Circular reasoning (Rolfe's solar energy system)

[Bam Bam]

This is just horrible. After yesterday's wall to wall sunshine and 41 kwh generation, autumn is here with a vengeance. I've cancelled my plans for a road trip up north this week. My panels have generated a whopping 3.4 kwh so far today and it's nearly three in the afternoon. There is barely enough solar gain to make the conservatory habitable with a sweatshirt on, and I just turned on the central heating.

It's a good job that Scotland and England are grid-connected because we seem to have opposite weather!

26kWh on Friday
10kWh on Saturday
17kWh so far today (Sunday) and several hours of sun left on the West array.

It's baking here - 29 degrees.

Was going to wash the car at lunchtime but its been too hot.

 

[Rolfe]

Lucky you! It's nearly five, and I've generated 4.3 kWh and exported 2.2 kWh. The panels have only briefly exceeded 1 kW a couple of times. Currently doing 412 watts.

Maybe one day Octopus will schedule one of their free electricity hours on a day when we're getting virtually nothing, because they think they'll be swamped with English solar, and it might be worth me doing something about it. While the 4.45 kWh exported yesterday from one till two made any attempt to take advantage wildly counterproductive, the same hour today only exported 0.35 kWh, which would have made plugging in the car quite an attractive proposition!

 

[Rolfe]

I've decided September is the start of set-it-up-and-forget-it. Which of course won't happen immediately, but here goes.

• Export margin set on the MyEnergi products so that they won't take any solar
• Eddi set to heat 4.30-5.30 am, then if more hot water is needed, either a manual boost on the Eddi or let the central heating take care of it
• Battery set to charge from midnight to 5.30 am, then to discharge to export from 9.45 pm with a nominal end point of 11.59 pm. (It will be done long before that.) The 9.45 time is provisional and subject to adjustment.
• No high-demand appliances to be started before the battery has finished exporting, which should be by 11.45 if I get this right. It's easy enough to check before switching anything on.
• Zappi might be an exception to this if I want a full 6-hour charge, but that's not going to happen more than once a week, realistically.
• No going near Octopus's "free hour" games unless solar export during that hour seems likely to be less than 1 kwh. (Half-price car charging doesn't have any strings attached though.)

I feel I've made a ridiculous meal of all this and been pretty dense at times, but in my defence the appliances don't always work as I expected them to. Things designed to make maximum use of solar generation are having to be beaten into submission to allow maximum solar export. And a bunch of it wasn't really very intuitive, at least to me.

 

[Archev]

I've decided September is the start of set-it-up-and-forget-it. Which of course won't happen immediately, but here goes.

• Export margin set on the MyEnergi products so that they won't take any solar
• Eddi set to heat 4.30-5.30 am, then if more hot water is needed, either a manual boost on the Eddi or let the central heating take care of it
• Battery set to charge from midnight to 5.30 am, then to discharge to export from 9.45 pm with a nominal end point of 11.59 pm. (It will be done long before that.) The 9.45 time is provisional and subject to adjustment.
• No high-demand appliances to be started before the battery has finished exporting, which should be by 11.45 if I get this right. It's easy enough to check before switching anything on.
• Zappi might be an exception to this if I want a full 6-hour charge, but that's not going to happen more than once a week, realistically.
• No going near Octopus's "free hour" games unless solar export during that hour seems likely to be less than 1 kwh. (Half-price car charging doesn't have any strings attached though.)

I feel I've made a ridiculous meal of all this and been pretty dense at times, but in my defence the appliances don't always work as I expected them to. Things designed to make maximum use of solar generation are having to be beaten into submission to allow maximum solar export. And a bunch of it wasn't really very intuitive, at least to me.

It’s been an interesting process to follow. Thanks for documenting the story so far. Looking forward to the book, perchance the documentary ?

 

[Rolfe]

As I said, I was using the thread as a scratch-pad for much of the time. And a very useful process it was.

The first completely uninterrupted run at this has been a complete success. The battery was already down to 83% when the export started at 9.45. Given the very useful observation that it discharges at 10% every 15 minutes, that gave an estimate of 13% at 11.30. In fact it was at 12%, so it was pretty close. (I suppose the speed of discharge might vary with temperature though.)

