# Solar Array Sizing: Off Grid Solar Power System Design – Step 3

Hi, welcome back. I'm Amy Beaudet from the altE Store. Thank you for watching the 3rd of our video
series on designing an off-grid solar system. Step 3 is to size the solar array. We'll discuss the different considerations
that go into determining how many solar panels you need. We're making good progress, videos 1 and 2
showed how to determine the loads and battery bank, now we'll do the solar panels. Just a quick reminder of the components that
make up an off-grid system. We're onto the panels. You may recall the loads list we did in our
previous video.

We came up with a total usage of 2191.5 watt
hours a day. So how do we go about this? Insolation, also referred to as Sun hours,
is not the number of hours a day that the sun is shining. It is the number of hours that the sun's intensity
is equal to Standard Test Conditions, or the brightest part of the day. For example, since the sun is twice as bright
at noon as it is at 9AM, 9AM to 10AM only counts as half an hour, whereas noon to 1PM
would be counted as a full hour. NREL also took historical weather conditions
into account for each area, so it is a pretty accurate prediction of how much sun you'll
get. This is a snapshot of Average annual sun hours
for PV mounted at latitude tilt, which is the angle that generally gives you the best
year round performance. You can see that here in Massachusetts, we
have an annual average of about 4.5 sun hours a day. However, when you size an off-grid system,
you must plan on worst case, which, unless it is a seasonal camp, would be winter. As you would imagine, the sun hours drops
off dramatically in the north.

So here in Massachusetts, in the Winter, we
drop down into the 2's. So designing my system, I'm going to look
at the sun hours for December. I'm designing my system to be tilted at 42
degrees off horizontal, so I can look at the numbers for latitude, which is 42.27 degrees
here. If I had it at a steeper angle, I would increase
my winter production, but decrease my summer production. Conversely, if I had it installed at a lower
angle, I would increase my Summer, and decrease my winter. Because winter in New England is pretty grim,
with December going days without ever seeing the sunshine, I will be planning on using
a generator when needed. But I'm still going to design this trying
to optimize my solar contribution, and only use the generator when needed. Let's face it, there is no such thing as a
perfect system with no losses. But in an off-grid system, there's a lot more
than you would expect. Due to things like soiling of the panels and
voltage drop in the wire, losses in batteries and electronics, and several other factors,
we'll be losing about 1/3 of the rated power from the panels.

So a 100W rated panel may actually give us
only 67W of usable power, or less, after all losses are factored in. Here's the steps to calculate the minimum
size array you need. You take the watthours that you calculated
from your loads list. You may recall in our example it was 2192Wh
a day. We divide it by the number of sun hours, we
are using 2.8 hours. Then divide it by the system inefficiencies,
or .67. This equals at least 1168 watts of solar needed. There are so many options that are available
for solar panels, selecting which solar panel you use can seem daunting. First step is to narrow down what your priorities
are. Do you want Made in USA, or other specific
country, or NOT made in a specific country? Do you want the panel frames to be black or
silver, and the cells to be black or blue? Is there a particular brand you want to use? Or is cost the only criteria you care about? For my system, I picked a nominal 24V, Made
in USA panel, SolarWorld's 315W monocrystalline panel. If the panel you select doesn't say what its
nominal voltage is, you can look at the specs.

A 24V panel has a Voc of about 46V, whereas
a 20V nominal panel has a Voc of around 38V. Now that we know how many watts of panels
we need, and which ones we are using, the rest is pretty simple. You do need to make sure that the number of
panels allows you to wire them so that the nominal voltage of the panels, either matches,
or is higher than, the voltage of the battery bank. In this example, I can wire them in 2 parallel
strings of 2, which gives me a nominal 48V array charging a 48V battery bank. Perfect! If I had determined that I needed 5 panels,
I would have had to increase the array to 6, to allow for even strings of 2 in series.Let's
run through that one more time.

A 48V system in Worcester, MA using 2192Wh
a day. 2192Wh divided by 2.8 sun hours divided by
.67 efficiency = 1168W needed, 1168W divided by 315W panels = 4 panels, 48V system divided
by 24V panels = 2 panels in series per string, 4 panels needed, divided by 2 in series = 2
parallel strings of 2 in series.I'm going to run through that example again, but this
time using a different panel. Nominal 20V panels are usually used for grid-tied
systems, not battery based, and as such their voltage does not easily match up with a typical
battery bank.

But with an MPPT charge controller, which
we'll discuss in the next video, you can use the grid-tied panels in a battery based system. 20V panels are generally more common than
24V panels, so you may be able to get them easier than a 24V. I'm going to use these SolarWorld 260W 20V
panels. Let's run the math. 2192Wh divided by 2.8
sun hours divided by .67 efficiency = 1168W needed, 1168W divided by 260W panels = 4.49
panels. Hmm, this is where it starts getting interesting. Let's put a pin in this, and come back. 48V system divided by 20V panels = 2.4 panels
in series per string. Well, obviously that won't work, so you'll
have to round up to 3.

Let's go back to the number of panels we need,
if we have to have strings of 3, we need the total to be divisible by 3. So we'll have to round up to 6 panels. So now we have 6 panels needed, divided by
3 in series = 2 parallel strings of 3 in series. That's it for the 3rd video for designing
an off-grid PV system. Watch the next videos in the series for how
to select the charge controller and inverter, using the numbers you came up with from these
sessions. Also watch more of our Video Series on our
web site, and peruse our selection of solar panels. We've got a team of highly trained Technical