Hi. I'm in process to going off-grid, but in the meanwhile I have a propane stove and a fairly cool house.
I was about to sell the outdoor propane grill that came with the house, then realized I could cook outdoors and save on energy - well, actually, be more comfortable by not heating the house up cooking.
I've done one breakfast and think I was wrong. The grill is very far from the flame, it took a long time to heat up - and I think I'd be better off just cooking on the stove with the vent fan running like always.
But I wanted to check. I did several internet searches, looking for the comparison in propane use between an outdoor propane grill and an indoor propane stove. This question is apparently not of interest to anyone. The number of wrong topics (butane vs propane, or natural gas or electric or charcoal) was astounding. NOBODY in the history of the internet has ever asked this question?
So I came here, thinking at least these are the right people. I didn't find my answer, but maybe somebody has thoughts. Until then, I'll cook indoors. (Until the wood grill and the solaroven are done - and probably after too.)
If you're cooling your house with electricity, or can't cool your house, and have the option to cook outside, the calculation that matters is how much electricity will it take to get your home back down to temperature again.
Either way, you're burning energy to eat. But in one scenario, you need to expend more energy to get that unwanted heat out of your once-cool living space.
I agree that perhaps the grill is much less efficient. I wouldn't cook directly on it, and if it had room to add more lava rock, I would do that. In addition, I think that I would use cast-iron pans on the grill, or better yet, an enamelled dutch oven. That would boost heat retention greatly, but you'd still want to plan to do one-pot meals and store leftovers.
The two suggestions I might make, if you have electricity, is to think about a slow cooker. The other would be an induction hotplate. Either would be more efficient than a propane grill in the heat, especially if your electricity is solar or wind-derived.
If it's expensive of difficult to cool your house down again, try to find ways to avoid it.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
BTUs are BTUs. From my perspective, it's the design of the appliance and how it's used that determine efficiency.
As designed, a stove would be much more efficient for cooking with pots/pans.
Grills are not designed to be efficient; they're designed to spread out the heat instead of concentrating it. If I ever needed to cook with a pot or pan, I would take out the existing top grill, remove the lava/ceramic rock or heat shield, and put in a smaller grate that sits very close to the burner. I might even add a vertical metal/brick heat shield to concentrate the heat on one side. I would only light the burners that I actually need.
propane stoves occasionally have a separate burner specifically for this sort of thing, which I imagine indicates that the grill itself is not well suited for anything but cooking food directly on fire (since as mentioned above, you're heating up a large piece of metal for no good reason).
Either some tinkering is in order (like Douglas says, if you dare) or I would consider a different option. Small rocket stove (even bucket-sized) might be a good call for your future cooking endeavors as you move offgrid.
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