The battery went right down to 4% this time, getting there about 11.38, and stopped dischaging. The grid took over powering the house, at 7p/unit. As I type the battery is sitting idle at 4%, the house is running off the grid, and I could turn on the dishwasher at 7p/unit if I wanted to. The battery will start to recharge at midnight, having been at its lowest permitted state of charge for only about 22 minutes.

This was a good test, because this evening was pretty wintry. The central heating was on, and that bumps the house load up a bit - when the freezer does one of its half-hourly sessions, it gets nearly to 500 watts. The rest of the time it's more like 200-250 watts though. I also had a cooked evening meal, although I only used the microwave and the kettle, and that took something out of the battery after dusk. So 9.45 was OK with just over five minutes to spare under these circumstances, which are about as bad as it's likely to get for a couple of weeks I think. If it takes another 20 minutes for the battery to discharge completely on a nice evening when the central heating isn't on and there's no cooking happening, then it will still work. I just have to wait these extra 20 minutes to turn on the washing machine. If I'm feeling cheapskate.

I just have to post these two graphs, of two consecutive days. First yesterday, sun splitting the sky, made £6.67 from export and used only £1 worth of mains electricity.

The odd camel's hump pattern in the early hours is the dishwasher running at the same time as the battery was recharging. Then an hour or so after the battery reached 100%, the Eddi got up to temperature just before the off-peak rate ended. The battery ran the house until sunrise, then grabbed the first of the solar excess to recharge. Almost perfect Ayres Rock formation all day, with a few clouds passing and a kettle boiling spoiling the line a little. The central heating wasn't on, I didn't cook, and then I went out. It was 8.35 before the battery dropped below 100% and it was still at 96% when the export started. The double export peak is just because I realised there was no point in reserving any of the battery as the house wasn't going to get it. It was at 11% at 11.30 and bottomed out at 5% at 11.50 (delayed because of the delay in exporting the last 5%). As it happens, a 9.30 start was perfect for this summery day.

Now look at today. Note that the scale is actually wider than the previous graph, but the solar still looks like it's hardly there.

Battery charged between midnight and 3.30 am, no dishwasher this time. Eddi heated the water from 4.30 am till it was at temperature. Oddly enough the battery didn't seem to have to work so hard to power the house from 5.30 am till sunrise, only dropping to 98% from 7.40 till 8.15. Don't quite know how that happened. Then absolutely pathetic solar, with the battery being called on several times to supplement it, then recharging when it got slightly brighter. Turned on the central heating just after 1 pm, you can see the base load increase. The battery had already started to drop below 100% by six, when the cooking dropped it even more, and as I said it was already down to 83% when the export started. You can see the central heating go off at 11.30, just before the house starts to draw on the grid.

So a 9.30 start is good on a bright day, but a 9.45 start is good on a dull day. That confirms 9.45 as the starting time, because it's better to run a bit into off-peak time before finishing the discharge, than to get there early and draw on peak-rate electricity. Also, dusk will get earlier as the month goes on. I'll leave it all as it is and see what happens over the next few days.

Now it's twenty past midnight, the battery is already at 16%, and I'm going to bed.

 

[Gadget Geek]

We lose roughly 2.5 minutes of daylight every day after the solstice if that helps.

 

[Rolfe]

We lose roughly 2.5 minutes of daylight every day after the solstice if that helps.

I think in fact there's a more rational way to work this out, which I feel a bit dim for missing. The evenings vary a lot depending on how cloudy it is, so the exact time of sunset is less important than the weather in determining where the battery charge is towards the end of the day.

Sorry, scratch-padding again.

Being logical about the export discharge, now that I've realised it seems very stable at 10% per 15 minutes, it's possible to work backwards. An hour and three quarters (9.45 to 11.30) is 70%. The absolute minimum I'd want the battery to get to at 11.30 is 5%. So the 9.45 start time should be good for any battery SoC above 75% at the beginning.

