Insane Propane vs. Microwave Mania

[Oey]

this is a energy use/cost question: i can't figure out whether it's cheaper for me to use the microwave or the LP gas propane stove. chatgtp says it is way cheaper to run the microwave.

electricity here is .44c kwh --- LP gas is $3.75 a gallon, roughly. with a common denominator, a therm i guess, what do you say if the microwave is 1200 watts and the stovetop uses about .28 gallons of LP per hour, or 10 cubic feet, according to chatgtp.

so for example, say I'm cooking beans for 2 hours on the stove. Chatgtp says I'd have to run the microwave 12.5 hours to use the same energy. whaaaat? doesn't make sense to me, a math nincompoop.

[maskedslug]

I think it’s about right. Napkin math, a gallon of propane can produce about 1 therm. In watt-hours, that’s around 29k or 1 day of your 1200 watt microwave running. 2 hours of propane is just a bit over half a therm, 12.5 hours is just a bit over half a day. Seems ‘bout right!

[loutfard]

Let's see:
- 1 US gallon = 3.785412 l
- 1 l = 7.08 kWh

Some basic math later:
- Running the propane stovetop costs $ 3.97/hour: $3.75/gallon * 3.78 gallon/l * .28 gallon. It converts 7.5 kWh of primary energy to heat: .28 gallon * 3.78 gallon/l * 7.08 kWh/l.
- Running the microwave costs $0.53/hour: $0.44/kWh * 1.2 kW * 1 hour. It converts 1.2 kWh of primary energy to heat.
- Converting the same 7.5 kWh of primary energy to heat would cost $3.31 with the microwave versus $3.97 with the propane stovetop.

Some other factors that come to mind:
- A microwave probably is (much) more efficient at converting eletricity to heat than a propane heater propane.
- For cooking purposes, a microwave probably directs the heat generated to the food to be heated more efficiently than a propane stovetop.
- Producing cheap solar electric is often easy. Producing your own propane is much much more involved.
- Cooking in a microwave requires less cookware.

In your specific scenario, it seems like a microwave is the cheaper choice.

[Oey]

thanks for that.

I was surprised to read somewhere that a microwave oven is 50% efficient. A gas stove range is 38% efficient. Is that right?

I just cooked a potato in the microwave in 3 minutes at 1200 watts. If I heated up the gas oven to cook it, I think I'd use much more energy. Even though that wasted energy could heat the room, as I don't set my thermostat above 50*F in winter.

[white belt]

The microwave is almost always going to be more efficient for heating/cooking small amounts of food. However, if you really want to reduce your energy consumption when cooking something that requires long and slow heat (like beans), I recommend looking into thermal cooking strategies. The basic idea is that you only need to bring the cooking vessel up to a certain temperature, then you can put it in a bunch of insulation for it to slowly cook over an extended period without additional energy input. The lower tech implementation is something like a haybox. Higher tech versions are vacuum sealed containers like the Thermos Shuttle Chef.

[Chris]

I was surprised to read somewhere that a microwave oven is 50% efficient. A gas stove range is 38% efficient. Is that right?

This is in line with what I've read. Gas stoves' problem is that so much heat is lost around the cooking vessel as waste heat.

Depending on what you're cooking it's hard to beat the efficiency of an electric pressure cooker. I cook 225g dry black beans in 30 mins, using 0.15kWh.

[Slevin]

Depending on what you're cooking it's hard to beat the efficiency of an electric pressure cooker. I cook 225g dry black beans in 30 mins, using 0.15kWh.

I always take 50 mins for black beans, but I also cook 1kg dry beans at a time.

[Oey]

that's a great idea. thank you

do you have a recommendation for what particular brand to buy/find? I kind of avoid metal cookware from China, because I'm not sure of the purity of metal. maybe I'm wrong about that, if someone knows better please chime in.

my cookware is old American made stainless steel pots and pans. a Farberware pot made in Brooklyn and All-Clad pan made here somewhere; and a heavy sauce pot, made in Italy, that I use for small amounts of rice. However, my ancient pressure cooker for the stovetop is an old China-made one that I can't seem to find a good replacement for. So yeah if you have experience there on pressure cookers, getting the beans cooked properly, would be great to know more.

[2Birds1Stone]

Even though that wasted energy could heat the room, as I don't set my thermostat above 50*F in winter.

To add something to consideration, cooking potato's on the stove for 45 min using LP and hoping to heat the house with it also likely means poor ventilation and a non trivial amount of unwanted toxins in the air.

