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Old 06-04-2009, 06:30 PM   #1
2 Rivet Member
1969 21' Globetrotter
Cambridge , Massachusetts
Join Date: Sep 2007
Posts: 50
Electric refrigerator check my numbers

For various reasons, I am interested in installing an electric only, compressor type undercounter refrigerator in the '69 Globe Trotter which I am currently remodeling. To this end I am looking at an economical approach using a Summit model CP-35 and at the high end, a Norcold model DE-490. The Summit, not designed specifically for Rvs, operates on 120 volts only and is energy star rated to use 290 kwh per year. The Norcold can operate on both 120 volts or 12 volts. It requires .83 Amps per hour when using 120 volts and 3.1 amps per hour using if supplied with 12 volts. My calculations are as follows.

For the Summit, 290 kwh/year divided by 365 equals .79 (say .8) kwh per day. Multiply this by 1000 to get 800 watts per day and then divide by 120 to get 6.66 amp hours per day if supplied with 120 volts (watts=voltsxamps). If supply is 12 volts, multiply by 120 and divide by 12 to get 66.6 amp hours per day, Finally, multiply this by 1.2 to account for inverter loss to get 80 amp hous per day. This all suggests that I need 160 amp hours of battery dedicated to the refrigerator for each day off city power if I want to avoid draining the battery below 50 percent.

For the Norcold on 120 volts, multiplying .83 by 120 and dividing by 12 results in 8.3 amps per hour when supplied with 12 volt dc. Why not 3.1 as listed in the Norcold specifications​? Oh well, lets assume 8 amps per hour to operate at 12 volts. Multiply by 24 to get 192 amp hours per day. Divide by 4 on assumption that refrigerator need only run 25 percent of the time, getting 48 amp hours per day. Finally, multiply by 1.2 getting account for inverter loss. The result is 60 amp hours per day which is similar to the result (66.6) for the Summit.

For all my other requirements while off city power, I expect to need about 60 amp hours per day resulting in a total of 60 + 80 = 140 amp hours per day. 140 times 12 volts equals 1680 watts per day and multiplying by 1.2 to account for loss in batteries gets to 2016 watts.. If I get a honda 2000 watt generator, is it reasonable to assume that I will be able to recharge my batteries by running it one or two hours per day? What would the inverter requirements be? Is all this in the right ballpark? Your comments will be appreciated.
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