Solar oven!

Really down to physics.
To cook a 4 oz potato takes the same amount of energy. It has the same specific heat capacity no matter what. You can cook it more efficiently though. So roasting it on your wood burning stove is not going to be the same as boiling it in a pot, or a microwave oven. They all loose different amounts of energy.

Then there is the carbon foot print, here a solar powered oven is probably best (PV or ST), once the embodied energy of the panels and oven are paid off, the rest is carbon neutral.
Anything that involves burning, either locally or at a distance, is not going to be carbon neutral.
If you want to get an idea of how long it takes to pay back the embodied CO2 of a potatao by using a wood burner, work out the average ring count of what you are burning. So if the average is 10 rings, then it is at least 10 years to grow the similar amount of timber (contentious I know).

Not sure, bet every type has some 'research' that makes the claim that they are more efficient.
All about watts in and watts out.
We could do a standard test.
Set the oven at 180° (gas mark 4), put a litre of boiling water into a pre-heated dish, leave in oven for 30 minutes. Allow to cool and measure the evaporation losses.

Microwaves heat only the food, traditional ovens get hot so are less efficient.

Here's a nice counter top oven: http://www.amazon.co.uk/dp/B00VE1CJPI

Here's a really efficient one: http://www.amazon.co.uk/dp/B00651T6AI (Russell Hobbs 18537 Halogen Oven )

Posted By: GarethCAren't halogen ovens supposed to be good mainly due to extremely fast warming times?

They are normally compared to conventional ovens, and the simple fact that they are much much smaller obviously helps. Not sure if they radiant heat is also a bigger factor than in a normal oven which is mostly convection?

Slow cookers are great if you have PV - you don't need a very sunny day to get a free meal. Trouble is browning the meat (if you are eating meat) at the start ;) That said I've found you can actually sweat onions in our slow cooker given enough heat up time.

It is odd, in over 35 years in catering (on and off, my Mother owned a restaurant, I like to keep my hand in) I have never seen a steam oven, not once. I wonder why that is? When worked for a bakers, about 34 years ago, it was all electric oven, all the bakers I see down here seem to be the same.

I don't think there is much difference between the energy usage of a microwave and an electric over, gas uses a bit more because it has to vent the combustion gases, electric fan ovens seem to use a little less, but that may be because you can cram more in them. I like electric fan ovens.
Gas is my favourite oven type, seems to cook 'more gently', but the temperature spread inside them is greater (case of know your oven).

If I was forced to have just one oven type for ever, it would be fan assisted electric, cheap to buy, consistent and easy to clean.
One day I will find a decent grill, one day.

So, to throw some different views in

New top of the range Siemens oven does the lot - fan oven, grill, steam and microwave. Steam seems to be an increasing feature - have never used it but hear good things. I get the impression common in high end commercial kitchens.

For me a fan oven works but must be "pyrolytic" - has a programme to get super hot and burn off all the dirt. Much better than using toxic oven cleaner. We only ran ours a couple of times a year and always in winter so all energy went to heat the house.

Will likely try a steam oven this time - meant yo be great for defrosting and reheating without drying out as well as cooking. We're going to skip a microwave this time. I've no woo health fears about them, and I'm sure they're super efficient but they don't cook well. Reheating drys things out and is uneven, defrosting part cooks. Actually cooking pale wet and unappetising.

It's not limited to microwaves. My understanding is that "over heated" food (not just burnt or microwaved) has been implicated in some studies.

Anything that is cooked over 140°C (the Maillard Process) is.
Boiling in water isn't, though some things change phase, like an egg. Liquid to solid. Which is strange when you think about it. It should get runnier as it gets hotter.

I could be wrong but this sounds awfully like changing food at a “molecular level”, what with breaking bonds between different parts of the protein, etc:

https://en.wikipedia.org/wiki/Denaturation_%28biochemistry%29#How_denaturation_occurs_at_levels_of_protein_structure

Whatever, my basic point is that saying that something is changed at a molecular level seems like pseudo-scientific way of making it seem more dramatic than it really is.

It is a slippery slope to a watery grave