Steaming vegetables in microwave

I made a short research on steaming vegetables in microwave to compare it to steaming on stove top. An interesting discovery I made is that steaming in microwave is more energy efficient when cooking in small portions. Large portions, on the other hand, are best made in stove top.

Can anybody help me to know what would account as large portions?

• This "rule" seems very vague. What stovetop types (resistant, inductive, gas) does it apply to? What size and shape of steaming vessel is used on stovetop? The amount of energy consumed will vary with these variables, so you can't find a break even point without knowing them. What is the original source for the claim? – rumtscho Jan 27 '14 at 16:36
• Without knowing where you read this, my best guess is that the author simply meant portions too large to fit in your microwave. – Anthm Jan 27 '14 at 21:49

I steam vegetables in large batches (up to 3 lbs) in covered Pyrex bowls in our large microwave. The microwave is 1800W, and steams the vegetables well in 4-6 minutes (depending on the vegetable...about 5 minutes for broccoli, which is fibrous). Bringing approximately 2" of water in an 8qt stockpot (the amount you'll likely need to heat in order to have the entire batch remain over steam before evaporating all the water)to a boil, plus steaming 3 lbs of broccoli in a steamer insert will take about 6 minutes on my stove on the 12,000BTU/hr gas burner.

So, that's 1800W x 5 min vs 12,000BTU/hr x 6 min

1W = 3.41BTU/hr

Converting the BTU/hr to watts that becomes

1800W x 5 min vs 3517W x 6 min

Since 1 W = 1Joule/second, the two energy expenditures are:

-Microwave: 1800J/s x 300s = 540,000 joules

-Stove: 3517 x 1,266,120 joules, more than twice the cost.

Stoves send a great deal of energy up the sides of the pans and into the air, where it is lost. Microwaves send nearly all their energy into the food. This means that in nearly every case, steaming in the microwave is more energy efficient.