Electric space heating is a beat-down offgrid, typically requiring over 1000w for small space heaters. Contact heating, on the other hand uses relatively little power and can be extremely effective.
measuring power consumption
Electrical heating devices don’t consume power the way we might expect or want. Consider a 60w blanket with three settings (HI, MED, LO. We might expect:
- HI 60w
- MED 40w
- LO 20w
but in reality it’s often something like this:
- HI 60w 100% of the time (steady draw)
- MED 40w 66% of the time (ON two seconds, OFF one second, repeat)
- LO 20w 33% of the time (ON one second, OFF two seconds, repeat)
Luckily the heating elements have some amount of thermal inertia and a person normally can’t feel the ON/OFF cycles. It does require observation and math skills or a watt-measuring device to get good averages. The electronic logic controlling these cycles probably requires a pure sine wave inverter. Older resistor-type controllers will work on MSW.
I use a kill-a-watt meter to measure power consumption over time.
Giantex electric heated mattress pad. Mattress pads tend to need less power than electric blankets because the body is pressing down on the heating wires.
- rated 95w. I observe an average of ~13w on lowest (where I usually have it) to 80w on highest, which no sane person could stand. I once ran it on high and measured 117F (!) with the temp gun under the covers.
- 8 heat levels. The highest I ever ran it with me in bed was level 3.
- automatically defaults to 10 hours, though it can be set to 1, 2, or Forever.
Nitpick: it defaults to 8 when you turn it on, perhaps for preheating.
At $15 the Sunbeam heating pad is a great value.
- rated 45w, I observed 43w on HI and ~10w on LO.
- 3 heat levels
- no auto power-off
Nitpick: 10w is a good power level, but the pad is so small it’s highly concentrated. I have to put a towel over it for comfort.
For light use I will recommend the Dr. Prepare heated vest which does an astounding job for 10w.
- usb-powered, so you can use it walking around
- I observed 10w on HI, 7.5w on MED and 5w on LO.
Nitpick: the USB wiring seems a bit flimsy and I worry it will break if I use it too much.
The Karacel rechargeable USB handwarmer was the biggest surprise of the lot. It claims to be a handwarmer adn a USB power brick and I assume it would be terrible at both. Instead, it’s good at both.
I wear it on a lanyard around my neck on LO and use my jacket zipper to regulate heat level. It gets so hot I can’t imagine anyone using it on HI unless they were in the Arctic.
- three power levels
- can be used to charge USB devices
This Sunbeam mattress pad was a failure for offgrid use. The first one was DOA; Amazon quickly shipped another.
The bigger issue was that this 180w-rated device usually pulled in the mid-200s when turned on, and frequently shut down my 300w inverter. Fine for a house, not fine for off-grid. I donated it to a Goodwill store.
The Wagan heated seat cover worked well enough, but felt flimsy like it could break at any time. I laid it out flat under my sleeping bag and it was comfortable on LO (~20w).
- two heat settings
- native 12v device