gemini://ur.gs/blog/finished/

Batteries are finally in. I ended up going a different route to the one planned in my last post; it turns out that it's actually quite hard to get an AC-coupled battery system that permits the solar PV to keep running when the grid is down. I found an article describing four different "levels" of grid independence; actually getting someone to fit, say, "Manual Whole House Backup" is just... difficult.

So I rethought, and decided to replace the existing Solax inverter with a Lux Power hybrid inverter and make do with a simple EPS socket. Since the PV and batteries are both DC-side, it's trivial to charge the battery from the panels this way. Removing the Solax inverter also felt good from a security perspective, since the Lux allows you to disable both the wifi AP and call home functionality.

Price-wise, it was fine - the hybrid is only a little more expensive than the AC-coupled inverter would be, and the Solax is now a spare that I can use for... well, I'll find a use for it. My sister wouldn't mind a ground-mounted solar PV install, it could be the hub of that.

Big Green Beard didn't come through for us in the end - they took the order details and then never got back in touch, despite several follow-ups. No matter. I found another distributor (Home Energy Group), and got an LXP3600 Hybrid with a 3.2kWh "Greenlinx" battery. Price transparency on the website was nice to have; overall a recommend.

Lead times were about a month; it finally went in yesterday, and it looks like it'll do the trick nicely. Today was a sunny autumn day, so we got to see it properly in action:

The yellow line is generation, while the red line is "power from the inverter" - which may come from PV or the battery. Green is imported power. The battery went from 10% to 100% charge between 9am and 2:30pm (not shown), then supported power use, keeping import down, until it ran dry around 17:30pm. After 2:30pm, a small amount of energy went to export (also not shown) but we captured most of what we generated. It was a heavy energy use day for us - dishwasher, washing machine, and dryer all ran in the afternoon - but we've kept the day's import to ~6kWh.

This inverter series has a nice, pre-existing project for local metrics and control, which is surprisingly similar to my solax2mqtt project in concept - lxp-bridge. Written in rust, it polls the inverter and converts what it finds into mqtt messages. It's much further advanced, though, and supports sending the inverter commands (e.g., "charge from AC"), has some home-assistant auto-discovery support, and can push the metrics out using the InfluxDB v1 API (which Victoria Metrics supports natively). Once I got it set up, it was pretty simple to cut over my existing graphs to support it.

The status value is a bitfield, which was fun to work out how to support in grafana. I ended up with this visualisation:

"FAULT" and "GRID OFF" will turn red if the bit flips, while the rest toggle between nothing and green. It's pretty handy.

This inverter also happens to send grid import and export numbers (it has a CT clamp wired in), so the Glow CAD may be obsolete, which would simplify matters. Haven't quite gotten around to disconnecting it yet, though.

The EPS circuit is up in the loft, it's just a simple double socket at the moment. I'll get that expanded to be a minimal consumer unit that takes over the lights, at minimum, when I can; if there's a grid outage in the meantime, I've got an extension cable I can run through the loft hatch. Given the difficulty of using the EPS, the capacity is fine for power cuts lasting multiple hours; it'll keep the lights and internet on for ages, and we could even microwave food! The UK is currently operating with a worst-case scenario assumption of a week-long powercut, but my approach to that is a bit more low-tech - we've got some wood stored in the garage, and a chiminea to burn it in. From that we can heat water for hot water bottles and/or cluster around it for warmth.

Given peak solar generation in winter just about fits in this battery, I've forbidden myself from getting any more until at least spring (HEG sell them for £1550 per 3.2kWh unit). I should be able to DIY the install, as well. Famous last words.

3.2kWh is also enough to completely remove any grid import during Octopus "saving sessions", which is what the no-grid bribes have been branded as. Currently they're £2.25/kWh and they calculate reductions against a baseline derived from your import in the preceding days, which makes big payouts harder than I thought. They're also relatively infrequent; there have only been two in November so far. Still, the batteries were never going to be a winning financial proposition.

The one remaining outstanding thing is getting paid 15p/kWh for export, which allows me to treat the grid as a 50% efficient battery - I finally got all the paperwork relating to the solar installation over to Octopus. They're doing the necessary behind-the-scenes work, and in perhaps a couple more weeks I'll be getting paid. That makes the PV diverter useless, but I've already turned it off anyway as it's much better to store the power in the battery until the heat pump needs it.

tl;dr: lots of learning, lots of waste, but I'm finally done with making changes to this house for the year. I am, in fact, finished!

Finished