Oven Efficiency

[iPlug]

Efficiency...Love heat pumps. We've got them to heat/cool our homes, the Tesla lineup, our hot water for home, and clothes dryers. We've got induction for stoves/cooktops. And of course we have electric vehicles.

The next area for disruption for home efficiency, sorely needed, might be our ovens. With high temperature deltas, heat pumps unfortunately don't work here with our existing technology. That's unfortunate as ovens use a lot of energy and they are really lousy in terms of efficiency.

A standard electric oven has a cooking efficiency of about 12%, according to the LBNL study, while a self-cleaning electric oven, which has more insulation in the oven case, has a cooking efficiency of about 14%. By comparison, standard gas ovens have cooking efficiencies of about 6% and self-cleaning models 7%.​

Efficient Cooking - GreenBuildingAdvisor.

Hopefully we will figure out how to make progress here. Even 25-30% efficiency would double the most efficient oven.

Maybe this calls for better insulation, specialized double/triple paned glass that better rejects conduction and infrared radiation...

In the mean time, perhaps it's best to use a microwave when the culinary outcome is acceptable:

(The cooking efficiency of microwave ovens is about 56%—nearly ten times that of a standard gas oven and more than four times that of an electric oven.)​

 

[Ampster]

Are convection ovens more efficient?

 

[iPlug]

Have read convection ovens use 20% less energy to achieve the same outcomes thrown around on a few websites that link to a dead US. Dept of Energy website.

If we apply that to the most efficient (14% efficient) self-cleaning electric ovens noted above, that gets us to almost 17% efficiency.

There appear to be commercial oven beasts that in convection mode do much better efficiency numbers over at energystar.gov.

 

[Daniel in SD]

How much energy does an oven use? Say, to heat up and run at 450F for 20 minutes?
I suspect to get a big improvement in efficiency you’d need to make the oven out of a material with less thermal conductivity than steel.

 

[iPlug]

Good question on energy use in a 20 minute cycle. Enough energy to require a 240V connection and on a 30-50A breaker for most electric ovens, AFAIK. Guessing 2-6 kWh energy used in this scenario.

Suspect if heating up an oven for this duration, much of that would end up heating the internal metals for at least the first 10 minutes. Then when opening the oven at 20min, the reverse happens and puts out a great amount of heat into the kitchen for some time after. Of course none of that ends up in the food. This might be slightly welcome in the winter (inefficient way to heat house), but only adds to cooling demand in summer.

 

[swaltner]

6 kWh is WAY over reality of what an oven uses for a 20 minute cooking cycle.

My oven uses 3.5 kW when the heating element is on. This is measured using an actual power meter, not a guess, based off just multiplying the amperage rating of the outlet with the voltage.

The pre-heat cycle to heat the oven up to 400F lasted 8 minutes. That's 0.466 kWh (3.5 kW * .133 hours).

Once the oven is up to temp, the burner element is no longer on a 100% duty cycle. It cycles on and off. The oven toggles between drawing 3.5 kW and drawing no power. If the burner stayed on 100% of the time, it would reach a temperature close to 880 F, which is what is used for the self-cleaning cycle. Here's a graph of the power draw of my house with the oven set to 400F.

Over a 5 minute period that seems to repeat, the oven's heating element is on about a 50% duty cycle for 4 minutes and then off for a minute. That means for each 5 minutes that the oven is on, it consumes about .116 kWh (3.5 kW * .066 hours * .5 duty cycle).

So, to preheat my oven to 400F and run it for 20 minutes, it consumes .93 kWh (0.466 kWh + 0.116 kWh * 4)

 

[iPlug]

Appreciate the hands on data. Agree, the 6 kWh max guesstimate is out of the range of possibility for any residential scenario. Our oven spec is max 7.4 kW, so max 2.5 kWh over 20min.

As you note the ~1 kWh actual energy used in your experiment is for a 20 minute pre-heat cycle including static hold after temp attainted, but the question requires a substantial cooking load.

The thermal heat sink of a typical oven cooking load would increase the energy use during that 20-min window significantly. For example, cooking a 6 lb frozen lasagna without pre-heat set to 400º F for 20 min would not get the lasagna close to goal temperature, even if running the heating elements more consistently. We could do a pre-heat before placing the lasagna in the oven, but would need to add the pre-heat energy used to the actual cook time energy used of the lasagna.

Got a frozen lasagna laying around to test? Can't share the food unfortunately, just the data.

 

[Unclepaul]

Do not believe that home ovens are significant contributors to home energy use. With time of use programs, people can just cook, do laundry etc when prices are lowest.

 

[iPlug]

Agree, electric ovens have been reported ~3-4% average home energy use, but topic of interest is that they are one of the most inefficient appliances in the home by a wide margin.

In terms of home energy use rankings, have seen breakdowns similar to this:
1) home air heating
2) home air cooling [#1 and #2 ~50% home energy use]
3) water heater
4) clothes dryer
5) everything else (refrigerator, washing machine, dish washer, stove top, microwave, oven, home lighting, TV, etc.)

In that sense, take home message would be to convert home HVAC and water heaters to heat pumps before solving the oven efficiency problem, but we have threads for that already.

 

[Empiro]

Is there any analysis done on where the energy is wasted? How much of the energy is due to the heat escaping the oven versus just heating up the oven itself? When you're done cooking, the internal surfaces of the oven are at 400 degrees, but the energy there will all be wasted unless you put something else in the oven right away.

At least right now in the winter, the heat will be dissipated into the home where it'll slightly lower the heating requirements.

 

[iPlug]

Is there any analysis done on where the energy is wasted?

Must be. These numbers are reported to come from the Lawrence Berkeley National Laboratory, but I have not been able to find their study online.

How much of the energy is due to the heat escaping the oven versus just heating up the oven itself? When you're done cooking, the internal surfaces of the oven are at 400 degrees, but the energy there will all be wasted unless you put something else in the oven right away.

Agree, it seems a very large part of inefficiency may involve the time to pre-heat then heat lost on cool down from the metal heat "sink" of the oven structure after the cooking cycle. Certainly some energy is lost through insulation, but the surrounding cabinets don't get hot enough for fire risk. Probably more is radiated via conduction and infrared from the door glass than side/top/bottom/back structures.

Commercial ovens are reported as having substantially higher efficiency than residential ones - my guess has been that most of this comes from them running in long cycles (most of day or virtually non-stop) so pre-heat and cool down penalty would be much smaller.

For the reason that this efficiency problem still exists with residential ovens suggests at there may be no easy solution. Some avoid the microwave as it does not produce the crisping and other desired cooking effects, but it is certainly much more efficient.

Would like to see efficiency numbers on smaller devices like the toaster/convection ovens and "air fryers".