Circular reasoning (Rolfe's solar energy system)

[Rolfe]

I think I'm over-complicating the issue. I'm looking at battery settings, but this isn't primarily a battery setting issue, it's an issue with the house. The fundamental problem is that the system prioritises the house to use solar rather than the battery. The house uses solar first, failing that battery, failing that grid. Even if the battery wasn't allowed to charge from the solar, the house would still use the solar first, and it's the house usage that's driving all this.

The house prioritising the solar is the cause of the battery remaining at 100% all day - it simply isn't being asked for the electricity it has stored. If it was being asked to supply the house, it wouldn't matter that it had an underlying instruction to charge from the solar if it could - it would never be able to do that, because the house demand is incessant.

The house should be instructed to take electricity in ascending order of its effective cost. The battery is always going to be the cheapest, because it's always charged at 7p/unit. The solar is next, with a notional cost of 15p. Grid is bottom, because at the time when solar is available, grid costs 23p. The house should only use grid when neither solar nor battery is available, which in practice will be 11.30 pm to 5.30 am, when the battery is set to charge.

So I think I have been asking the wrong question, or rather I have been asking two questions where one is enough. Because all the settings on the inverter relate to the battery, it's natural to try to find a battery setting that will fix this. However, it's not a matter of telling the battery not to charge from the solar. It's a mattery of telling the house to run from the battery in preference to the solar.

Yes, set up like this it does end up looking like two separate systems. One system that uses cheap off-peak electricity to power the house all day by storing it in the battery, and another system that generates electricity from solar and exports it as a cash-generating exercise. They could even be separate, except that if the day comes when the export tariff is below the off-peak tariff, you'd want to switch the priorities back agan. Or you might want to let the house use some solar if you had a day when you anticipated that the demand would be greater than the capacity of the battery. But as a default, at the moment, telling the house to ignore the solar really ought to be a default position that can be configured.

Unless anyone can tell me why this isn't possible?

 

[Bam Bam]

I don't really see where we disagree.

I think of it like buckets.

You've got a bucket of electricity in your battery filled up overnight at 7pm. Say its 9kWh, but it can be topped up by solar (which might be more efficient because a) its DC so there no conversion losses and b) the battery can take solar above 4kWh if there is space).

Then you've got a bucket of solar generation, you don't know how much this is going to be per day, but you know by about 7pm.

These are your incoming buckets. You could supplement with grid buckets as well but this is to be avoided.

Then you've got your bucket of usage. This might vary a little depending what you are doing, but you are filling the EV (and maybe doing other big ticket things) overnight so it doesn't impact on these buckets.

You want to:

• Fill the overnight bucket (battery) as much as possible on cheap energy.
• Export as much as possible during the day, from solar and surplus battery that isn't required to run the house
• Get nothing (or as good as nothing) from the grid outside the off-peak.

All of these aims make perfect sense to me.

For the daily export you can't exceed the buckets that you get from the overnight battery + solar for the day - the amount you need for the house.

On a sunny day your export might be 20kWh, on a sunless day it might be 8kWh.

As far as I can see it doesn't matter what time of day you discharge the battery. Do some battery export in the morning, discharge the rest in the evening.

If you tell the battery to discharge to 50% during the day (say, 8am to 4pm), and then discharge the rest at the end of the day, why wouldn't that work for you? It would mean setting the discharge time settings twice per day, which would be a pain.

 

[Rolfe]

You have answered your own question in the last sentence. I want this to be a set-up-and-forget-it system. It's not that if I have to worry even once a day whether the export discharge has started at the right time, which I can't see how to avoid at the moment.

It's especially annoying if I'm cooking. This evening I actually forgot it and it doesn't matter a lot, because it's the night the chippie van comes to the village and I went out for a fish supper. So all I did was boil the kettle for a cup of tea. I think the late solar still covered that, because the battery didn't fall below 100% till 7.30 pm. I had thought about starting the export early to save the hassle of it over-running, because I want to run the washing machine tonight. But I forgot. It set off at 9.45 as scheduled. It probably won't be done until 11.50-11.55. So I won't be starting the washing till midnight, and I have to get up early tomorrow.

