Field report · battery & off-grid protection

The night a $400 inverter failure took out my water heater, stove, and a two-month-old heat pump.

A whole-house surge protector never tripped — correctly. Here's the protection gap that a surge protector can't cover, why it quietly destroys appliances on battery and off-grid systems, and the dumb hardware that actually stops it.

Get the $400 protection audit Read what happened

01What happened

In the middle of the night, on battery, with no sun on the panels, one of my house inverters hard-failed internally. Not a fuse, not a fault code — a component crossed battery DC straight to the AC output.

The battery bled out through that fault, and the 120-volt line that feeds half my house quietly climbed to roughly 165 volts and stayed there for hours.

Dead

Water heater transformer

Dead

Stove

Dead

Heat pump (2 months old)

Dead

A handful of fan motors

For the following week I kept replacing 24-volt control transformers one at a time, thinking I had a string of bad luck. I didn't. I had one event with a long tail.

02Why nothing caught it

This is the part that should worry anyone running battery backup or an off-grid system.

I had a whole-house surge protector. It never tripped — correctly. Surge protectors are built to clamp spikes measured in microseconds and kilovolts. A sustained 165 volts for hours is a completely different kind of event, and it sails right under an SPD's trigger threshold.

The witness problem

My monitoring software was down that night, for unrelated reasons. But even if it had been up: the software path and the protection path were the same path. If the thing watching the voltage is powered by the voltage that's failing, it isn't protection. It's a witness.

03What actually stops this

The fix isn't smarter software. It's dumb hardware that doesn't need software to work:

A voltage relay on each leg, set to trip around 130 volts.
A contactor on each leg, powered by a low-voltage coil rail that's separate from the bus it's protecting.
Wire it so the relays gate the coil power in series — if either leg goes out of range, or a relay itself loses power, both contactors drop and the house is isolated from the bus. Fails safe, not fails open.

No PC, no WiFi, no cloud dashboard between the fault and the disconnect. Parts cost: a few hundred dollars. The equipment I lost that night cost thousands — before counting the heat pump warranty I had to fight for.

04The part most people get wrong

A surge protector and an over/under-voltage cutoff solve two different problems. Almost every battery-backup and off-grid install I've looked at has the first and not the second.

If your inverter can fail internally and backfeed a sustained over-voltage — and any inverter can — an SPD gives you a false sense of coverage for the exact scenario that actually destroys appliances.

The offer

Protection & monitoring audit

$400 flat · battery-backed & off-grid systems

I look at what's actually installed — inverters, batteries, panel, protection.
I identify the gaps, like the one that cost me a heat pump.
You get a plain-English writeup of what it would take to close them.
If you want it built, that's a separate conversation — but you'll know exactly what you're buying before you spend a dollar on hardware.

Pay $400 and book Questions first? Email

Checkout asks for your service address, phone and a line about your system. You get a confirmation page and a receipt; I reach out within one business day to set the date. Detroit metro and surrounding. Remote review available for systems outside the area.