Solis inverter + battery + solar: Optimal settings to export to grid at specific time without running out?

[mikecambs]

I'm on Octopus flux export + import. Cheap import 2-5am and best export price 4pm-7pm.
Have 5kw solar, 7kwh batteries and inverter.
I've set up batteries to charge from 2am-5am.

What i'm trying to do is export all my 'spare' battery charge between 4-7pm for export payments. One easy way is to set up the battery to dump its full charge between 4-7pm. Since the import price before 4pm and after 7pm is slightly lower than the export price between 4-7pm, I think this is financially roughly equivalent (but slightly better) compared to saving the charge and using it in the evening afer 7pm.

However, if the battery is already low on charge at 4pm, this 'dump everything at 4pm' method will result in the battery running out (.e.g at 6pm) and me using the grid at peak rate (bad).

So one solution is for me to set up the battery to charge up from the grid between say 2-4pm. However, depending on the charge % I need that timer to ideally start earlier or later (which I'm unsure is possible). To guarantee a full battery at the start of 4pm, i'll need to start charging it at around 2pm which will often be pointless if the battery is already at 99% and will be topped up by solar anyway (although in this case, it'll cost me very little since its already nearly full), but it seems suboptimal?

Is there a trick I'm missing here? Is my maths correct? It seems like ideally I could set the battery to only charge up to 90% between 2-4pm and then let solar do the rest if the sun comes out but I'm not sure if this is possible. Or I don't bother charging from the grid between 2-4 and only dump excess power into the grid between 4-7 but again I'm unsure if this is possible either?

 

[SeaGal]

Since the import price before 4pm and after 7pm is slightly lower than the export price between 4-7pm, I think this is financially roughly equivalent (but slightly better) compared to saving the charge and using it in the evening afer 7pm.

What are the rates you are getting? Bear in mind that a round trip from 230V AC grid to 48V battery and back to 230V AC house will result in 2 conversions in the inverter with a round-trip efficiency of approx 85%. So, for £1 of charge, you will only get back 85p worth of electricity whether you use that to power the house or export.

With regard to your other question, sounds like you need some external controller to manage the timing and rate of charge and/or discharge depending on the SOC in early afternoon. I do something similar for charge up / discharge overnight. Will depend on whether you have the programming skills and an inverter than is controllable in that manner.

 

[jgh]

From the graph, it looks like a Solis - hence controllable with some effort.
Have a look what this guy did: YooToob

I've not gotten that far myself; still feeding a set current from battery 16:00 - 19:00 to dump about 40% of my capacity, and occasionally changing the charge current for the 02:00 - 05:00 slot depending on the forecast (if I remember).

 

[mikecambs]

> What are the rates you are getting? Bear in mind that a round trip from 230V AC grid to 48V battery and back to 230V AC house will result in 2 conversions in the inverter with a round-trip efficiency of approx 85%. So, for £1 of charge, you will only get back 85p worth of electricity whether you use that to power the house or export.

Ah, i didn't take into account the round trip efficiency. The rates are really close.
Peak Export 4pm-7pm: 29.9p/kwh
Off-peak Import 5am-4pm: 29.22p/kwh

Full rates - Import

02:00 - 05:00	17.53p/kWh
05:00 - 16:00	29.22p/kWh
16:00 - 19:00	40.9p/kWh
19:00 - 02:00	29.22p/kWh

Export

02:00 - 05:00	6.53p/kWh
05:00 - 16:00	18.22p/kWh
16:00 - 19:00	29.9p/kWh
19:00 - 02:00	18.22p/kWh

This means:

1. I shouldn't charge the battery from the grid at all except during the overnight cheap rate. Even if I charge the battery with day rate then export during peak export rate, i'd still be losing money.
2. I should only ever export if I can guarantee I won't use that energy by 2am, since drawing from the grid in the offpeak evening is more expensive than peak export price (due to efficiency loss).

Basically it seems like its not worth exporting (unless I have sufficient excess) and its not worth importing during the day just to export at peak. Seems like a shame the electricity company hasn't set up the prices to factor in the efficiency loss. It looks like they want to encourage battery owners to help balance the grid by exporting at peak time but it looks like this isn't viable for me unless I have a large battery that can store more than enough energy to cover my house usage between 4pm-2am?

 

[rpdom]

I've looked at Flux, but have decided that I'm better off with Agile for import and Outgoing for export. Agile may not be as cheap as Flux most nights (when it's cheaper it's great), but it tends to be cheaper from 10am to 4pm and 8pm to 2am. So on cloudy days I can top up my batteries before the 4-7 peak rate cuts in.

Also I get a 15p fixed rate for export at all times, even when import is much less that that.

If I had more battery capacity I might be better off on Flux, but my 4.8kWh won't get us through from 5am to 4pm even on a dark day.