Since the day-length is getting shorter and some days can be really terrible (like today), the sensible thing is to leave it as it is. It would have been no chore to delay switching on the washing machine until 11.40 pm yesterday, and the loss due to powering the house from the battery rather than the grid for 10 minutes was a whole 0.56p. I could move the start time a little earlier, it can be configured in one-minute intervals, but I think what I want to do is see how September goes if I don't change it at all. How often (if at all) does the discharge crash the 11.59 cut-off on nice evenings, and how soon do we get to the point when the battery is low enough to discharge before 11.30 - which I think I've now established would be below 75%. I'm basically working towards the point where I can ignore it, not even look at the app, and be confident that it's working within the parameters set.

I probably need to think about intervening on days when I charge the car though. If I have a schedule set to start at 11.30 I could lose several pence in lost export if the battery is relatively high at that time. It's not much, but this is all about tuning the system to pence either way so that these turn into pounds in the longer term. However, if I'm setting a charging schedule anyway, it should be no big deal to look at the battery % around 9 pm and decide what to do.

Yesterday's sums are in. Although I only exported 2.34 kWh solar, I also exported 6.42 kWh from the battery. I imported 13.76 kWh, to recharge the battery and run the house overnight, including the water heating. The import cost 96p and the export netted £1.31, so amazingly I'm still in profit even on such a horrible day.

Today is even worse than yesterday though. Apart from a very slight lessening of the gloom around 10 am, the solar has been struggling to support the house load all day. The battery has dipped (and recharged) several times and is now on 98%. I've turned the central heating on again. Fortunately the rain has mostly held off, with a few downpours, or I'd have been forced to decamp into the living room to escape the drumming on the conservatory roof, which would have meant turning on the radiators in there, and doing the piano maintenance for when the room is going to be heated.

Generation so far today has been just 2.1 kWh, of which only 0.3 kWh was exported - during that brighter half hour in the morning I think. Nevertheless this has more or less supported the house through the daylight hours with the help of the battery, although that now seems unlikely to get back to 100% after I boiled the kettle over an hour ago. I suppose I'm in for much worse during winter when the days are much shorter and it can be even duller. My friend in Glasgow says her ten panels sometimes don't generate a thing for days at a time.

Every time I think I'm there I realise some other wrinkle, but it's just about done.

 

[Rolfe]

My advice would be not to advocate for people doing it, that's what seems to upset them.

My position is that I can't stop my system from running the house and recharging the battery all day from the solar. So unless it's very dull I come into the evening with the battery at 100%, and it only drops a few % until it starts charging on the grid again! This is not my choice, I would rather run the house on the battery during the day and export all the solar, but that's how it works. My only recourse is to export this excess power once the solar has gone.

You are allowed to export your own solar which was stored in your battery, according to the barrack-room lawyer, and to a large extent that's what I'm doing. If you dig down into daily usage there's likely to be a bit extra left over from the previous night's grid charge, but hey, what am I actually supposed to do? Take half an hour every evening to calculate how much solar the house and battery used during the day and stop the export once that figure has been reached? They're getting the power for 15p at a time when they're selling it at peak-rate price, so I'm not sure why they'd be upset about it. Isn't everybody doing this?

 

[Rolfe]

The most miserably dull day since I got the system is drawing to a close. The solar barely supported the house. Export was minimal. And yet, as the evening arrived, the battery didn't fall much. It had been at 98% since I boiled the kettle about a quarter to three, and it was still there at five to seven. (I think the cloud cover was diffusing the light across the sky so that the panels, normally in shade at that time, were getting more than you'd expect.) Then I changed all that by cooking dinner, in the oven this time. Bang went 9% of the battery in half an hour. (I also managed to draw a small amount from the grid by being careless and switching on the kettle while the oven was drawing power, and that was more than the battery could handle. I worked out that it cost me 2.5p. Coincidentally, this was the price of 1 kWh of electricity when I was at school. An old sixpence.)

The battery was thus at 82% when the 9.45 export started, much the same as it was last night, and it was on 8% at 11.30. That's a bit faster than 10% every 15 minutes, but it was fine. Export stopped at 5% this time, at 11.33, but then flipped down to 4%. Fine, but there's a lot of variation in how much I use when cooking. If I'd had the hob on as well, I could have missed the window. I don't really want to have to skimp how much electricity I use when cooking, or check the damn app every evening to assess when I ought to start the export, but maybe. If only it was possible to export everything and simply run the house on the battery this wouldn't matter, the battery would be low enough most nights for it not to be worth exporting the rest. But no, it doesn't work like that.

