Circular reasoning (Rolfe's solar energy system)

[Rolfe]

Bear in mind there are differing opinions on that. My system was wired the way you don't like, and after operating it like that for a bit I decided not to change it as there were several advantages and I could work round the disadvantages fairly easily once I realised what they were. For example I have to do a 100% charge tonight which will undoubtedly get me a schedule more than the normal 6-hours off-peak period. I need to make sure this is continuous (apart from anything else I'm trying to do the "long charge" tonight and get it right this time) and then set my home battery on charge for the scheduled time period.

But then, obviously there are people who prefer it the way you recommend. Swings and roundabouts.

The other thing I need to remember to do (which I forgot last week) is to set the home battery to start its export discharge earlier, to stop by the start of the scheduled charge - assuming that's earlier than 11.30 which it may well be.

 

[johnb80]

Bear in mind there are differing opinions on that. My system was wired the way you don't like, and after operating it like that for a bit I decided not to change it as there were several advantages and I could work round the disadvantages fairly easily once I realised what they were. For example I have to do a 100% charge tonight which will undoubtedly get me a schedule more than the normal 6-hours off-peak period. I need to make sure this is continuous (apart from anything else I'm trying to do the "long charge" tonight and get it right this time) and then set my home battery on charge for the scheduled time period.

But then, obviously there are people who prefer it the way you recommend. Swings and roundabouts.

The other thing I need to remember to do (which I forgot last week) is to set the home battery to start its export discharge earlier, to stop by the start of the scheduled charge - assuming that's earlier than 11.30 which it may well be.

I only know of one person that actually wants it to run this way and that of course is you!

RobF actually said in his post that he was hoping that Zappi would be updated to have battery control, I was just explaining that that was not right thinking and to achieve what i believe he asking for was the way I described with a full explanation of how it works.

 

[Rolfe]

I read what he said somewhat differently. But if you're right, then he definitely has to get it wired as you say. (I do know someone else who prefers it the way I have it, he switched it back after changing it.)

Since I can't get Octopus to give me a scheduled charge for tonight (some sort of bizarre phone glitch) I'm quite glad to be able to let the car have some home battery for an hour, so as to reduce the amount of peak-rate import.

 

[Coulomb]

I.e. the Zappi gets its information from the point as close to the grid as possible?

No, the Zappi needs to get its power ahead of the Current Transformer, so that the inverter's CT doesn't see the Zappi's power load.

Yes, the Zappi gets its information (from its own CT) right at the grid connection. It's still aiming for zero export when the car could be using that energy.

 

[johnb80]

No, the Zappi needs to get its power ahead of the Current Transformer, so that the inverter's CT doesn't see the Zappi's power load.

Yes it does, the CT/Meter for the Inverter needs to be after the Zappi's feed which is what BamBam said and as per my diagram.

Yes, the Zappi gets its information (from its own CT) right at the grid connection. It's still aiming for zero export when the car could be using that energy.

If set in ECO mode of course.

 

[johnb80]

I read what he said somewhat differently. But if you're right, then he definitely has to get it wired as you say. (I do know someone else who prefers it the way I have it, he switched it back after changing it.)

I stand corrected, I now know of two

Since I can't get Octopus to give me a scheduled charge for tonight (some sort of bizarre phone glitch) I'm quite glad to be able to let the car have some home battery for an hour, so as to reduce the amount of peak-rate import.

And that can still occur when wired correctly.

 

[Robf]

Thanks all

It is a very valuable and helpfull dicussion for me (and others), I have about a 9 months to a year before I need to make a decision. I would love to access also the car battery for house, but following other discussions this just might be a step to far.
One of my older daughters is a doctor of physics out in Boston, having worked on the collider in Cern, I will try and persuade her to see if she can get some of her colleagues to come up with an ideal solution

 

[johnb80]

Thanks all

It is a very valuable and helpfull dicussion for me (and others), I have about a 9 months to a year before I need to make a decision. I would love to access also the car battery for house, but following other discussions this just might be a step to far.

Thats not difficult to do, use an AC Adapter from the car to feed a battery charger, the battery charger is plugged into your house battery and it will feed at a max 3kW into your system until the cut off point of the car. Works like a dream.

J

One of my older daughters is a doctor of physics out in Boston, having worked on the collider in Cern, I will try and persuade her to see if she can get some of her colleagues to come up with an ideal solution

Plug into next doors supply

 

[Bam Bam]

One of my older daughters is a doctor of physics out in Boston, having worked on the collider in Cern, I will try and persuade her to see if she can get some of her colleagues to come up with an ideal solution

I don't think it is a problem of physics, it is a problem of consumer products and manufacturers (particularly of cars) not really having incentives to facilitate it.

They are guaranteeing their batteries based on miles travelled and time spent, not the number of cycles taken (which increases if the car battery is used for stationary storage).

