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Kyocera 120-Watt Solar Panel

Mobile Power and Draw

Solar, Alternator, 120V and AGM Battery Power

Flip a switch and a light comes on. Submerge some bread into a toaster. Drill a hole in the wall for the heck of it.

With electricity from the grid these mundane activities require little consideration for the power draw beyond that they should be extinguished when finished.

But when power comes in limited batches it is essential to conserve it. It soon renders a scrupulous miser of its user. Well, that's what happened to me anyway. The day the battery power ran dry I realized my frivolous ways were to cease...until I'm back on the grid anyway.

What's all this for?

While travelling and working out of and around my van a number of valuable utilities require considerable amounts of electricity from a 12-volt system.

Lifeline GPL-27T Deep-Cycle AGM Battery

This Lifeline GPL-27T Deep-Cycle AGM Battery weighs in at 65 pounds and is rated at 100 amp-hours.

  • Laptop : 4.7 amps
  • 65W Peltier-Based Cooler by Koolatron : 5.3 amps
  • Black & Decker Drill : 39 amps
  • 2 hp Wet/Dry Vacuum : 60 amps

Others require much less...

  • 6 Ceilings Lights : 1.2 amps
  • 1 Passenger Reading Light : .9 amps
  • Charging a Smartphone : .4 amps
  • Charging the GPS : .5 amps

Watts, Amps and Volts

To put things in perspective the relationship between watts, amps and volts must be understood:

Watts = Amps X Volts

Yep, that's it. Watts is a measure of electrical power independent of voltage. On a 12-volt system a 100-watt light bulb draws about 8 amps, whereas on a 120-volt system the same bulb draws less than 1 amp. Similarly, a typical household refrigerator might surge to 2400 watts when its compressor kicks in and level out to 1200 watts. On a 12-volt system this 100- to 200-amp draw would require heavy duty 1-gauge cable composed of numerous tiny copper wires. On a 120-volt household system the 10- to 20-amp draw allows the electrical wires to be comparably narrow and simple. The higher energy requirements of households and businesses necessitates the 120-volt and 240-volt standards. But the power flowing through the wires is the same.

Battery Storage

Good batteries are at the core of many stable electrical systems, and deep-cycle AGM batteries are a preferred modern choice. For my application I selected two Lifeline GPL-27T rated at 100 amp hours. This is the standard rating based on a 20-hour rate. While this is subject to many conditions it is an industry-standard comparative measure of battery life.

Electrical Outlets

Some utilities, like cabin lights, are wired directly to the batteries with only an intermediate fuse required on the circuit. Just about everything else gets plugged in. The laptop can be fed with either a 12V or 120V adapter, the latter of which must pass through an inverter to convert from 12V direct current to 120V alternating current.

1200W DC-to-AC Inverter

Inverters are ranked by maximum wattage they can handle. In my case it's a Xantrex 1200W unit with a couple of outlets. The maximum draw is 100 amps from the batteries, delivering up to 10 amps at the new voltage. This has been sufficient for all my needs so far.

Cigarette Lighter Plugs

Utilities like the laptop, Smartphone, GPS and Cooler can also be run directly from the batteries via standard vehicle cigarette lighter outlets. For convenience I've installed two more in the cabin besides the two up front in the dashboard.

Direct 120V Outlets

When plugged into the grid the 120V source can be used instead of draining the batteries. This is typical of most RVs and motorhomes when at a site with hook-ups. This electrical source can also be used to charge the battery bank.

Charge Controllers

From left to right: the SunSaver-10 solar charging controller; the Iota Engineering DLS-15/IQ4 15-Amp power supply and AC charger; and a 100-AMP shunt to tie it all together.

Charging Options


The alternator powered by an internal combustion engine usually has ample amperage to charge both starting battery and house batteries. My Ford E250 van is equipped with a heavy-duty 115-amp alternator easily able to push 30 amps into the house bank if necessary. However, this is usually only an initial charging rate for semi-drained batteries. Using a special charge controller designed for deep-cycle marine batteries, the alternator's capacity is moderated to a range of 2 to 15 amps to charge the batteries in cycles to help prolong their life.

Kyocera 120-Watt Solar Panel

This Kyocera 120-Watt Solar Panel has generated up to 7.2 amps at 13 volts under ideal conditions, but can unexpectedly dip to less than 1 amp when the sun is obscured by weather or trees, or simply not pointing directly at the panel anymore. *sigh*

Solar Power

Besides 120-volt "shore power" and alternator juice, the next option is finnicky solar power. Granted, solar power is an excellent renewable source of clean energy and hot water. For remote housing in sunny areas it's quite ideal. But for mobile applications it can often be labor-intensive. Although it might only yield optimal results for a few hours of the days, often that's enough for conservative demands. With this 120W Kyocera panel I might get up to 7.2 amps under ideal conditions. Solar panels typically output up to 20 volts in bright sunlight on a cool day. Most economical solar chargers will clumsily convert the amperage at that voltage even though the final voltage has been reduced to the bank's voltage. For example, a 120-watt panel outputting 6 amps at 20V might be knocked down to 8 amps at 13 volts by the controller, which is about 104 watts real output, with 16 watts lost in the conversion.

And those are ideal conditions. Panel pointed at bright sunlight continuously. Professional installations will rotate panels to follow the sun's path throughout the day. In mobile applications the panel is typically affixed to the roof of the vehicle. In my case I go out and tilt the panel when the amperage starts to dip a tad. Balance it on a tree or a boulder or against a tire. Once in position it might yield 6 or 7 amps for an hour or two, then dip to 1.3 amps while my power consumption remains steady at 6 amps.

The Trimetric

The last component of this electrical system is the Trimetric by Bogart Engineering. Using a heavy-duty 500-amp shunt between the negative terminals of the house battery bank and the van's chassis, the Trimetric measures power draw in amps and watts, and monitors the voltages of both the house and starting batteries. This would be fine, but the Trimetric also counts how many amps have been removed from the bank since last full charge. And also includes a sort of odometer of battery life, since all batteries—no matter how expensive—will slowly lose the ability to hold charge and eventually die. Knowing these figures is an important part of tracking battery performance and optimal utility, and practicing energy conservation.

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