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Old 02-22-2013, 02:28 PM   #1
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2013 22' FB Sport
College Place , Washington
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Load Calculations

I am trying to calculate approx loads to predict whether I should be purchasing solar first or a generator. The largest loads appear to be the following:

Refrigerator: Dometic RM 8501 ??
Furnace: Atwood 1912 (3.4amps at 12v)

I can't figure out what the load is for the Dometic based on the info in the manual. And I am not sure what a valid guess is on the furnace. Mostly edge season and summer camping, so perhaps 15 minutes an hour?

Thanks for the input.

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Old 02-22-2013, 03:52 PM   #2
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No-brainer. Generator first. Solar is mainly a trickle/float charger for your batteries, not a source of power for appliances. Plus it won't help if you camp where you've got tree cover or several days in a row of overcast.

Generator, on the other hand, can be a substitute for shore power all by itself for AC as well as charging the batteries. And it works perfectly well in shade or overcast.
I thought getting old would take longer!
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Old 02-28-2013, 11:15 PM   #3
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For my solar load calculations I went to the extreme on everything. For furnace amps I calculated the heat degree days for my location, my approx. R-value etc. then I took the coldest day I expected and arrived at a worse case scenario for amp use. The fridge amps depend on if the draw is only from control circuitry or from a 12v heating element. If you plan to run fridge on just DC power the panels will most likely not keep up as even the most efficient fridges use around 5 amps per hour X 24 hrs ( not enough roof to supply power from panels alone) Don't worry, humans have survived for thousands of years before the advent of generators or solar panels, if you get creative you'll survive just fine !
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Old 02-28-2013, 11:46 PM   #4
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For 110-volt electrical appliances, a device similar to the one below can provide the data you seek: P3 International P4460 Kill A Watt EZ Electricity Usage Monitor: Home Improvement

For 12-volt appliances, use an ammeter or multimeter to measure current. Hook-up instructions vary with the type of meter used, so consult meter operating instructions to determine how to connect test leads.

As an aside, has lots of anecdotal information on solar panels and generators. Use SEARCH to find discussion threads and specific posts on these subjects.
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Old 03-01-2013, 02:07 PM   #5
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2013 22' FB Sport
College Place , Washington
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I was just trying to figure out if the refrigerator EC load while on LP was negligible or not and looking for ideas on a way to calculate a reasonable guess on the furnace.

Sounds like the DC load from the refrigerator is negligible since the manual leaves it out of the manual on LP mode?
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Old 03-01-2013, 04:09 PM   #6
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Western , Massachusetts
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We had a Trimetric 2020 and measured the load on a smaller Dometic on our last trailer. The DC load was 0.3 amps.

Hope that helps.
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Old 03-01-2013, 05:12 PM   #7
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We have a 5 cuft Dometic. Our Trimetric monitor measures a current draw of 0.4 amps to run the DC circuit board. I don't remember what the furnace draws exactly.

Funny how people can have opposite opinions about generators and solar. We have zero intentions to run AC with a generator and we have absolutely no desire to haul a generator around. Given that we can go about 5 days on the two plain-Jane flooded Group 24 batteries (no furnace or inverter), solar will give us the ability to go as long as we'd practically need.

Now: 2007 Safari SE 23' "Anne" towed by 2011 Dodge Durango "Herman"
Before: Argosy Minuet and T@B, towed by various Honda Odysseys
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Old 03-01-2013, 11:29 PM   #8
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July & August, southwest desert, boondocking, 115+/95 degrees F. --
2 Honda 2000's, air conditioning.
Or, head for the Rockies!

Holiday season, Connecticut, boondocking, 32/15 degrees F. --
1 Honda 2000, furnace and/or heat strip in air conditioner.
2 Honda 2000's, furnace and/or heat pump.
Or, head for Arizona!

Or, stay home...
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