As Jon notes, the V2L equipped cars can already do this up to a point. You can plug something into it and reduce your house load, and even wire it up fully if you are handy and have the right equipment.

But I can see why car manufacturers are wary.

 

[Rolfe]

I thought I'd just do an update. I've come to a different, more realistic and more sensible way of thinking about cost and export.

In effect, every day I have ~9 kwh from my battery (at 7p/kwh) plus whatever the panels generate that day to play with. ALL of that could potentially be exported for 15p/kwh, irrespective of where it came from. So there is really no difference between surplus solar and surplus from the battery and it doesn't actually matter where it's exported from.

I've had a look at the numbers since I started exporting the battery at the end of the day, and in more than 50 days there have only been three days when I've exported less than the total solar generation for the day. There were specific reasons for that (playing the "free hour game", letting the car have some of the battery, and starting the export too late), and as far as I can see absent these circumstances I can always export more than I generate. Therefore I would be wanting to export the battery anyway, whether or not the house was running off the battery during the day, so the whole thing is a nothing-burger. I'm only left with the hassle of choosing the time to start export every day, which prevents the whole thing being set-up-and-forget.

It's a paradox in a way. If I had no solar and was simply living off cheap-rate electricity stored in the battery, all my usage would cost 7p/unit so long as I didn't drain the battery during the day (unlikely). However, with the solar and the potential for export, suddenly every unit used turns into 15p not exported. So it can be thought of as all my daytime usage "costing" 15p rather than 7p. But that's an effect of having the advantage of the solar "cash cow" which is paying off my system a lot faster than I'd be paying it off simply from savings generated by a battery-only system. So basically I need to get over that mindset.

I've also looked at costs. In the ten weeks since I got the export tariff there have only been two in which my credit for export has been less than the cost of my import (including the standing charge). Both of these weeks involved a lot more car charging than I usually do, for various reasons. I'm currently about £67 ahead of the game over these ten weeks, which straddle the equinox, which is moderately encouraging.

Obviously this will get less and less favourable as we approach the solstice, and I wouldn't expect to be ahead for the next ten weeks or the ten after that. It will be interesting to see, though. What I'm wondering is, is it possible that over a whole year I'm actually making more from export than I'm paying for import? It depends to a large extent on how many sunny days we have, and how much I need to charge the car, but given that I'm ahead mid-August to mid-October and can expect to be very substantially ahead from May to July (the months I lost this year due to the G99 being so long delayed) there seems reason for hope.

 

[Bam Bam]

I'm currently about £67 ahead of the game over these ten weeks, which straddle the equinox, which is moderately encouraging.

What I'm wondering is, is it possible that over a whole year I'm actually making more from export than I'm paying for import?

I am hoping that we will be able get through to March with our Summer credit, but we will have to see.

We almost managed last winter, having a February bill of less than a fiver (I think an Electroverse test with the then-new car put us over).

This winter we have the MG4 rather than a petrol car (plus "Pebbles" has a daily car commute now rather than both of us working from home) so we will be using a LOT more electricity. However, having an EV means we can now access cheaper overnight tariffs during the winter months so can run the house off cheaper energy.

As it happens I was thinking about your set up earlier today, strange as that sounds. I still think that when the sun comes back you'd do well to export fully in the morning. Then you'd be able to fill the battery back up with power that would otherwise get clipped in the middle of the day. Then export again in the evening.

 

[Rolfe]

I see the logic, but the practicalities escape me. Once the time comes I might be able to work something out.

I've currently got a direct debit on for £25 per month but every month I'm more in credit. I think I'll leave it alone until into the New Year at least though, so I can see how it pans out.

 

[Rolfe]

I did a few back-of-the-envelope sums and it seems as if, since I got the export tariff, I've paid a total of £15 for my electricity. That covers some of the darkest months of the year. I noted that just October, November and December came to £85 altogether (August and September were each £35 credit), so at a rough guess maybe January, February and March will be about the same. So £170 spend to see me over the winter, including car charging. I'm certainly going to do a lot better than £170 in credit over the summer months, so I guess I better start remembering to use my Electroverse card on public chargers to mop some of it up.

 

[Tangoed Man]

I did a few back-of-the-envelope sums and it seems as if, since I got the export tariff, I've paid a total of £15 for my electricity. That covers some of the darkest months of the year. I noted that just October, November and December came to £85 altogether (August and September were each £35 credit), so at a rough guess maybe January, February and March will be about the same. So £170 spend to see me over the winter, including car charging. I'm certainly going to do a lot better than £170 in credit over the summer months, so I guess I better start remembering to use my Electroverse card on public chargers to mop some of it up.

Make sure you factor in the lump sum cost of £750 per year to capitalise a solar/battery installation that everyone always forgets …… Also, you never get back the original ‘investment’, something else that everyone always forgets.

