...So I am standing there with my coffee thinking why am I installing this oversize inverter, xantrex 120vac transfer switch, xantrex remote on/off switch etc ( got em all for real cheap) when I could use some of the appliances I saw at this truck stop?
Do these 12 volt appliances work good?
All I guess I really wanted to use was my 950 micrwave to heat food and drinks up....
To the original question...
Don't forget wire thickness. You'll need some very big, expensive/heavy cables to move the microwave's 100 amps with little loss. (Your inverter needs close to 12v to operate.) That's the one reason to get an inverter, unless you're going to put that 12v coffee maker right next to the battery bank. Running an inverter means current goes down to 1/10th of what it was before, and the cables get much smaller. Put another way, Power Loss = Current X Current X Wire Resistance, so lowering current has a massive effect on the wire resistance you can use. It's why power is moved around the country at very high voltages on high tension wires.
The other reason is that there are a lot more appliances out there that work with 120v AC.
Seconding what others have said...
Compared to the other parts of your system, your inverter is too big and will have too much loss. (Full disclosure, we have a 2000W Xantrex on our 50' boat and I would consider trading/buying with you, but our 2000W with 5000W peak serves us excellently, so I'm not sure I'd trade/buy anyway. Money-- especially shipping costs -- could be better spent elsewhere.)
We do exactly what you are talking about -- we cook potatos and pop microwave popcorn and heat things in our microwave. But for heating in general, it's better to use fossil fuels, unless you are prepared to replace that energy. Our wind generator keeps the batteries topped up (with the fridge off) so that we have energy to spare. But still we only use the microwave sparingly.
I bought a common, 120v household toaster (because I love toast in the morning). And it pulls 160 AMPS!!! from the batteries. That's more than the mircowave uses. Toast takes maybe 5 minutes and I would watch the Victron battery monitor as the batteries drain down. (Your 40 amp alternator means running the engine at a good rpm for 20 minutes to replace the energy!) I went and bought a toaster frame that fits over the propane stove and gave the toaster away to a homeowner. Now I make toast faster and don't feel like I'm blowing the energy budget everytime I make toasted bread. Lesson learned is that heating with electricity doesn't make sense, except for re-heating in the microwave. We do jiffy-pop popcorn instead of microwave popcorn, and we haven't microwaved potatos since that first time - when we had the engine running. Instead, use electricity for non-heating appliances like stereos and TVs
(Useful note: low power microwaves suck at making microwave popcorn.)
But I understand the attraction of microwaves. I like melted cheese on chips, for instance. Just make sure you 1) can store enough energy to not hurt your batteries when you run it, 2) create enough energy to replace the power, 3) have thick enough cables between the batteries and the inverter, and therefore 4) generally have a spot for the inverter very close to the batteries.
P.S. We had the 120v Christmas lights on last night, run off the inverter. Up the backstay and down a line tied to the bow pulpit. It was beautiful.
P.S.S. If you have ice, you can run a blender. All the naysayers will flock to your boat and change their tune -- just see 1 to 4 above.
P.S.S.S. Bravo on getting a full sine wave inverter(I think that's what you have anyway). The power to our microwave went from 60 amps to 120 amps with the full sine wave inverter. That makes a huge difference in the effectiveness of the appliance.