A quick response (a fuller one will follow).
Take a look at the thread Discharging to grid when set to Consumption Type Home Demand where I’ve been posting recently. DavePickard is doing what you’re trying to.
His approach is to set a charge period from 2 am to 4 pm and charge to the max from the grid. The way the system works for this isn’t quite what I expected, but it’s well suited to the Octopus Flux tariff, provided you’ve got your Export MPAN.
What happens is that from the start of the Charge Period the system is in Charge Rate Mode. When the battery gets full, the system switches to Battery Idle Mode. In this the battery neither charges nor discharges. Load is met from PV with any surplus PV exported (at day export rates). This is fine unless at some times (i. e. when the sun goes in) Load< PV. Then the balance to meet Load comes from the grid (at day import rates). Unless your Load is low, you’d probably want to be in Eco Mode at such times.
So there’s a balance to be struck between
-
having the Charge Period extend to 4 pm (so the system remains in Battery Idle Mode until then), in which case
- when PV < Load you’re buying from the grid at day import rates; and
- when PV > Load you’re selling at day export rates
-
switching to Eco Mode at or after 5 am, in which case:
- when PV < Load you’re using the battery to meet Load which isn’t being met from PV; and
- when PV > Load, the first call on the surplus PV is to recharge the battery; only once it’s full is the surplus exported at day export rates
If you switch to Eco Mode at some point after 5 am, you:
- avoid buying electricity at day import rates when PV < Load;
- sell less surplus electricity at day export rates (because when PV > Load, the surplus is first used to recharge the battery);
- may end up having less charge at 4 pm unless you top up the battery at day import rates in the run-up to 4 pm.
I doubt whether there’s much to choose between these. If your Load is sufficiently low that it’s usually met from PV even when the sun isn’t out, I’d stay in the Charge Period until 4 pm. As the excess of Load over PV when the sun isn’t out increases, so does the attraction of meeting it using electricity either purchased at off-peak rates or which you’ve stored from when the sun was out. At some point you decide that you’d rather be in Eco Mode. You then face a further decision as to whether you want to recharge at day import rates to sell after 4 pm at peak export rates.
Those decisions will depend on what the losses are (which I’m not sure about) and how much you think the various approaches are going to knacker the kit and reduce its future abilities.
My own inclinations are that I’m not in the business of buying electricity at one price, storing it and selling it when it’s higher. I’ll leave that to the big boys: Gresham House had 640 MW of charge/discharge capacity at September 2023; by now it’s probably approaching 1 GW with 2 GWh of batteries.
My interest is in reducing how much electricity I consume from the grid and how much I pay for what I do import. My tariff (Cosy Octopus) gives me two 3-hour periods a day of half-price electricity and a flat-rate export tariff. So I aim to have enough charge at 4 pm to get through at least to 7 pm without having to import. That’s not difficult now that the clocks have changed and there’s less need to heat the house. I get no benefit from exporting between 4 and 7 pm, so am happy to export when the battery is full and there’s surplus PV, whenever that happens.