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Friday, November 14, 2008

Calculating Your Boondocking Power Needs

One of the most-asked questions of boondockers is, "How many solar panels do I need?" The question is somewhat like, "How much money do I need to RV?" It's an individual thing--based on your consumption. We'll try and make it a bit easier to get a handle on, dealing with a boondocker who wants to run 12 volt equipment--and isn't thinking about adding an inverter to run power-hungry inverter--we'll cover that topic later.

First let's talk about measurements: One of the most important units of measure for RVing is the amp--the unit of electrical use. Turn on a "pillow style" light and you're using roughly 1.5 amps for every hour the light is on. Taking a shower? For every hour the water pump runs you'll typically use about 5.0 amps. Here's a chart of common devices used by RVers:

Device                                                              Amps per Hour

  • Incandescent light, single 1141 bulb          1.5
    Incandescent light, double 1141 bulb        2.5
    Incandescent light, single 1003 bulb         0.9
    Incandescent light, double 1003 bulb       1.8
    Fluorescent light, single 8 watt tube          0.7
    Fluorescent light, single 15 watt tube        1.2
    Fluorescent light, double 15 watt tube      2.0
    Radio/CD player (12 volt)                          2.0 - 5.0
    Furnace fan                                                   2.0 - 7.0
    Water pump                                                 3.0 - 6.0
    Vent fan (single speed)                               2.0
    TV, black & white (12 volt)                        1.0 - 4.0 Color sets higher
    Refrigerator on LP gas mode                    .25 - 1.5 (The lowest figure is to operate the control board. Accessories like cooling fans or use of door seal heater--for high humidity result in higher consumption. The board is on 24-hours a day)

By now you're probably wondering: Why are we worrying about amps, when solar panel output is measured in watts? That's because there's an intermediary involved: The house batteries. Your RV batteries act as a middleman. During the day when the solar panels are putting out power, the batteries act like a banker, storing up the energy. When there's little or no power produced by the panels, the batteries supply your electrical needs.

Battery storage capacity is measured in amp-hours. When setting up a solar system, it's best to look at the battery needs first. Rule of thumb: Calculate your average daily power needs, then double it. The result is the minimum required amount of battery capacity. As an example: An RVer whose daily power needs are say, 20 amp hours, double that, the RVers will need battery capacity of 40 amp hours.

Now we bring in the solar panels: For every amp-hour of battery capacity, provide 1 watt of solar panel production. In our example, our RVer will need a minimum of 40 watts of solar power. This allows a bit of occasional cloudiness, and keeps the batteries from being discharged down to such a level that they'll lose much of their effective life. This all assumes that your solar panels will "see" a good six hours of bright sunshine every day. Incidentally, a 20 amp hour need is, in our opinion, a pretty frugal RVer!

As we mentioned earlier, all of this is predicated on the thought of an RVer with simple needs--using 12 volt equipment only. Adding an inverter to provide shore power will make your calculations a bit more complex, and we'll hit those in a future column.

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