[loutfard]

A pressure cooker combined with a hay box probably is the most efficient way. Bring the pressure cooker up to pressure for n minutes less than it takes to get the food ready. Switch off the heat. Put the pressure cooker in the hay box for n minutes.

If I'm not mistaken, even Jacob himself used/uses this technique.

[white belt]

A pressure cooker combined with a hay box probably is the most efficient way. Bring the pressure cooker up to pressure for n minutes less than it takes to get the food ready. Switch off the heat. Put the pressure cooker in the hay box for n minutes.

I think the most "efficient" method using electricity is likely to use an induction cooktop to bring the contents of a thermal cooker inner pot (stainless steel) to a boil, then put the inner pot in the thermal cooker to cook the rest of the way. This is really just a higher tech version of what you are suggesting, but it won't be as fast as just using a pressure cooker. The downside is a thermal cooker can be as expensive as a pressure cooker, but it is less versatile. I've done a little bit of cooking with a thermal cooker, but plan on doing more this summer when I'm settled into a stable housing situation with DW. Here is a related article about doing it on a sailboat: https://theboatgalley.com/thermal-cooker/

I put efficient in quotation marks because using electricity to cook isn't necessarily going to be efficient. From an energy perspective, it's hugely inefficient to turn heat from burning fossil fuels into electricity, only to turn that electricity back into heat when you cook at your house. Maybe you know all of your electricity comes from cleaner sources like hydro, wind, nuclear, etc but that infrastructure still has an enormous footprint. So some of it is going to come down to how you are getting electricity. One could have a solar panel system to power an induction cooktop, but it will need to be sized on the larger end to handle the load.

The lower tech and more sustainable solution is to just repeat the steps above but instead of using an induction cooktop or pressure cooker for the first step, just go outside and use a small wood camping stove with some twigs to bring the contents to a boil. That uses a sustainable fuel source and will have a much lower carbon footprint than any other method. The upside is you only have to be out there for maybe ~10 minutes depending on conditions, then you're free to have it cook the rest of the way indoors without having to tend to anything. The downside is it isn't always convenient due to weather and location, which is why I can understand using other methods indoors.

Related article: https://solar.lowtechmagazine.com/2014/ ... king-pots/

[loutfard]

I have researched this a bit further. Since 2022, Kuhn-Rikon offers an induction compatible double-walled vacuum insulated pressure cooker in the Durotherm pro series. That should be the most efficient way of electric cooking, even more so if combined with solar.

If cooking on anything else but electricity, any another good thermal cooker will do. The Kuhn-Rikon Durotherm (not pro) series pots have an excellent reputation. They have vacuum insulation but are not suited for induction. They have been available since the mid-1970's and pop up in second hand sales every now and then.

Also, vacuum insulated water cookers are a thing. If you use a water cooker a lot, it might be a good idea to get one of those should your current one die.

[Oey]

yes thanks for the info. i've been thinking of buying a new pressure cooker, however i don't want a china-made unit. i just don't trust that the steel won't have bad impurities that would eventually enter food cooked in it.

[loutfard]

After setting up monitoring of our solar electric production and electric consumption, I've looked into this even further.

Somewhat random observations:
- We are charged ~0.3€/kWh for getting electricity from the network and get 0.07€/kWh fed into the network.
- Average cooking energy used is about 175 KWh/year on an induction system.
- Optimising for cooking just when the sun is shining strongly enough, we could shave up to 40€ (.23€/KWh* 175 kWh) off our yearly electricity bills. Additional benefit as far as I'm concerned is less of our money going to nuclear or natural gas.
- The entire year for a few hours round around noon, our solar panels should be able to provide enough electricity to cook using a slow cooker, a well-insulated double walled cooking pot or pressure cooker. As long as the weather is clear, and no other large electricity consumers are active at the same time, that is.
- Baking on a pan on solar only should be just about possible year-round around noon too.
- Our microwave has a "750W" sticker on the front, but turns out to consume ~1.3 KW.
- Our induction hobs are much more efficient than I thought. Medium heat is under 1 KW, full above 2KW.
- We use a bowl with lid inside the vacuum cooker. This holds heat very well. Great for cooking after switching off the hob.

Conclusions:
- Little financial benefits of shifting electricity consumption away from the grid. Up to ~3€/month in the very best case.
- Cooking in line with the sun and at low power is worth it from an ecological perspective.
- Operating the cooking hobs at medium heat as opposed to an initial full heat blast avoids quite a bit of electrical consumption.
- Avoiding 500km of electric car driving will avoid a similar 75 KWh of electricity consumption. Less km for a fossil fueled car.