Um, forever! I have it plugged into the cig socket. It CLAIMS that it pulls 2.1A at 12V, but I haven't measured it yet, which I can easily do and plan to. If the claims are true (they're usually over-rated), then it's pulling about 25W, which is a little less than my laptop pulls when pulling DC-DC (no inverter). Thus, I'm not sure I'd recommend using it too long without solar power or keeping a good watch on the house battery voltage.
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2006 RB50 6.0 Diesel
Flatplate water heater (mounted INSIDE), portable butane stove (no propane)
SMB 4WD
Amsoil
In theory you'd only be using it while driving, so running the van batt down shouldn't be an issue, right?
Correct! Although I have kicked around the idea of getting a nice lower back massage after a long day of skiing... One of these days I'm going to have to get off my butt and install a solar panel.
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2006 RB50 6.0 Diesel
Flatplate water heater (mounted INSIDE), portable butane stove (no propane)
SMB 4WD
Amsoil