Mick and Janet said:
I have seen a few solar panels for charging your 12v car battery. The
really cheap ones output 200-500 milli amps whilst the larger panels output
10watts. How do you convert the 10w into amps?
I have a series of 12 garden lights that come on at night. I was hoping
to get rid of the power inverter and run them direct from a 12v car battery.
At the moment there is a 70w transformer to power 10 small lights. Assuming
they come on for 3 hours a night, thats 210w a day that is needed.
How do I convert those 210w back into amps, so I can work out how big a
battery I need, and how big the panel needs to be.
The basic formula is W = I*V (Watts = Current * Volts). In order to charge
a 12V battery, "12V solar panels" actually produce more than 12V. They're
usually 14V or higher. It should say on the panel when you buy it what
actual voltage to expect. So, assuming the 10W panel you saw has, say, 15V,
you would plug that into W = I*V to get I = 0.6667A (or 666mA).
You have a slight misunderstanding of wattage. A watt value cannot be
increased by multiplying by time. An analogy I like to use is thinking of
wattage as a speed measurement. If you're going 60mph for 3 hours and you
want to know how fast you're going after 3 hours, it's still 60mph.
Similarly, if you're using 70W for 3 hours, you're still using 70W at the
end. If you want to measure a quantity of energy, however, that's when you
multiply by time. Just like in your car, if you want to measure how far
you've gone, you multiply your speed by time. So here the analogy is
Wattage is to Speed as Energy Used is to Distance Driven. So, by
multiplying 70W by 3 hours, you get 210Wh (watt-hours), which is very
different than 210W. A value of Watt-hour is an actual quantity of energy
while a Watt represents real-time power usage.
If you already knew all that and you just forgot to add the "h" onto the end
of W, then my apologies for such a long winded explanation. So for your 70W
system, you need to make sure you have a battery than can easily handle a
210Wh drain every night. If you're using a 12V battery, it'll have to put
out 5.83A (5.83A = 70W/12V) for 3 hours a night, which is 17.5Ah (17.5Ah =
5.83A * 3 hours). Assuming you intend to charge that battery during the day
with a solar panel, you need to make sure your panel can produce at least
210Wh during the course of the day. So assuming 4 hours of good sunlight,
you'd need a 52W panel to do the job. That number will vary depending on
how much sun you get in your area.