On the other hand, a couple of days ago when I used the oven, the export finished only two minutes into the off-peak period. That's OK, but it was close, and if I had used the hob as well I would have been drawing peak-rate power before 11.30. This is where I am at present, and I can't see how your idea would make it any better.

Setting multiple windows of discharge is only going to cause more hassle, not less. And by the way, the battery discharges from 100% to empty in about 2 hours 15 minutes. There is no way to set it "to discharge to 50% during the day (say, 8am to 4pm)". If I set it to start discharging at 8 am it will be empty about 10.15 am. Then what? The house goes on taking solar I suppose, but then I have to set it to charge again, and bye-bye export while that's happening, and if it's not a good day for solar (like Monday wasn't) then it won't charge and there'll be nothing for the house to run on after dark. What does that achieve? I can't see that it achieves anything.

I want to use the power I put in the battery overnight to power the house directly during the day. For 7p/unit. If I happened to have anything left at the end of the day I could always export it if I wanted to, but it wouldn't matter if I didn't, it wouldn't be that much. It would be truly set up and forget. And if would minimise losses. The power would go where it was intended to go in the first instance, solar to grid, grid to battery, battery to house. And it would let me run the washing machine during the day on 7p electricity rather than having to wait till midnight. And if would be an easier life for the battery.

But I'm just venting now. I don't think there's a better work-round than what I'm doing. I'll just have to hope it doesn't go wrong and cost me money too often, because I'm living for the day when I can say, I have this great system that works in the background and I never need to look at it.

 

[Bam Bam]

Setting multiple windows of discharge is only going to cause more hassle, not less. And by the way, the battery discharges from 100% to empty in about 2 hours 15 minutes. There is no way to set it "to discharge to 50% during the day (say, 8am to 4pm)". If I set it to start discharging at 8 am it will be empty about 10.15 am. Then what? The house goes on taking solar I suppose, but then I have to set it to charge again, and bye-bye export while that's happening, and if it's not a good day for solar (like Monday wasn't) then it won't charge and there'll be nothing for the house to run on after dark. What does that achieve? I can't see that it achieves anything.

What I had in mind is that you could set it to discharge to 50% rather than empty (or 6% here for mine). That might take an hour on full discharge.

But after this point my thought was that it would still aim to be discharging to 50% when it gets further solar during the day.

It isn't quite set and forget if you do it like this, as you might still like to export whatever is left at the end of the evening.

I don't see how you can achieve that without some kind of clever automation that takes account of how much solar you've generated and how much your house has used each day.

 

[Rolfe]

What further solar? I can't be sure there will be further solar, especially here, as winter sets in. My requirement is set-up-and-forget, and you're not convincing me that even more fiddlling is going to achieve that.

Bam Bam, which tariff are you on?

 

[Bam Bam]

What further solar? I can't be sure there will be further solar, especially here, as winter sets in. My requirement is set-up-and-forget, and you're not convincing me that even more fiddlling is going to achieve that.

Bam Bam, which tariff are you on?

We're on Intelligent Flux, which works for the Summer (20p export (up to 24p peak)!), but not much good in the winter.

Plus Octopus just control the battery so you don't have to think about anything.

Would love to switch to intelligent Go, but will have to do with "dumb" Go over the winter until Octopus integrate with either MG (seemingly unlikely) or Givenergy EVSE (likely, but no indication by when).

Wondering exactly when we should switch over. Maybe later this month when the solar starts to drop off.

Would be great if there was a Flux/Go combo but I think Octopus haven't got their heads around that yet.

 

[Rolfe]

I think the Flux and the Agile tariffs are a lot more hands on, and from what I've picked up, it's not such a big deal that the house takes the solar.

