The negative-price shield

Why prices go negative

When lots of rooftop solar floods the grid on a mild sunny day, wholesale electricity prices can fall below zero — and on a plan that passes wholesale prices through, that means you pay for every kilowatt-hour you export. Most inverters have no idea this is happening and cheerfully sell into it all afternoon.

What the shield does

Every five minutes, solarprod checks your live Amber feed-in price. If it’s negative, your inverter is switched to “never export” — solar still powers your home and charges your battery; the surplus just stays put instead of being sold at a loss. The moment the price comes back above zero, your normal export setting is restored automatically.

While prices stay negative, the shield keeps re-confirming the “never export” setting every few minutes. Some inverters — Tesla Powerwalls included — can quietly revert to exporting after a small grid disturbance, undoing a setting you chose only minutes earlier. The shield catches that within five minutes and puts its protection back, every time, for as long as the price is negative.

One consequence: changing the export setting by hand during a negative-price window looks exactly like one of those reverts, so the shield will switch it back. If you want manual control while prices are negative, turn the shield off first — it’s one toggle on the inverter control panel, and every switch the shield makes is logged.

“My app says the price is zero, but the shield is on”

Price displays usually round to the nearest cent, so a feed-in price of −0.4c can show as 0c. The shield works from the unrounded price, so it can be active while your retailer’s app shows zero — it reacts to what you’d actually be paid (or charged), not the rounded display.

“Shield overridden” — what that means and how to fix it

After the shield switches your inverter to “never export”, it checks that the setting actually took. If your system keeps exporting anyway, the shield shows the red “can’t protect” banner and stands down rather than keep sending settings that aren’t being honoured. There are two known causes.

Another controller is managing the battery— usually a retailer’s automation such as Amber’s SmartShift, which quietly wins over settings made by anyone else. Two controllers can’t drive one battery: either turn the automation off in the retailer’s app (you keep their prices — only the battery control changes hands; afterwards, check the rate plan in your Tesla app, as SmartShift replaces it and may not put yours back), or keep it and switch the shield off with the toggle above.

Your site doesn’t allow export control at all — on some Powerwall installations Tesla doesn’t provision owner export control: there’s no “Grid Export” option in your Tesla app, and export settings from any source are stored but never applied. The fix is a quick Tesla support request: “please enable Grid Export control for my site — the setting doesn’t appear in my app”. Once Tesla enables it, nothing is needed here — the shield re-checks at the next negative-price window and the banner clears by itself when protection is confirmed working.

What it needs

A supported inverter — Tesla Powerwall or a Deye hybrid today — and your Amber Electric account connected so solarprod can see your real prices (tick “Use Amber” on your retail plan, or connect it from your account page). Then it’s one switch: turn on “Protect against negative feed-in prices” in the inverter control panel and the shield is on guard from the next price check.