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Old 09-10-2016, 08:01 AM   #13
CaptTom
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City: Southern Maine
Country: USA
Vessel Model: Prairie 36 Coastal Cruiser
Join Date: Dec 2012
Posts: 1,904
I have a 1980 Onan MDJE, 7.5 KW. Also has some oil in the tray after running for a while, although I haven't pinpointed the source yet. It isn't enough to worry about yet, but I know the day will come when I'll face the choice you're facing.

I can heartily agree with the comment about these things being serious rattlers. With the sound shield and an AquaLift muffler, it's whisper-quiet outside the boat. But the genset is mounted directly on the stringers, so the vibration inside is pretty bad.

As for sizing, I think the builder just added up all the potential loads and sized as if they'd all be used at once. I have two, 30A shore power circuits. I'd have to max out both (60A x 125VAC = 7,500W) to max out my generator. I've never done it.

I like TwistedTee's approach better.

I have two, 15,000BTU air conditioners, a 100A charger, 1,500W water heater and electric range/oven. I never use more than one 30A shore power cable, split to power both sides.

I can get by with 20A, or even 15A shore power, but it requires some load management.

My typical genset use would be after 12-24 hours on the hook, with the batteries down to 70%-75% SOC. The charger starts pulling 10A (1,250W) for a while when first powered up. The water heater will pull 12A (1,500W) for a half-hour to 45 minutes. By then the batteries are in "accept" mode and the charger draw is negligible. Even if I add in 1,500W for cooking, I can't come up with the need for anything larger than a 5KW genset.

The wildcard here is your air conditioner use. With a 5KW generator, I'd be worried about starting the air conditioners with too many of the other loads going. I don't do this now, mostly because running the genset that long would rattle all my fillings out. But if I had a quieter unit, I'd just manage my loads. Maybe shut down one air conditioner while running one of the other big loads. Some would find this inconvenient. I don't.
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