PV solar
I wrote this out before and the ether has eaten it. Perhaps it will show up somewhere else.
So lets try this again! As the good Captain sez,,,"Do the math!" Let us start with a some basic assumptions and a bit of a primer about solar panels. In addition to doing the math on the load side, you have to do the math on the PV side. Solar panel output is rated under what we call Standard Test Conditions, or STC. This is so that one panel can be compared to another. In addition to the voltage and the amperage of a panel, it has to be compared with similar sun and similar temps. Contrary to intuition, PV panel efficiency drops off considerably as the PANEL temp rises. My panels put out way more than their ratings at -40, but way less at +80f. The average panel is rated at ~80% at STC In addition there are battery charging losses that are not insignificant. The rule of thumb is that for every amp/hour (ah) you take out of a battery, it takes ~1.25 ahs just to stay even. (the perkett effect?) Solar panels are constant voltage (relatively) devices. They operate at a fairly constant voltage, with the amperage (current) changing according to sunlight conditions. As such there is another considerable loss to calculate between the PV panels STC output voltage, ~17vdc for a 12 vdc (nominal) panel. The voltage above the battery voltage is lost. Add in charge controller losses, wiring losses etc, and you end up with a REAL panel output projection.
So lets take all this, and use real numbers. Lets say for the sake of this conversation you have a 100 watt, panel. In general we consider anything above 50% to be doing well, that would be 50 watts of output. Let's see how we get to that number. 100watt panel @ 80% efficiency=80 watts, charging efficiency 80 watts@ 80%=64 watts, controller, wiring losses, say 5%= ~60 watts. Still not to bad. (The numbers are technically different if you do the math in a different order, and in fact the charging ef should come last, it will make the numbers worse, but I'm tired of writing). Now consider this, if you have a 60 watt, 17vdc panel, would put out ~3.5 amps right? But wait, it is not putting out 3.5 amps into 17 vdc, but rather 12.? -14.5 vdc. So at 12.6 vdc (full charged battery at that same 3.5 amps would only be ~44 watts!!!!. This is because the panel cannot compensate for te battery voltage that is lower than the STC panel voltage. There are expensive MPPT controllers that can help this, but you can never eliminate it, and they are expensive for such a small installation.
(BEWARE BZ PRODUCTS!!!)
So now you are left with a 100 watt panel that really only puts out 44 watts. Getting pretty shaky. Let's see what we can do with these 44 watts,,,,oh wait, we have to factor in what happens if we have less than perfect solar conditions. A clear, cold day might produce the 80% STC rating. Now factor in warm weather, haze, normal stuff AND most importantly on a mobil mount, and that my drop off to nothing. Lets be generous and say 80%, so now our panel is down to ~35watts. One important thing to note is that even the simple shading of part of a panel by a branch for example, can drop the panel output to near zero. This is because most panels are wired in a series, or series/parallel array. Any shading on any individual cell will serve to block the entire series!
So now we have our 35 watt (net) panel up and running what can we do with it 35watts divided by 12.6 volts equal 2.77 amps, under all the conditions set forth above. Give an average of 4 hours of sun a day (remember, if the panels are not at right (and correct!) angles to the sun, their out put will drop substantially, so if you are not going to re-aim your panel every 15 minutes you should count on 4 hours). 4 hours times 2.77 amps equals a whopping 11 amp/hours of charging, or put another way, enough to run a good cd player radio for ~10 hours! Put another way, it is about the equivillent of running the 90 amp car alternator for ~ 6 minutes!
So, long story short,,, Do the math.
If you take the the OP's ~20 watt panel, you could run the radio for about 3 hours.
Icarus
PS I'm still lurking about, even though I am not as active as I was in the olden days!
PPS. Just so that I don't give the mis-impression that solar energy is a loser. Quite the contrary. I am whole hog into and Pro-solar, but in this context it pays to "do the math" A modern grid-tie PV system of 2kw or better can under good site conditions yield efficiencies of 80-90%. The rub comes in when you start adding batteries. The cost of the batteries roughly doubles the cost of a stand alone system. ( a grid tie system needs no batteries, AND through MPPT (Maximum Power Point Tracking) inverter technology can convert to grid voltages/phases with efficiencies better than 95%). The scale of the loses is way less as the system size increases.
Point of fact, grid tie systems can be ALMOST competitive with grid power under ideal circumstances. (not the least of which is the political climate for energy policy). When you see the price of grid power double in the coming few years (as it has with most energy!) then it will become competitive in most areas of the country. If you are interested in reading more, I invite you to visit
http://www.wind-sun.com/ForumVB/index.php. This is a forum for users and those that are interested in building systems and learning, a forum that I am fairly active in.
Just for another comparison. I live ~ 6 months of the year on a small island in Northern Canada. We live completely off grid. Our solar system is tiny by most standards. 200 watts of panel, 500 amp hours worth of batteries. We use about 30 ah/day,,,360 watt hours! That for all lighting, water pumping, radio, satellite internet,,,no TV though! Just shows you can live well with little energy use!
T
T