And it's a wrap for the day the sun forgot.

I'm going to bed.

 

[Gadget Geek]

Have you considered switching off the solar panels if you want to export and use your battery on a sunny day?

 

[Rolfe]

Have you considered switching off the solar panels if you want to export and use your battery on a sunny day?

That's a long way above my pay grade! I have no idea what you're talking about, never mind how to do it. If it even can be done.

 

[Gadget Geek]

It will be a simple isolator switch, probably situated near the inverter or the generation meter.

 

[Rolfe]

What does it do, though? When you said turn the panels off, I thought, but what use is that, I want the export credit. Then you seemed to imply that I could still export the solar. I'm confused.

Here's the thing. I have a 9.5 kwh home battery. The house is perfectly capable of running on that from 5.30 am till 11.30 pm, especially when it's possible to shift things like the washing machine and the dishwasher to the off-peak period. It would be more advantageous to me to do that, at 7p/unit, than to use the solar that I could be getting 15p/unit for. But I don't know how to do it.

The battery is sitting at 100% most of the time. It charges 11.30 till 5.30. The solar kicks in soon after that, even at this time of year, and little if any is used between 5.30 and dawn. Then it stays at 100% all day. Even if some charge is used by the house, the battery immediately recharges on the solar. Then there's a bit more discharge after dusk until 11.30, but again it's not much.

So at the end of the day I'm left with a battery that has been almost permanently full, having used up maybe 3-5 kwh of solar (at a cost of 15p) rather than using the battery at 7p. Yes, it's pence, but it's every day, and pence every day add up to pounds over the year.

The answer to this is to export what's in the battery at the end of the day before it's recharged. That makes up the loss, but it's a pain in the neck. I'm currently breaking my brain over when to start the export so that it finishes at or just after 11.30, but there's no easy formula. It depends on the time dusk falls, which depends on the weather, and on how much cooking I decide to do in the evening. It's not the set-up-and-forget system I was hoping for, or not if I don't want to lose up to 50p a day for a lot of the year.

If I could just stop the house and the battery grabbing the solar whenever it's there, it would be so easy. Day to day running would be purely off-peak electricity into battery, run house on that till 11.30, battery recharges, rinse and repeat. The solar panels would become purely a cash cow, with the tariffs as they are at present. If you can tell me a way to do that, I would be forever in your debt.

I'm sorry John, I don't mean to hijack your thread, but this is something I'm struggling with, and if I sniff a possible solution I get a bit excited.

 

[Gadget Geek]

Ah, I think I see what you're after now. You would like a three-way switch so solar goes purely to export, house runs off battery, and nothing from grid during the daylight hours, then export rest of the battery at the end of the day.

I'm presuming you are trying to keep the battery at 100% to force the solar into export?

 

[Gadget Geek]

Rolfe as moderator: I moved some posts from the "Tomato Energy" thread to this one, to keep it all together. Thanks to @Gadget Geek for moving the discission to this thread.

 

[Gadget Geek]

What app do you use to control the battery?

My Powervault has options to alter how the battery behaves;

"Only Discharge" would be the option I would use for the battery to run the house, which would cause any solar to go to export.

Does your app have anything like this?

 

[Rolfe]

The settings are on the inverter section of the app.

• "Eco" reads "This setting will use your battery or solar to power your home's electric demand." As far as I can see this would simply cause the home to run on the grid if I turned it off, and what use is that?
• "Timed Charge" is obvious, I have that set for midnight to 5.30 am, so that's fine.
• "Timed Discharge" says "This module will trigger your battery to cover your home's electric demand only at the times you specify." I can't quite figure out what that's for. However, just thinking about this, is it possible I'm misunderstanding the word "only"? Dunno.
• "Timed Export" is also obvious, and that's the one I'm breaking my brain over.

I'm now wondering if I've misunderstood what "timed discharge" means. Maybe I need to try something.

