Hi I want to know if there is a way to simply toggle the use of solar? Reason being I get a cheap rate at night so can fully charge my batteries for the day and I get a really good rate for solar rather than it charging the batteries maybe something I can do as and when ie use when battery is running low or just switch when my batteries are at a certain percent? I’m using ESS with set charge at night
Hi @Daza
I think I know why you're asking this question and here is my thinking as to why it's not really as beneficial as you might think compared to just letting the batteries come up to 100% before naturally exporting any remaining excess energy.
When you collect energy via the MPPTs, the conversion efficiency is very high, "up to 99%" in fact although that will vary a bit depending on power levels. This is a good few percent higher than the AC-DC or DC-AC (charging and inverting) efficiency you'll see through the Multiplus-II which is going to be more like 88-95% depending on power levels. You need to factor in these losses rather than just looking at the 7.5p/kWh import versus 15p/kWh export rates on Intelligent Octopus Go.
First some assumptions:
So, let's illustrate this by considering firstly what happens if you just let the batteries charge first followed by export. In this case the 1.5kWh that discharged between cheap rate end and solar PV start is recovered first and then the balance of the solar production is exported:
Now, let's consider what happens if you somehow export solar first and then charge the batteries up in the following cheap rate period. In this scenario you'll need to charge that extra 1.5kWh to get back to your 100% point by 05:30hrs:
So the net difference is £0.73 - £0.64125 = 8.875p per day not 11.25p per day as it would be if you didn't take account of efficiency losses. This scales linearly with total solar forecast so you can think of it as 8.875/6 or approx 1.48p/kWh of solar.
And as a qualitative point, I live in the countryside with an overhead 11kV power line feeding my house so power cuts happen 4, 5, 6 times a year so having that extra 11% in the battery during the day is worth giving up the small amount of potential additional income.
Another qualitative and potentially quantitative point - having that extra 11% in the battery might prevent you running short of power before the cheap rate kicks in again and 1.5kWh at £0.30 is going to cost you 45p which wouldn't have to happen very often to blow your potential gains out of the water. It also might delay the point at which you need another battery module and therefore save on capital expenditure.
Just some things to consider. :)
@Craig Chamberlain Cheers mate the reasoning that I have an AC inverter so no conversion needed I just wanted to chuck it all down the line, but now I seem to have issues now I’ve played about with schedule charge, the inverter doesn’t want to seem to 0 my smartmeter with peirods of 112watts granted for 4 or so seconds but no addition load on the house and it’s set to -30 grid point at the time and that’s even with the CT clamp, it’s driving me mad now lol which has just seem to sort.
Yeah I would rather the solar go to the grid at 15p, your house seems like the at rest load is low, mine is about 400watts to 600watts, mainlyfrom network equipment server and smart speakers so can’t turn them off, my house is in the low high 80s around 8am after toaster kettle and water pump for the shower which Thames water would give decent prea so I didn’t need a mains pump, hence wanting the extra battery so I would’t have to rely on any solar hence wanting to export the lot. But I suppose if I get an extra battery it wouldn’t take long to charge as I’ll be in the high 90’s I guess, still that option would be bliss though.
Yeah, I forgot your solar was AC coupled so yes it makes sense to just export it if you don't need the extra battery energy. In my case I'm better off storing it, even if I do then later decide to dump some battery power back into the grid, perhaps during the 4pm - 6pm peak period to either score points with Octopus or just generally help the grid.
I don't have any specific advice to give you regarding your 112w issue but general advice is to walk away for a while and come back to it fresh minded. Sometimes then you spot immediately what was obscured before. Good luck!
My solar forecast turned out to be pessimistic today so I've still got 94% battery SOC at 5pm and have pushed 6.3kWh to the grid earning almost £1. Not bad for late November in central Scotland! :)
@Craig Chamberlain dam you must have a big array, I burnt all my solar keeping the batteries charged which for me doesn’t make sense as they will see me through till 1130 anyway and wasting 15p to charge them when I do it for 7.5p at night plus the wasted conversion from the AC inverter panels DC inverter AC Batteries DC, would rather waste that conversation at night but then again it’s only AC to DC.
Yeah this phantom draw every now again is doing me in I just want that’s smart meter to read 0Watts it’s annoying, I’ve rebooted the Cerbo GX thinking it’s gotten confused but maybe it’s the CT that is faulty hard to know what the behaviour is without a CT and the loop on the bridge wire cut that does the internal sensing when there is no CT clamp.
My array is 6.8kWp (16 x 425w Trina Vertex S panels) so is a decent size but it was a lovely sunny day so 11.9kWh is well above average. I've attached my 30 day solar data below which shows 208kWh total so just under 7kWh per day. Compare that with August which is when I got my export tariff and started exporting properly and I exported 764kWh. That would have been higher in July and a good bit higher again in June but I only got my PV operational on 19th June and didn't really export much until I got my tariff.
I agree you're doing the right thing by exporting as long as you don't need the extra resiliency of having more charge in the batteries, and assuming you aren't running short of charge towards the end of the day.
As for the phantom draw thing, my advice would be to forget about the CT clamp and just use proper energy metering. I started with a CT clamp because I wanted to have the fastest ramp up and down to avoid inadvertent export until such time as I had my DNO approval and export tariff. I didn't want to be exporting energy for free due to slow ramp downs. Since I switched to using an ET112 as my grid meter, yes the ramp up/down is slower but overall stability and metering accuracy is much better. Actually, reading your last sentence again I'm not clear whether you are using the external CT or the internal one.
@Craig Chamberlain much bigger than my 3.2kW array hence takes longer to charge the batteries that’s why I just wanted it to go to the grid, batteries will be down to 30% - 25% with no sun come 2300 to 2330 that’s where the other 5kW will come in handy, I’m using an actual external CT but it’s still programmed as an internal as it connects to the Victron, where as the ET’s are to be listed as externals. May go back to the ET112 I ditched it to get a faster response but if it’s allowing that draw then may as well get an accurate picture of what I’m drawing, got someone from technical giving me a call Monday so hopefully they can shed light on it once I give the access.
Looking at the octopus app it appears to be the same as before I’ve just had my old smart meter replaced and this has a much better display so I guess I’m noticing it a lot more but would love to see the smart meter seeing 0 a lot more than the spike that I’m seeing but every half an hour is either 0.01kWh or 0.02kWh.
I’m down south and only about 9 degrees off of true south I’ll look at adding another array this time on the garage so we could get some extra power coming in. Possible I may swap out the 5000 for 10000 to have both arrays on backup Although not had any power cuts really only an issue with the grids transformer. Only 2 in 9 years that I was at home to witness.
