Solar power system design Step 3-Calculate the solar panel power

How to calculate the power of solar panel? This is the 3 steps for solar power system design. We are getting close for the solar power system design.

Step three is to size the solar array. We'll discuss the different considerations that go into determining how many solar panels. 

We're making good progress blogs. Step One and two 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.

Now, you may recall the loads list we did. In our previous video. We came up with a total usage of 21, 91 . 5 watt hours a day, rounding up to 21, 92. How do you go about doing this?

Insulation 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 with the brightest part of the day.

For example, since the sun is twice as bright as noon as it is, at 9:00 am, 9:51 am only counts as half an hour, whereas noon to 1:00, pm would be considered a full hour. Also took the 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 your best year round performance.

You can see here in Massachusetts, we have an annual average about 4.5 sun hours a day. However, when you size it off grid system, you must plan on worst case, which, unless it's a seasonal camp would be winter. As you would imagine, the sun hours drop off dramatically. Here in Massachusetts, in the winter, we drop down into the twos, designing my system.

I'm going to look at the sun hours for December. I'm designing my system to be tilted at 42 ° off horizontal. I can look at my numbers for latitude, which is 42 . 27 ° 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 didn't decrease my winter.

Because winter in New England is pretty grim with December going days without ever seeing sunshine, i'll be planning on using a generator when needed.

But I'm still going to design is trying to optimize my solar contribution and only use a generator when needed. Let's face. It is no such thing as a perfect system with no losses.

But enough grid systems is a lot more than you would expect. Do the things like soiling of the panels and voltage drop on the wire, losses and batteries and electronics, and several other factors will be losing about 1/4 of the rated power from the panels

So a hundred watt rated panel may actually give us only about 80 watts of usable power or even less after all the losses affected in. Here's a steps to calculate the minimum size array. You take the water hours that you calculate from your loads list.

You may recall our example was 21, 90 21 hours a day. We divided by the number of sun hours. Here we're using 2 . 8 sun hours then divided by the system inefficiencies . 67. This equals at least 1,168 watts of solar needed.

There are so many options that are available for solar panels, selecting which panel to use may seem daunting.

First step is to narrow down what your priorities are. Do you want made in the USA or other specific country or not made in a specific country? Do you want the panel frames be black or silver and the cells to be black or blue? 

Is a particular brand you want to use or is cost the only criteria you care about. For my system, I picked a nominal 24 volt made in the USA panel, solar world's 351 monocrystalline panel. 

If the panel you select doesn't say what it's nominal voltages, you can look at its specs. A 24 volt nominal panel has avoc of about 46 volts, or is a 20 volt nominal panel, is avoc of about 38 volts.

Now that we know how many lots of panels we need, and which ones were 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 two parallel strings of two, which gives me a nominal 48 volt array, charging a 48 volt battery bank. Perfect.

If I had determined that I needed five panels, I would have had to increase the rate of six to allow for even strings of two in series. Let's run through that one more time. A 48 volt system in Worcester mass using 21, 90 21 hours a day, twenty one ninety two one hours divided by 2 . 87 hours divided by . 67 efficiency equals eleven sixty eight. What's needed?

Eleven sixty eight watts divided by 315 watt panels equals four panels. 48 volt system divided by 24 vote panels equals two panels in series per string.

Four panels are needed divided by two in series equals two parallel strings of two in series. I'm going to run through that example again, but this time using a different panel, nominal 20 volt panels are usually used for grid tide systems, not battery based.

As such the voltage does not easily match up with a typical battery bank, but with an mppt charge controller, which we'll discuss in the next blog, you can use the grid tide panels in a battery based system. 24 panels are generally more common than 24 volt panels, so you may be able to get them easier than a 24 volt.

I'm going to use the solar world to 61, 20 volt panels. It's on the math. 21, 92, what hours divided by 2 . 87 hours divided by . 67?

Efficiency equals 11, 60 8. What's needed? No change there. Eleven sixty eight watts divided by 261 panels. It was 4 . 49 panels.

This is where it starts to get interesting. Let's put a pin in this and come back. 48 volt system divided by 20 volt panels equals 2 . 4 panels in series per string.

But obviously, that won't work. So you'll need to round up to three. Let's go back to the number of panels we need. If we have to have strings of three, we need the total to be divisible by three.

So we'll have to round up to six panels now that we need six panels. We divide that by three in series, and that gives us two parallel strings of three in series.

That's it for the third video, for designing an off grid pv system, what's 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 blog series.

Wang has over 10 years of experience in off-grid projects. And know how to wire the system well. Making every installation step easier.