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 Sales Reps available to help you plan your
system, give us a call.
Thanks!