Inverters and transfer switches
An inverter turns battery power into 120V. Big loads on an inverter pull huge amps from the battery.
The short version
- A microwave drawing 1,200W (check its input rating, not its cooking watts) pulls roughly 110+ amps from a 12V battery.
- Inverter alarms on startup usually mean low battery voltage under load, not a bad inverter.
- Transfer switch picks shore or generator. No power past it = check its relays and connections.
- Know which outlets the inverter actually feeds. Often it's only some of them.
On the job
- DC at the inverter terminals under load (not at the battery). Big fuse, lugs, disconnect.
- Read the remote: Low Battery clears at 12.5 V; High Battery over 16 V; Overtemp; AC Overload; Stuck Relay.
- No AC out with DC good: output breaker, the GFCI it feeds, the sub-panel.
- Shuts off under load: watch DC voltage at the moment it trips. Under 11 V is battery or cable drop.
Numbers
- DC amps from inverter: watts ÷ 12, plus ~10% for losses
- Magnum low battery clears: 12.5 V
- High battery cutout: 16.0 to 16.9 V
- Idle draw: 0.5 to 2 A left on
Shop tip
Residential fridge works on shore but alarms the inverter? Check battery voltage at the inverter while the compressor starts.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.