Right now I want to get that washing on and go to bed, but because the battery was at such a high SoC when the export started I'm going to have to wait till nearly midnight or lose even more export. (On top of the house load for 25 minutes, which isn't a lot to be fair.)

 

[Bam Bam]

I think the Flux and the Agile tariffs are a lot more hands on, and from what I've picked up, it's not such a big deal that the house takes the solar.

No - its the same rate in and out.

I'm still not sure what the 'house takes the solar' comment refers to.

Surely the house should be fed from the inverter (whether battery or solar) rather than the grid outside the cheap period. What difference does it make how much the sun is shining at the time when the house "takes" some?

 

[Bam Bam]

I think the Flux and the Agile tariffs are a lot more hands on, and from what I've picked up, it's not such a big deal that the house takes the solar.

Agile is very hands on, unless you get one of those third party apps to control it all for you.

Someone built a clever service apparently, but I haven't investigated.

'Dumb' flux isn't that hands on - you just set it to charge overnight and discharge during the afternoon peak.

Intelligent flux isn't hands on at all - I guess it is tentacle on.

 

[Rolfe]

No - its the same rate in and out.

I'm still not sure what the 'house takes the solar' comment refers to.

Surely the house should be fed from the inverter (whether battery or solar) rather than the grid outside the cheap period. What difference does it make how much the sun is shining at the time when the house "takes" some?

If I could figure out what that last paragraph means, I might understand it better. Are you saying the inverter can't simultaneously take both solar and battery at the same time? That might explain my issue. Is it the case that while the inverter is taking solar and passing it on to the grid, it can't simultaneously take anything from the battery to power the house, but has to power the house from the solar without any way to alter that?

 

[decrep]

I was wondering what controls the house?
Can whatever controls the house decide where it's power comes from?

If it's the inverter, then probably it takes battery once there is no solar. I think that would be the normal setup.

This is a long shot, can the leads be swapped, so the inverter thinks the battery is solar and solar is battery? Need to think about this, may just make things worse!

EDIT, just had a 10min think about this, and I don't think it's possible, the inverter would be treating the battery and solar very differently. It would need a major hack of the inverter to get what you want.

 

[decrep]

I think this is why, I end up making my own stuff, what I want to do, isn't possible with off the shelf stuff.
You may just have to live with what you've got.

 

[Coulomb]

Are you saying the inverter can't simultaneously take both solar and battery at the same time?

All inverters can blend solar and battery power, that is, they can use them together in any proportion. Most but not all can blend AC-in with the other power sources, but usually only when in a particular mode. For example with the Axpert inverters that I'm familiar with, they can only blend AC-in with other sources when using SUB (Solar before Utility before Battery) output source priority.

 

[Coulomb]

This is a long shot, can the leads be swapped, so the inverter thinks the battery is solar and solar is battery? Need to think about this, may just make things worse!

EDIT, just had a 10min think about this, and I don't think it's possible,

You are right in that you can't swap battery and solar ports. A battery is typically 50V, while solar is typically 350V. A battery can be charged, and while it's possible to push power into an array of solar panels, this doesn't do any good.

So the solar and battery ports of any inverter are very specialised, and must never be swapped.

 

[Rolfe]

It may be that that what I want to do is actually impossible due to the way the inverter works. It does necessitate an annoying work-round though.

 

[johnb80]

All inverters can blend solar and battery power, that is, they can use them together in any proportion. Most but not all can blend AC-in with the other power sources, but usually only when in a particular mode. For example with the Axpert inverters that I'm familiar with, they can only blend AC-in with other sources when using SUB (Solar before Utility before Battery) output source priority.

Surely ALL inverters do this as part of grid tied operation?

 

[Bam Bam]

If I could figure out what that last paragraph means, I might understand it better. Are you saying the inverter can't simultaneously take both solar and battery at the same time? That might explain my issue. Is it the case that while the inverter is taking solar and passing it on to the grid, it can't simultaneously take anything from the battery to power the house, but has to power the house from the solar without any way to alter that?

