Here I will show how to make some electric ovens more efficient by adding insulation. My YouTube video has the long story if you're interested, but here I will present one toaster oven which I was able to improve greatly.
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Step 1: SAFETY FIRST!! YOU ARE RESPONSIBLE FOR YOUR OWN ACTIONS.
It is very important that you not do this unless you understand the inner workings of an oven. Every oven is different and you will have to use your own judgement to determine how to safely implement these methods. Misplaced insulation could cause equipment damage or fire.
Step 2: Finding Your Way In
You must first determine the easiest way in to your oven that will give access to all areas that could potentially be insulated. In the case of our toaster oven, it was the back.
Step 3: Assessment
Next, inspect the interior of the oven to see where there is already insulation and where there are components that should not be stuffed with insulation. Here you can see that this oven has a fiberglass batt on the top. It also has one on the left side, but they don't even meet in the corner! You should never trap wiring between insulation as it could overheat. Make sure that you can move it to the outside, or don't insulate there.
Step 4: ROXUL
Roxul building insulation is the best product I know of for this purpose. You can use small, otherwise useless scraps to stuff a space like this and get it pretty tight without being overly compacted. It takes about 2100 deg F to melt.
Step 5: Fully Stuffed
Here is my finished product. Obviously the back had no insulation and I remedied that thoroughly. I also stuffed the corner where the existing batts didn't meet, the bottom, and the other side. The other side had the convection components so I had to leave enough space for the fan motor to breathe.
Step 6: The Results
I tested this oven before and after modifications using the same procedure. I measured how long it took to preheat to 350 deg F and then left it on for 30 additional minutes. On the unmodified run it took about 15:30 to preheat. With the insulation the time was reduced to about 12:30. For the total run I achieved nearly a 37% reduction in energy consumption.