I have a 100 watt panel on the roof of my diesel suv, I didn't buy the panel for this application, it was purchased for a standalone house power system which has since been replaced by a mains power connection. It seemed a waste to have a panel sitting around doing nothing so I fabricated a reasonably aerodynamic roof bar for my suv and mounted the panel on the roof. First off I connected the panel via a charge controller to the battery, the effect was glaringly obvious. The battery was always at full charge, so when cranking the engine the starter motor spun the engine over faster than it did without the panel connected, which made for less cranking time to get the engine started. Once started (this is a diesel engine with power hungry glow plugs) the battery volt gauge/meter didn't sit on the usual 12.5 ish volts for the first few minutes, the voltage now sat around the high 13 volt mark, and reached the peak 14 volts (or there abouts) in about 2 or 3 minutes compared to around 10 minutes while running no panel. On a sunny day with no computer to run (only a 12 volt fuel cut solenoid) the alternator was effectively relieved of its load. I didn't check for fuel savings, but the engine didn't feel as sluggish from a cold start. I'm now considering the purchase of a dual battery controller, which could first direct the charge to the battery, then could switch the charge over to a block heater which would shorten warm up times. A hidden bonus is longer battery life via reduced sulphation, of course there is the reduced aerodynamic efficiency, but I can't detect this via my present "cruise fuel use indicator" (the boost gauge).
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