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Old 09-10-2016, 07:14 AM   #11
twistedtree's Avatar
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 5,125
I think I would calculate it in two different ways based on how I see generators getting used.

1) When air con is in use, you will need the gen on all the time. In this case your load will be the AC units plus what ever other loads you will want at teh same time, or that you can't conveniently control. I would probably figure in the two AC units, plus the stove or the hw heater. The stove and HWH are probably similar loads, and you could control use to only one at a time without too much hassle. I''d size that for maybe 75% of the gen capacity to allow for misc stuff.

2) When you don't need AC, I think the gen will get used a couple of times a day to recharge batteries, make hot water, etc. In this case the loads will be the HWH plus your batter chargers. Some people like lots of charging capacity to load the generator and speed up recharge. You also might want to be cooking at the same time, so you might want to figure in the stove at the same time. Once again I'd size this for about 75% of gen capacity.

1 and 2 may come out about the same number, or they might be very different. But I think you will start to triangulate on the right size.

You might also want to look at your min load which is probably in case (1) above when you are asleep with AC running and it's cycling on and off, so maybe figure one AC unit running. Long lightly loaded gen runs are not great, but in my experience unavoidable if you want to use the AC, and there are times when AC is a must for whimps like me.

On bigger boats, I know a lot of people with two gen sets, one big and one small. These are typically boats that run a gen all the time, or most of the time, and the little gen runs at night, if not most of the time when operating in case (1). Then the big gen gets run when at anchor with battery charging, laundry, water making, hot water heating, etc. all happening at the same time. But I bring this up only to illustrate that there are lots of operating strategies for gens and you'll need to pick what works for you. Two gens would be silly on a 40' boat.
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