ETA. Nope, scrub that. I enabled "timed discharge" and the house is still taking solar. Hasn't changed anything. So I don't know what that's really supposed to do, other than prevent the house from using the battery at certain times. So I seem to have a way to get the house to run only on the mains, and a way to get it to run only on solar or mains, but no way to get it to run only on battery or mains.

ETA again. I turned "eco" off, and again nothing changed. The house is still running from the solar, not the grid. So what on earth is that for?

I've got settings that seem to do nothing, or at least to do things I can't see any reason to want to do, but nothing to do what I want to do. I must be missing something here, surely?

 

[Gadget Geek]

• "Timed Discharge" says "This module will trigger your battery to cover your home's electric demand only at the times you specify." I can't quite figure out what that's for. However, just thinking about this, is it possible I'm misunderstanding the word "only"? Dunno.

If you set the time on this setting for when you want your solar to go to export, it will exclusively use your battery to run the house . . . no solar and no grid.

 

[Rolfe]

If you set the time on this setting for when you want your solar to go to export, it will exclusively use your battery to run the house . . . no solar and no grid.

I just tried that. It didn't work.

I originally read that as, this setting will only use the battery to cover the house demand during the times specified. Which I couldn't see the point of. However, as I was typing it out, I realised it could perhaps be taken to mean, this setting will cover the house demand exclusively from the battery during the times you specify. Which would of course be paydirt. But when I turned it on, the house went right on running on solar. I don't get it.

 

[Rolfe]

Ah, I think I see what you're after now. You would like a three-way switch so solar goes purely to export, house runs off battery, and nothing from grid during the daylight hours, then export rest of the battery at the end of the day.

I'm presuming you are trying to keep the battery at 100% to force the solar into export?

Not quite. I'm having trouble explaining myself, although it seems so obvious to me.

As the system is set up, the solar goes first to the house. Then when the house has all it wants, the solar goes to the battery. I can't stop that happening. If the house is using more than the available solar can supply then the battery will come in to cover the rest - but as soon as the demand is over, or the solar has increased, the battery will use the solar to recharge. The result is that from dawn to dusk everything I use in the house comes from the solar.

I'm using solar to power the house, when I could be exporting it for 15p, and I have a battery full of 7p electricity that I can't use! The battery is spending almost its entire life at 100%. It charges 11.30 to 5.30, then drops only a little (if anything) between 5.30 and dawn, then stays at 100% till dusk. It drops a bit more between dusk and 11.30, but if I didn't export it at the end of the day again it would only be a small dip before it starts to recharge at 11.30.

I want to stop the house and the battery from taking any solar. I want them to behave as if the panels didn't exist. I want to fill the battery (and charge the car and use heavy-duty appliances) from 11.30 till 5.30, then run the house on that 9.5 kWh from 5.30 till 11.30, rinse and repeat. In this way the panels become purely a cash cow. Given the price I can export for, and the price of off-peak electricity, I don't need the panels to power anything, I need them to make money.

Near the start of this thread someone said, why not just get the battery and the car charger and the off-peak rate, you don't need the panels. I answered, have you seen my roof? It would be a crime. And it would be, and the panels are making a fair bit of money. But he was right about the house not needing the solar. It's more profitable to export it. If only.

When I didn't have an export tariff it made perfect sense. Run the house and charge the battery during the day from the solar, then use that solar, now in the battery, to power the house during the night. Fine. It worked. Here's an example.

During this time I wasn't using grid electricity at all. I ran the house on the solar, using the battery to deal with night-time, and the grid got the rest.

Now I have the export tariff however, I want to reverse that. I want to see the battery discharge during the day, using up the charge it got during the night, so that all the solar goes to export, not just the "excess". I should have a period of charging from 11.30 to say 3 am, a period when the battery is at 100% till 5.30 am, then a gradual discharge as the house uses the battery right through till 11.30, when it can charge again at 7p.

Instead I'm seeing the battery almost permanently at 100%, because it still charges in the daytime as well as overnight, with only two short periods of discharge just before dawn and just after dusk. (Or I would, if I didn't force a discharge in the late evening.)

This is so nuts I still feel I must be missing something, but I've been through every setting I can see, and can't find anything. And I read someone else saying he didn't know how to stop his battery charging on the solar.

I think I need coffee.