Our first Axle event, and teaching the system to see the next one coming

Six days after signing up to Axle, the email we’d been waiting for landed at 18:32: “You are now in VPP mode.” Half an hour later, at 7pm, the battery started pushing power to the grid. Our first paid grid event.

What actually happened

The dispatch ran the full hour, 7–8pm. The battery discharged at a rock-steady 5.5 kW, exporting 4.27 kWh to the grid and dropping from a near-full 99% down to about 66%. At Axle’s £1/kWh that’s £4.27 for an hour of doing nothing but letting the grid borrow what the sun put in earlier.

Update — Axle has now settled it: £4.27. Their figure landed in our account a couple of days later, and it’s an exact match for what our own inverter logged exporting during the hour (we integrated the grid-export trace ourselves and got 4.27 kWh too). That’s reassuring: the VPP pays on metered export, and our meter and theirs agree to the penny. It’s now showing on our stats page and homepage — and from here on, every event auto-publishes from our own meter the same night, no waiting on the email.

No drama, and the dispatch itself needed no help from us. The battery was already full because Predbat routinely charges it for the evening peak, so there was plenty to give.

The surprise: it pulled 5.5 kW, not 8

Our inverter can discharge at 8 kW, and our grid connection is uncapped. Earlier the same day the panels alone were exporting 8.5 kW at midday. So why did the event top out at 5.5?

Turns out the rate is Axle’s call, not ours. They command the battery from their cloud and decide how hard to pull. And 5.5 kW is suspiciously close to 90% of a 6 kW inverter, which is exactly what we started with, before it was swapped for the correct 8 kW unit on signup day. Best guess: Axle still has us on file as a 6 kW system. We’ve emailed them to update it. If they do, future events could dispatch nearer 8 kW, roughly 50% more per event.

The real lesson: you can’t prepare during an event

Here’s the part we didn’t expect. The moment you enter VPP mode, that 18:32 email, the inverter hands control to Axle’s cloud and your own control goes dark. We watched it happen in Home Assistant. The setting that lets our software charge or discharge the battery flipped to “unavailable” at 18:32 on the dot, and stayed there.

That has a sharp consequence. If the battery hadn’t been full when the event armed, there’d be nothing we could do about it. No last-minute top-up, because by the time you know you’re in VPP mode, you’ve already lost the controls. Reacting to an event is too late.

So we rewired it to look ahead

Two changes came out of tonight.

1. It now tells us, and steps aside by itself. We found the exact signal the inverter raises when VPP mode starts, and built an automation around it: the instant an event arms, Home Assistant notifies us and moves our optimiser out of the way so it can’t fight Axle’s dispatch, then restores everything automatically when the event ends. A late-night failsafe makes sure the battery is always freed in time for cheap overnight charging. No more doing it by hand mid-event like we did tonight.

2. Predbat now sees events before they start. This is the important one. We connected Predbat straight to Axle’s event schedule, so it learns about a dispatch ahead of time and treats those slots as worth £1/kWh. Because that’s such a big number, the optimiser will now make sure the battery is charged and ready before an event begins, back when we still have control, instead of hoping it happens to be full. That’s exactly the window the lockout takes away.

The one thing we don’t know yet is how far in advance Axle publishes each event. Tonight the email gave us 30 minutes. The data feed may show it sooner. We’ll find out at the next one, and report back here, hopefully with a fuller battery and a bigger number.