Sorry that paragraph obviously generated a lot of head-scratching.

Source for load: Either inverter or grid.
I just meant that the power from either solar or battery is going via the inverter. If you don't want the inverter to cover your house load then the grid will have to do it.

Your house demand can be met by the inverter (using solar and/or battery) or it can be met by the grid (or a mixture if the inverter can't cover it all).

I tend to just ignore the house when thinking about it because it just "is what it is." We are going to use electricity for things and the inverter will decide whether it will cover it (from solar or battery) or leaves it to the grid to cover the load.

Same with the solar - I can't control the sun.

So I'm easy breezy.

But then it is all very easy with intelligent flux. You pay the same rate import and export so it makes no difference what time your battery runs out or anything like that.

But IF is only a good tariff in the Sunny half of the year and not great with an EV because there is no cheap overnight. So I will switch at some point this month.

Optimising to sell all cheap electricity stored in the battery
One can try to squeeze out every drop from the battery each day but the amount left over in the battery will vary depending on load and solar.

It therefore involves a lot of manual tinkering.

Unless you have an automation service that does it for you, like this one:

 

[Rolfe]

[Cross-posting with @Bam Bam, I'll have to take time to read, watch and consider that last post.]

I did some basic early calculations on cost.

In the 12 weeks before I got the export tariff I spent £5.20 on electricity. (This was mostly down to a couple of episodes where the car took some charge overnight due to settings on the Octopus and the MyEnergi apps I didn't realise were there.)

In the 4 full weeks since I got the tariff (including the first week, which only included three days of export), I have spent £26.20 on electricity. But I have received £77.15 in export credits. So I'm ahead of the game by £50.95 over just the first four weeks, which weren't great for solar. If I add in the £5.20 I'm still ahead by £45.45. It will be interesting to see how long I stay ahead of the game as the days shorten and the sunshine isn't so plentiful.

I seem to be drawing a regular 13.4 kWh or thereabouts from the grid to recharge the battery and run the house and the Eddi during the off-peak period, which costs around £1 a day. (This has gone down by about 0.5 kWh since I turned off that hifi system, which would suggest the thing was taking about 2 kWh in 24 hours.) This is on days when I don't run any heavy-duty appliances after 11.30.

I generally send back about 7 kWh of that in the evening, depending on how much I use between dusk and 11.30-11.50, the time the battery has discharged by. So I have about £1 in export income before I start with the solar. At the moment then it looks as if the arbitrage is washing its face before the solar even comes into it. (Still excluding washing machine, dishwasher and car charging, also using a lot of power for cooking in the evening eats into it a bit.)

The solar can be anything from bugger-all (the worst so far was 0.31 kWh on Monday) to a lot. Saturday was 39.75 kWh.

Battery export will have to support the house for longer and longer in the evenings of course, and there may even be days when the morning drain on the battery can't be recouped from the solar if it's extremely dull, but to be in this position when we're coming up to the equinox is quite encouraging.

 

[Bam Bam]

Battery export will have to support the house for longer and longer in the evenings of course, and there may even be days when the morning drain on the battery can't be recouped from the solar if it's extremely dull, but to be in this position when we're coming up to the equinox is quite encouraging.

Good to know that, bigger picture, its doing its job!

 

[Coulomb]

Surely ALL inverters do this as part of grid tied operation?

Not all inverters are grid tied. I research mainly the group of inverters that Voltronic Power call "off-grid" inverters. These can and usually are connected to the grid, and are not supposed to export power to the grid, but occasionally do briefly when large loads come off.

The inverters powering my house at present are totally not capable of exporting at all. They do synchronise to the grid, and connect their DC-AC converter to the grid, but only for drawing power from the grid, in order to charge the battery from utility/generator. The DC→AC converter in this case operates in reverse, becoming a DC→AC unity power factor rectifier. At such times, the loads are powered entirely from AC-in, and solar power is used only to charge the battery.