Circular reasoning (Rolfe's solar energy system)

Well, here's the plan. The daytime settings worked perfectly. From 7 am to 5.30 pm the battery was set to export to 80% (that's arbitrary, it just has to be higher than the amount of charge I expect to pick up in that time. That directed all the solar (above the house load) to export, until clipping started, when the excess was directed to the house battery. Today that brought the battery up to 56%, which is plenty and to spare. When clipping ended the battery stopped charging and export of the solar continued, and the battery kept the charge it had gained. Didn't have to change anything, it all worked perfectly.

At 5.30 pm (when the solar can no longer meet the house load) the export setting ends and the battery then powers the house through till 11.30 pm. Anything left over can be exported for profit just before 11.30. So far so good.

However, what happened during the night, during the time when the battery would normally be charging, was a load of random fluctuations that weren't terribly helpful, and I still ended up importing a little peak-rate power. So now I think what I need to do is export down to say 12% or maybe 15% in the late evening, rather than exporting everything. Then if I keep that export limit throughout the night, that should preserve that amount of charge to run the house 5.30 am till sunrise.

If this works then it's close to being set and forget, any time there's a nice day. Nice days being so infrequent sometimes that I'd better write it all down so I don't forget.

Everest said:

I would guess that is down to your inverter's settings. It is common for an inverter to have a minimum SOC% specified below which the battery power will only be used for powering the inverter and not for powering the house. IME those default values are typically in the 10% to 20% range.

Hence, you may need to have more SOC in the battery before it can be used to power the house.

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It usually powers the house down to 5%, so it still doesn't quite compute. Also, there are times when the battery % seems to increase with no clue where that came from, so it's a mystery. Anyway, I'll set it only to export to 15% this evening and hold that until 5.30 am, when there should be plenty to power the house for an hour and a half. It will be interesting to see how much it takes. Then the switch to exporting to 80% at 7 am will let it charge on the excess above the clipping, rinse and repeat.

Rolfe said:

I could probably do it with the Zappi if I fiddled with the settings, but the granny charger is easier for that job. The Eddi is another possibility but to be honest I heat that for 7p/unit just before the end of the off-peak tariff every night and it usually does me for the day,

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"
16.2 Device Limit
Sets the maximum current that the zappi will draw (including when boosting and FAST mode). This is useful if the supply current is limited, for instance, if zappi is connected on a 16A circuit instead of a 32A."

OK thanks.

Well, this worked rather well.

I was away from home, and left the system set up, knowing that a sunny day was forecast. I had exported the battery before 11.30 the evening before. Then the battery was set to charge from midnight and export (down to 15%) from 3.15, finishing at 5.30 am. That gave it enough time to do that cycle before the off-peak period ended. Partly that cycle is to gouge a few more pennies from the system, but it also has the side-effect of meaning that the battery is low for a shorter period than it might otherwise be, in case of a power cut. (I don't have my EPS yet but I'm getting one.)

The battery actually ended up at 14% at 5.20 am, after which the house drew on the grid for ten minutes until the end of the off-peak tariff. It then drew on the battery, because its export period had ended. However it had only dropped by 1% by the time the solar started to take over the house load. I had set the battery to start exporting down to 90% from 7 am (about when the solar takes over, I note there was just a tiny hint of the battery starting to charge from the solar before that happened.) That stops the battery taking any solar below the export limit, but it then takes the solar after that, eliminating the clipping.

It did that from 9.50, and between then and 3.10 pm, when the clipping stopped, it charged up from 13% to 75%. Not bad. That would appear to be 4.5 kWh (maybe, I'm getting some very strange readings on the GivEnergy app). The solar went on exporting and running the house, and was still running the house at 7 pm when I returned home. I cancelled the battery export at that point, because obviously when I got in and started switching things on the battery would be needed to cover the increased house load. (It was close to being needed anyway.)

And that's that. I'll probably export whatever is left of that 4.5 kWh at the end of the evening, then start again. However, as the car will be on charge, I'll cancel the home battery cycling, as the export from that would just get gobbled up by the car. I also need to turn the Eddi back on to get hot water in the morning. I had it off while I was away, and have just let the central heating heat it a bit this evening.



Solar generation appears to have been 49.3 kWh for the day, but as the GivEnergy app and web site is giving some very peculiar readings I'm not sure how much was directly exported.

Rolfe said:

Well, this worked rather well.

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Glad to hear it.

Rolfe said:

I'm getting some very strange readings on the GivEnergy app

Solar generation appears to have been 49.3 kwh for the day, but as the GivEnergy app and web site is giving some very peculiar readings I'm not sure how much was directly exported.

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I also had bizarre readings today that did not make any sense.

It said we had put 20kWh into the battery (but it is a 9.5), and that we had somehow bought over £5 of electricity without having the car plugged in and also that it was pretty much our best ever day for solar even though it is only April.

Mine thinks that in the half-hour period from 1 pm, I exported 23.19 kWh solar to the grid. It also thinks that in the half-hour from 7.30 am I put 9.49 kWh solar into the battery. (The solar is only just covering the house load at 7 am.) It thinks the total generation for the day was 63.58 kWh, which is impossible. Nevertheless the daily tally along the top reports that total was 49.3 kWh, which is plausible. Hopefully they'll sort it out at some point.