Yes, it's an either/or and I have mine wired directly to the battery with a 100w solar panel on it for the last week and it's a game changer. I've not seen it drop below 12.4v on cloudy day and usually it's at 12.7v when I check it. I'll never big-tent camp without this thing. It's only been getting to about 20 every night so I've run it on low (1.4) the entire time and the rent is almost too hot. Wife stayed in camp a couple days and heater ran for 3 days non-stop. So far since Saturday I've used about 2 gallons of diesel (plus came in with a full tank). Get you some cinder blocks or something to set it up on. The exhaust pipe gets HOT.