AR15.Com Archives
 how many amps from a solar panel?
snakeshooter1  [Member]
6/20/2008 4:36:34 PM
how many amps will a 45 watt solar panel put into a battery bank? I'm trying to understand and relate solar panels to a normal 110 volt battery charger.
Paid Advertisement
--
cleb  [Member]
6/20/2008 5:02:38 PM

Originally Posted By snakeshooter1:
how many amps will a 45 watt solar panel put into a battery bank? I'm trying to understand and relate solar panels to a normal 110 volt battery charger.


ohms law watts = volts * amps

so

45 = 12 * x

45/12 = x

x = 3.75

now those are under ideal circumstances... you can see less or more depending on the panels and the conditions.

I should also note... the panel is probably putting out more the 12 volts so you would have to adjust the amps down to compensate.
snakeshooter1  [Member]
6/20/2008 5:15:20 PM
so a hundred watt panel would actually make about 7 amps of power per day?
PhilipPeake  [Team Member]
6/20/2008 5:32:45 PM

Originally Posted By snakeshooter1:
so a hundred watt panel would actually make about 7 amps of power per day?


Hmmm...

Ok, lets back up a bit.

Amps is a measure of current.
Volts is a measure of electo-motive force (the "push" behind the current).

V x A = Watts

Watts is a measurement of power (work done).

Look at a light bulb and its power is marked in watts -- a 60w light bulb consumes 60watts of power when it is fully lit.

Watts is a measurement of instantaneous power -- how much power is the light bulb using now.

Since Watts = volts x amps we can achieve the same power with different voltages and currents.

E.g.

1amp x 100volts = 100watts
2amps x 50volts = 100watts
100amps x 1volt = 100watts

When measuring total energy used, we measure by multiplying watts by time.

So a 60w bulb run for 1 hr would consume 60 watt-hours.

Your home electricity bill is tells you how much power you used in Kw hours (1000w-hours).

Now we have the basics, lets go back to your question.

If we have a solar panel which delivers 100w of power at 12v, the current is 100/12 = 8.34 amps.

If you can get that same output for 10 hours of the day, you will get 100w x 10hr = 1000w/hrs stored up in your battery.

So you could run a 1w bulb for 1000hrs, or a 10w bulb for 100 hrs etc.

Now, there are a few things to take into consideration:

100w is the best you can get from the solar panel -- doesn't mean you will actually get that.

The intensity of the light varies throughout the day, so you will probably only get the max power around midday.

Batteries are not 100% efficient - put 1000w-hrs in, and you will get somewhat less out.

Batteries are not really linear -- if you have a 1000w-hr charge, you may get something like 999hrs runtime for a 1w bulb, but you will not get 1hr runtime for a 1000w bulb.

Also, if we are talking lead-acid batteries, you don't want to discharge them more than ~ 50% or they don't last long -- which means that the practical capacity of a 1000w-hr batter is really only 500w-hrs.
Lowdown3  [Member]
6/20/2008 5:47:22 PM

Originally Posted By snakeshooter1:
so a hundred watt panel would actually make about 7 amps of power per day?


It would produce about 7 amps per hour of sunlight.

Down here we get approx 5 good hours of sunlight on a full sun day. That's "average." You could get all technical with everything if you wanted to.

Keep in mind also that the smaller panels still have more "shading" issues than the newer and larger panels do.
Gunlover66  [Member]
6/20/2008 6:02:00 PM

Originally Posted By snakeshooter1:
how many amps will a 45 watt solar panel put into a battery bank? I'm trying to understand and relate solar panels to a normal 110 volt battery charger.


My Siemens 55 watt panels put out between 2.87-3.10 amps. They are rated at 3.17 amps. I have 8 of them wired for 12V. According to my Trimetric meter I averaged 146.16 amp hours per day for the last seven days.
Paid Advertisement
--