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Old 05-11-2013, 12:29 PM   #23
twistedtree's Avatar
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 4,223
I think you guys may be overly optimistic on the power draw calculations, especially for AC. If you run the AC 24x7 then once the boat is cooled down you might be able to keep it cool with a 50% duty cycle 24 hrs a day. If you only run it 10hrs per day, I think you will get more like 100% duty cycle as it works to cool the boat down. Also, 12k BTU doesn't seem like much AC for a 54' boat. If you take a look at your AC units you should be able to see what they collectively draw, and from your current use perhaps you can get a sense of the duty cycle.

In general AC is a brutal load on solar/batteries. Electric cook tops are too, but they tend to only run for 15-30 minutes or something in that range (no pun intended), but AC is relentless. Your fridge and other loads should be manageable with solar, especially if you get yourself changed over to LED lighting.

There are tables where you can look up the equivalent number of hours of full sun light for different parts of the country. For DE, I'd guess it's around 5 hrs. That number takes into account weather patterns and expected shading over the course of a year. You could get weeks of clear skies, but you could also get weeks of overcast. The 5hrs is what it all averages out to. If you are only powering in the summer, then you will get bteer numbers since the 5hrs also accounts for seasonal variation.

Anyway, with 5hrs of equivalent full sun a day, that means that 2kW worth of panels will produce 10kW-hrs of energy per day. But that assumes you can capture 100% of that energy, and you usually can't. As batteries become charged they accept a slower and slower charge which means available power is being forfeited. I'd probably de-rate everything by 80%, yielding 8kW-hrs produced per day. That's still pretty good power, but I think you are going to be hard pressed to get very satisfactory AC performance out of it. Maybe if all you cool is your stateroom at night? That's probably what DeckOfficer is envisioning.
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