’s pretty pointless otherwise), the choice of flooring is absolutely key. We went with engineered oak wood flooring through most of the house, with slate/ceramic in hall and utility etc. The wood is specially designed to work with UFH. However slate/ceramic tiles work a million times better. Get nice and warm quickly, and thus the room is quickly warm. The wood take HOURS to heat up (days to properly stabilise). It’s an error that will probably cost us thousands over the next few years, because the pump is expensive to run and you can end up running almost 24/7 in the winter if the system is not perfectly designed and configured. Our so called expert and experienced installer told us none of this, and probably don’t know. Despite having every accreditation under the sun.

Some of the new generation “dry” (electric element, rather than water pipes) UFH systems coming into the market soon look better and are more efficient. Coupled with a huge solar array and battery for night time, they could be better, easier to maintain and silent. That the way I would go personally, even if it meant being more grid reliant in the dark winter months (which we are really anyway).

Also take great care where you site the pump AND where you run the pipes. Noise is still an issue, especially at night when you try and sleep. You will hear it, despite all the claims of the modern pumps being quieter.

Our wood burners are in now, so we will use them as much as we can to avoid using the ASHP as much as possible.
 
Last edited:
Good real world experience that, thanks. Shame you've had to learn the hard way.

I just can't believe direct electric can be more efficient than a heat pump at anything greater that -2 outside- the whole point of them is the COP. I fully agree about maintenance and noise though. It'd be some battery to carry you through overnight- but then that's what Economy 7 tariffs or the new Agile tariffs are for right?

If I ever get to move (thanks CV-19), I can do a mix of UFH in the new place, but some of the floors are just too nice to take up so will have to be FCUs or oversized rads. Fortunately there's a ready made external plant room for machinery and heat pumps to hide in. It's really a decision about ASHP vs the hassle of GSHP...

Wood burners are always a good shout if you can grow the fuel!
 
Good real world experience that, thanks. Shame you've had to learn the hard way.

I just can't believe direct electric can be more efficient than a heat pump at anything greater that -2 outside- the whole point of them is the COP. I fully agree about maintenance and noise though. It'd be some battery to carry you through overnight- but then that's what Economy 7 tariffs or the new Agile tariffs are for right?

If I ever get to move (thanks CV-19), I can do a mix of UFH in the new place, but some of the floors are just too nice to take up so will have to be FCUs or oversized rads. Fortunately there's a ready made external plant room for machinery and heat pumps to hide in. It's really a decision about ASHP vs the hassle of GSHP...

Wood burners are always a good shout if you can grow the fuel!

Thanks, yes I can grown the fuel as I own some woodland next to my house, far more than I could ever use (I’m still working my way through some stuff I cut over a year ago as we don’t need to burn much in our 2 burner). Next year I’m going to see if I can make even more use of it.

As for electric UFH, I get what your saying. I’m not got much info, just what I was told by a friend who works in that field. But the thing with ASHP is that you have to power a huge fan and compressor, that only works well at certain temps (and has a built in electric water heater to help when it struggles anyway) that then only manages to heat the water to about 48 degrees max, which then runs through miles of pipes. I’m told the new electric ones work on very low voltages/power elements and low temps. Combined with solar and some buffering capacity it’s meant to be far cheaper to run that traditional electric UFH (especially without solar). I just repeating third hand info, so take it was a massive pinch of salt, and I’m happy to be shown to be wrong. But what I do know is there is so much room for error and loss, that a poorly thought and planned ASHP and UFH system, even small changes, can mean the difference between it running for hours more each day and huge hit on your electric bills.

I’m looking into battery options again. We were offered a discounted Tesla Powerwall 2, but it’s was still so expensive that it would take far to long to pay for itself, if it ever did. However I see there are much cheaper options now, and the LG ones look promising. I maybe could use an agile tariff like the Octopus one to charge it off the grid during the super cheap hours, as well as solar.
 
Last edited:
Regarding the UFH, floor coverings really do make a massive difference to speed of response and the flow temperatures required to achieve a feeling of comfort.

In my own build (completed more than a decade ago now) I chose to have hard flooring throughout, and designed the house to utilise wet UFH upstairs as well as downstairs (screeded floor upstairs thanks to beam and block 1st floor). Tile and stone are great as they warm quickly and transfer heat well in winter but also feel cool in summer too. I’ve also used thin engineered oak flooring (plus some laminate in the kids bedrooms and a playroom), with the engineered boards only being 10mm thick, with a thermally transmissible underlay to help get a reasonable response, but it is still a lot slower than the stone and tile floors. When the time comes to replace the engineered oak I may replace it with stone or porcelain tile for the improved response/output.

I live in Scotland and have never needed to run my heating 24/7 apart from a few very cold snaps (-15 overnight and -5 through the day, for 5-7 days at a time), but that is because I designed my heating to run with a lowish flow temperature (it’s never been any higher than 38 degrees leaving the heat pump), and also attempted to keep the house relatively air tight as much as possible, with internal dense block walls and wet plaster finish for additional thermal mass to smooth the house reactions to changes in temperature (both internal and external).

This does all mean the heating still has a very slow response compared to radiator systems, but it remains comfortable and relatively efficient compared to friends who have oil boilers feeding radiator heating. And that is with comfort temperatures of 21-23 degrees in winter, we’re not a family that enjoys sitting in cooler temperatures with thick jumpers on!

My system also has full room zone control using Honeywell/Resideo Evo Home, so room temperature control is very accurate too.

