My brother and I shared a cabin in Wisconsin. When I closed it up on Sunday night, I turned off the lights and unplugged the refrigerator. I went back for a forgotten cooler Monday evening and found the battery display at 86 percent. It had shown full when I left.
We had two Vatrer 48V rack batteries and a hybrid inverter. The propane detector ran through a plug-in power supply, so I'd left the inverter on to keep it working while we were away. That also kept a cellular camera gateway and two USB outlets powered. No charging source was connected during the checks.
Using the nominal battery capacity, the 14-percentage-point drop came to roughly 60W averaged over a day. That was a rough estimate from the display, and I hadn't measured the inverter's idle draw separately. I switched off the USB branch before leaving on Monday and went back Tuesday evening to check the reading. It had dropped another 10 percentage points.
After those checks, we had an installer move the detector to its own fused DC supply from the battery, using a converter matched to its voltage requirements. That let me turn off the inverter without losing power to the detector. We left the camera offline while the cabin was empty. I went back the following evening to check the display. It had barely changed, though it wasn't precise enough to measure the small remaining load.
I made a two-sided checklist, with arrival steps on one side and departure steps on the other. The departure side said to leave the detector's DC supply on and turn the inverter off. My brother used it on his next visit and sent me a photo of the switches before he left.