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.