We do have a wood burner, although with having to pay for wood to burn it’s not as cheap to use as the heat pump, and the combination of fresh air supply and chimney does make the room feel a little cooler because of the increase air changes. That did it is a useful non-powered heat source that is essential during power cuts in winter.

And I disagree about direct electric underfloor heating being more efficient than well designed wet underfloor heating via a heat pump. I have electric carbon film UFH in a room above my detached double garage, fitted over a very well insulated floor and high levels of wall and ceiling insulation, and again with thin engineered oak flooring. It’s quicker to warm up from cold compared to my wet system, but also uses the same amount of power at full output as my heat pump even though it covers only 10% of the floor area! Doing a whole house this way would be very expensive and would cause issues with my electrical loadings for the incoming supply (my 100A singe phase supply would not be adequate if I followed the standard electrical load calculations with allowed diversity applied).
 
Regarding the UFH, floor coverings really do make a massive difference to speed of response and the flow temperatures required to achieve a feeling of comfort.

In my own build (completed more than a decade ago now) I chose to have hard flooring throughout, and designed the house to utilise wet UFH upstairs as well as downstairs (screeded floor upstairs thanks to beam and block 1st floor). Tile and stone are great as they warm quickly and transfer heat well in winter but also feel cool in summer too. I’ve also used thin engineered oak flooring (plus some laminate in the kids bedrooms and a playroom), with the engineered boards only being 10mm thick, with a thermally transmissible underlay to help get a reasonable response, but it is still a lot slower than the stone and tile floors. When the time comes to replace the engineered oak I may replace it with stone or porcelain tile for the improved response/output.

I live in Scotland and have never needed to run my heating 24/7 apart from a few very cold snaps (-15 overnight and -5 through the day, for 5-7 days at a time), but that is because I designed my heating to run with a lowish flow temperature (it’s never been any higher than 38 degrees leaving the heat pump), and also attempted to keep the house relatively air tight as much as possible, with internal dense block walls and wet plaster finish for additional thermal mass to smooth the house reactions to changes in temperature (both internal and external).

This does all mean the heating still has a very slow response compared to radiator systems, but it remains comfortable and relatively efficient compared to friends who have oil boilers feeding radiator heating. And that is with comfort temperatures of 21-23 degrees in winter, we’re not a family that enjoys sitting in cooler temperatures with thick jumpers on!

My system also has full room zone control using Honeywell/Resideo Evo Home, so room temperature control is very accurate too.

We do have a wood burner, although with having to pay for wood to burn it’s not as cheap to use as the heat pump, and the combination of fresh air supply and chimney does make the room feel a little cooler because of the increase air changes. That did it is a useful non-powered heat source that is essential during power cuts in winter.

And I disagree about direct electric underfloor heating being more efficient than well designed wet underfloor heating via a heat pump. I have electric carbon film UFH in a room above my detached double garage, fitted over a very well insulated floor and high levels of wall and ceiling insulation, and again with thin engineered oak flooring. It’s quicker to warm up from cold compared to my wet system, but also uses the same amount of power at full output as my heat pump even though it covers only 10% of the floor area! Doing a whole house this way would be very expensive and would cause issues with my electrical loadings for the incoming supply (my 100A singe phase supply would not be adequate if I followed the standard electrical load calculations with allowed diversity applied).

It’s a new system that’s in development, not executing systems, that apparently works very differently. I’ll try and find out more.

As for your flow temp, thats pretty low. I think my house is probably too large. Have you measured the floor temp using a IR gun? How warm do your wood floors get?
 
None of my timber floors exceed 28 degrees surface temperature as “measured” with my thermal inaging camera. I’ve no high temperature limits on the wet system, but do have a limit stat/floor probe for my electric UFH.

My house is approx 200m2, with two manifolds (one for each floor). It was properly designed (proper drawings showing pipe layouts/spacing and flow rates). House size should be no barrier provided maximum pipe lengths are not exceed and required flow rates are achievable. You tends to get problems where installers make it up following as they go using average recommended pipe sizes/spacing a rather than taking account of anticipated floor coverings/makeup/thicknesses and do not carry out proper heat loss calcs based on actual house construction.
 
Can’t remember exactly how big the solar array is, I’ll get back to you.
AP, have you got a device inside that tells you how much you have charged each day?
We have 16 panels =4 kw system. For the past 4 weeks we have been averaging 14-17Kw per day.
The system is Normally wired from your Inverter to you Consumer unit, with either 2.5 or 4mm Twin & earth cable.
If it is wired that way, the device inside will tell you the amount of charge instantly, & records the whole days charge amount.
 
AP, have you got a device inside that tells you how much you have charged each day?
We have 16 panels =4 kw system. For the past 4 weeks we have been averaging 14-17Kw per day.
The system is Normally wired from your Inverter to you Consumer unit, with either 2.5 or 4mm Twin & earth cable.
If it is wired that way, the device inside will tell you the amount of charge instantly, & records the whole days charge amount.

If you mean like a smart meter, no. But it is wired that way and there is a meter under the inverter saying what’s it’s generated.

I have done a lot of meter readings recently and with the ASHP not in use now we don’t use much in the day and are making good use of the Solar. In fact when it’s sunny the IBoost display shows that as well as heating the water tank we are exporting a fair amount. But we use a lot through the night, as much as many people use in the day, as the house is effectively split into two dwellings, with my mother living in the other side and I don’t think our usage profile is unusual. So I’m stating to think that a battery might worth looking into. Very expensive though, especially when you factor in a new inverter and fitting.
 

The latest video from AVForums

Is 4K Blu-ray Worth It?
Subscribe to our YouTube channel
Back
Top Bottom