Units are important. There are a couple of people on this thread with near saintly patience. Kudos to Puddin' Tain and GeorgeB who have been getting it right.
Units are important. If you are using units like amps/hr (or /min or /sec) you don't understand. If you use amps to measure power (W, kW) or energy (Ws, Wh, kWh, or (unfortunately since it adds voltage dependance) Ah) you don't understand. Carry the unit terms through your calculations and you'll see you the error of your ways. This is Jr High School algebra and you should understand it.
I don't intend to offend anyone directly. I hope to help you understand just how important this material is, and that if you get the units straight you will be an electrical rock star.
Originally Posted by anonymous
If a microwave is used sparingly, as they nearly always are, the DC amp usage is pretty low. An inverter is needed. But your 1200 watt example need not be that high. A small microwave at 700 watts AC will use about 70 amps DC through the inverter. 5 minutes of use to heat something up or make popcorn or whatever will use 70 divided by 12 or a bit less than 6 amps DC.
This is a great example. Lets walk through it.
First remember that watts are a measure of power that is NOT voltage dependent. That makes them great for dealing with voltage conversions that happen at inverters or battery chargers. Let's use my little Tappan microwave instead of the undefined one above. It is sold as a 600W microwave because that is the output of the microwave transmitter in the oven. If you read the label on the back you'll see it draws a maximum of 850 watts; the difference is due to the efficiency of the transmitter, losses in the power supply, and the control board, clock, etc. So we use 850 watts.
Actual RMS AC voltage (what you read on a volt meter) for US spec appliances should be between 117 and 120 VAC at 60 Hz (Hertz is just the frequency of the alternating cycle (AC) power). Let's use 120 to keep things simple.
850 watts (volt·amperes neglecting power factor which isn't relevant until we talk about refrigerators, air conditioners, or other device with significant rotating loads) / 120 volts = 7 amps ON THE AC CIRCUIT supplied by the inverter. We'll go back to watts to work our way through the voltage conversion in the inverter since it is voltage independent. My Mastervolt 2kW inverter has an efficiency of 95% - actually pretty decent. 850 watts on the AC side turns into 850 watts/0.95 = 895 watts on the DC side. If my batteries are fully charged and providing 13.6 VDC the microwave will draw 895 watts / 13.6 V = 66 amps. That's a lot of amps. If I heat up a casserole for 8 minutes I'll use 66 amps * 8 minutes / 60 minutes/hr = 8.8 Ah from my 675 Ah battery bank, about 1.3% of capacity. Not bad at all.
Lets look at a spaghetti dinner on an induction cooktop. Assume 3 minutes at 1500 watts to boil the pasta water and 11 minutes to cook the pasta at a lower setting - say 600 watts. We also heat canned spaghetti sauce (I'm making this simple so we don't have to saute meat and mirepoix, and sauce and paste, yadayadayada) at about 300 watts for the same total time.
3 min * 1500 W + 11 min * 600 W + 14 min * 300 W = 15300 W·min (aren't watts wonderful?)
Through the inverter 15300 W·min/.95 efficiency = 16105 W·min
Again with fully charged batteries that means 16105 W·min / 13.6 V / 60 min/hr = 20 Ah
Not the end of the world, but that does mean that if spaghetti is representative of a cooked meal and you make two a day aboard and that if refrigeration draws about the same 20 Ah you could cook with propane and have two big refrigerators and a separate freezer and come out ahead. If that's a good trade-off for you, fine - just make the decision with your eyes open and don't kid yourself that there is some magic efficiency changing the rules of physics.
Now one watt is roughly 3.4 BTUs, so a 1500 W induction hob is equivalent to a 5100 BTU propane stove burner. It can't be different. Force 10 and Eno propane cookers (for example) have large burners that run between 8000 and 8500 BTUs. The smaller burners hover around 3400 BTU. So in our spaghetti example above, the propane stove will boil the water faster than the induction hob, but that only reduces the initial 3 minutes. It isn't relevant to heating the sauce or cooking the pasta at reduced heat.
In fairness, most propane cookers have an electrical solenoid in the propane locker that draws a surprising amount of power. Many draw about 1 A the entire time they are switched on. So our spaghetti dinner on a propane cooker consumes 14 min * 1 A / 60 min/hr = 0.23 Ah. Not much, but it should be counted.
All the efficiency claims of induction are relative to electrical resistance heating, not gas.
Exercise: look up and understand current (amps), power (W), and energy (Wh, Ah, BTU).
In my opinion (all the above is fact - this is opinion) propane is the best choice for cooking aboard although it does carry some small risk. If you use a gas powered generator like the Honda to top up your batteries to support electric cooking the risk is similar. If you use a diesel generator, a diesel main, or alternative energy the risk is slightly higher for propane.
If you are so concerned with the explosive potential of propane as to rule it out you can make inductive cooking work but your battery bank better be big (off hand, 1200 Ah or so) and you should have the charging capacity to match unless you eat out a lot.