 

[johnb80]

Make sure you factor in the lump sum cost of £750 per year to capitalise a solar/battery installation that everyone always forgets …… Also, you never get back the original ‘investment’, something else that everyone always forgets.

Its not really that cut and dried though. You should take into account what the capitol cost would have earned in interest 'v' what you earn/save by having solar. Average interest rates for savers in 2024 was 2.5%, assuming a £12,000 install cost that would have been £300 in interest which is all you have lost. Now take Rolfe's savings in energy bills away from the £300 and Im sure that will be a very positive outcome.

 

[Tangoed Man]

Its not really that cut and dried though. You should take into account what the capitol cost would have earned in interest 'v' what you earn/save by having solar. Average interest rates for savers in 2024 was 2.5%, assuming a £12,000 install cost that would have been £300 in interest which is all you have lost. Now take Rolfe's savings in energy bills away from the £300 and Im sure that will be a very positive outcome.

It’s not a discursive point; it is simply arithmetic.

 

[Bam Bam]

Make sure you factor in the lump sum cost of £750 per year to capitalise a solar/battery installation that everyone always forgets …… Also, you never get back the original ‘investment’, something else that everyone always forgets.

I also don't understand your point, unless it is that we have paid up front for X amount of electricity for the next however many years.

That is a fair way to look at it. But once you've committed to that it is a given.

It is an investment in capital. Of course one could invest in a savings account or nvidia stocks instead.

If the house gets sold then the installation adds to the value of the house.

Which investment will pay off depends on how different prices change in the future (bank interest rates vs. stock market vs. electricity prices).

Given that we will need electricity (and we will have more electric in the future) it is also a hedge against future price rises.

 

[Exdro]

Quite a dilemma until you break the circle.

People often say like you do in (3) but in my experience it is often not the case or not wildly different. In my case, my Octopus Go Intelligent is 25.7p peak which seems competitive to most other standard tariffs. The real killer of your loop though is a combination of a tariff like Octopus Go Intelligent, your solar and your battery. Here's the result of mine:

View attachment 25667


I use very little peak rate energy, my house runs on the batteries and solar most of the time during peak time window. As you can see it's saved £70.81 this month which equates to £5.44 per day / £1988 per year. assuming similar savings your break even point should be around 7 years. Theres also the great smug factor during a power cut when you carry on as normal



Your wife is right to be hesitant. I do have an Air - water heatpump and several air to air heat pumps, I also have my gas condensing boiler still in the system too. The heatpumps (either type air-water of air-air) run with a COP of around 4 that means 1 kWh of electricity gives you 4 kWh heat. If your electricity tariff is say 25p kWh each kWh of heat from heatpumps is costing you 6.25p. Compare this to gas which I pay on average 4.3p kWh taking a boiler efficiency of 90% would mean each kWh of heat via the gas boiler cost 4.7p or 1.5p less than a Heatpump taking the COP of 4, when it gets cold that COP value drops, I have seen mine drop to 2.8 at times.
I set up my house with Home Assistant and programmed it to make decisions based on temperatures, gas and electricity costs. It chooses the cheapest heat source based on current conditions. Over last winter not once was the air-water heat pump chosen, the air to air was used a few times but the bulk was running gas.
I did a DIY install of the heatpump, buying it second hand but unused. I didnt get the grant because I wanted to install it myself and I wanted to retain my gas boiler. I paid under £2000 for it but even at this level I will never break even unless gas prices rise significantly.
My advice, ignore the sales guff about heatpumps, look at the amount of gas or oil you used for heating, work out how many kWh of heat that is being used to heat your property. Take that figure, divide it by 4 (the COP) and thats the amount of electricity you will need to run the heatpump. Now do a simple comparison, have a cuppa and admit your wife was right


See my reply to BAM BAM above before you make that jump.

That’s great if you are in the gas grid.

 

[Rolfe]

Make sure you factor in the lump sum cost of £750 per year to capitalise a solar/battery installation that everyone always forgets …… Also, you never get back the original ‘investment’, something else that everyone always forgets.

Well, according to the sums that came with my quote, I should get back the original investment. But that was really what the circular reasoning part was about. I decided to take the leap, and I also decided not to break my brain doing sums about the done deal. I was paying about £120 a month for electricity (pre-EV) and £70 to £140 a month for petrol. That has been converted to what looks to be a profit figure over the year.

The way I look at it is, I needed a new car. I could have spent over £40,000 on a new GTi. Instead I spent just over £40,000 on an MG4, solar panels, home battery and car charger. Now I have free home electricity and motoring fuel and some spare money on top. Works for me.

 

[fnegroni]

I hope the government understands that the time for social experiments is over.
Every building new and old must have solar plus battery plus charging.