How many batteries do I need for my solar panels?
people start sizing their solar setup by figuring out how much energy
they use in their home, then choosing enough panels and batteries to
provide that much juice. We're coming at the issue from a
completely different direction.
We plan to buy
a few solar panels
now and then as we can afford them, to hook up the appropriate number
of batteries, and to slowly work various
parts of our household off the grid. For example, our first
panels might be just enough to power our laptops and a couple of
lights, so we'll wire those gadgets directly into the solar setup,
sticking to DC if possible. Since it's a bad idea to hook old and
new batteries together, we'll probably keep each little system
separate, which will cost a little more but will provide backups.
Which is all a long way
of saying that we have two 45 watt solar panel kits (each of which
consists of three 15 watt panels) and we're trying to figure out how
many batteries we need to support those panels. My first step is
to determine how much energy we're likely to get from the panels on an
You can either size your
system based on the average peak sun hours (which will give you an
over-estimate for the winter and an under-estimate for the summer) or
based on the winter peak sun hours (a worst case scenario). If
you wanted to ensure you had enough power even during the shortest days
of the year, you'd want to use the winter peak sun hours in your
calculations, but I think it makes more sense in our system to use the
average peak sun hours, which is roughly 4.2 for our area.
The amount of energy
your solar panels will produce per day can be calculated using this
Solar panel output = Solar panel rating (watts) X Sun hours
Solar panel output = 90 watts X 4.2 hours
Solar panel output = 378 watt-hours
Most solar systems
recommend that you multiply your solar panel output by by 3 (or 4 or 5)
when sizing your battery bank so that you'll still have juice after
several days of cloudy weather. Since we're not planning on going
off-grid anytime soon, I think I'll stick to the bare minimum figure
above, though, and just plan on plugging our appliances back into
on-grid power when our batteries get low.
Next, you can calculate
how many watts the battery of your choice will hold. Batteries
are generally rated by volts and amp-hours, which allows you to
calculate watt-hours as follows. (This example is a typical golf
cart battery: 6 volts and 200 amp-hours.)
watt-hours = Volts X Amp-hours
Battery watt-hours = 6 volts X 200 amp-hours
Battery watt-hours = 1,200
That sounds great,
right? I'd just need one battery for three days!
Wrong. Deep cycle batteries lose a lot of life if you discharge
them below 30 to 40%, so the amount of usable energy in the battery is
watt-hours = 0.6 X Battery watt-hours
Usable watt-hours = 720 watt-hours
It's still looking like one
golf cart battery would be enough for nearly two days, but there's one
more factor to consider. Since we want to plug 12 volt DC
appliances directly into the system rather than losing efficiency by
converting from DC to AC, we need our battery system to be 12 volts,
not 6 volts. That means we need two batteries wired together in
series to boost the voltage.
As a side note, we got
an advertisement from Harbor Freight as I was researching, and the
company wants to sell me a "solar battery" for $75. Was it a good
deal? The battery is rated at 12 volts and 35 amps, so it would
hold 420 watts, or 252 usable watts. The benefit of the battery
is that it is already 12 volts, but I'd still need two of them since
the battery isn't even enough to soak up the energy from our solar
panels for one day. If I'm doing my math right, the Harbor
Frieght battery would cost 30 cents per usable watt, versus 21 cents
for the golf cart battery (using a rough estimate of $150 for the cost
of the golf cart battery). Of course, I'd need to factor in
longevity to really get an idea for which set of batteries would be a
better deal, and there's no information about Harbor Freight's battery
life span on the internet. Since our nearest golf cart battery
supplier is closer than our nearest Harbor Freight, we'll probably
settle on the former.
Our chicken waterer is perfect for chicken
tractors since it never spills on uneven ground and is easy to fill
without crawling into the tractor.
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