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OFF GRID
SOLAR POWER SYSTEM SIZING

Designing an off-grid solar system to provide electricity where there is none can be an intimidating project. This article will help make it more do-able.

Step 1 - Estimate How Much Energy You Use

The first and most important step is to figure out how much energy you need. Do not skip this step. If you just guess, or buy random items that are on sale, you will have a solar system that will not fit your needs, and will have wasted money instead of saved money.

Go online to a Loads List Calculator and enter everything you plan to power, how many watts it uses. You can find the power rating of appliances on the UL label on the equipment you want to power. If it doesnít list watts, just do the math, volts x amps = watts.

For example, if you are looking to power your 40 watt (W) laptop for 3 hours, enter 1 laptop, 40W AC, for 3 hours. It will tell you that that uses 120 watt hours (Wh). Add 2 LED lights at 6W each for 4 hours, thatís another 48Wh. Letís also charge your phone for 2 hours, 4W x 2 hours = 8Wh. Add them all up, 120Wh + 48Wh + 8Wh = 176Wh needed. That wasnít so bad, was it?

Step 2 - Solar Battery Sizing

Next is to figure out how many deep cycle solar batteries you need to store that energy. Luckily, thereís an online solar power system calculator for that too. You enter the number of watt hours you need to make (176Wh for our example). Enter many days without sunshine you need the system to store power in the battery (3 is common), and what temperature the room will get down to where you are storing the batteries. Batteries can work in the cold, but they canít hold as much energy, so you have to oversize it to compensate. We selected 50F (10C), as weíll be storing them in the basement.

Finally, you must select the voltage the battery bank will be. 12V is common for very small systems, for boats and RVs, and when you have 12VDC loads you need to power. 24V is a nice medium size system, it easily allows for up to around 2000W of solar without much complication. 24V is great for cabins. Larger systems are best with a 48V battery bank. It allows for a large solar array and a large battery bank, while keeping the amps running through the wires at a manageable rate. We chose a 12V battery bank for our example so I can charge my cell phone with a car charger plugged into a cigarette socket.

The off-grid calculator determined that at 12V, we need at least 121 amp hours (Ah) for my system. This also took into consideration that even deep cycle batteries donít like to have more than half of their energy used. So it figures on a 50% depth of discharge (DoD). this means it doubles the size of the battery bank from what you need, so that half of the energy will always remain in the battery, even after 3 days without the sun recharging them, keeping it healthy. We rounded up to a 135Ah 12V sealed battery.

Step 3 - How Many Solar Panels Do I Need?

Now weíll determine how many solar panels you will need. First, you have to determine how many sun hours a day the solar panels will enjoy, in the worst month that you will be using the system. Our calculator (linked above) lists common cities in the USA, and inputs the winter sun hours. But if you are in another country, or will be using the system seasonally, and not in the winter, you can go to other sun-hour charts to find the sun hours you need, also known as ďinsolationĒ. Note that sun hours is not the number of hours the sun is up each day, but the number of hours the sun equals the intensity of the sun at noon. Except in the sunbelt, that number can be around 3 hours or less in the winter. In the summer, it can be as much as 6 hours, or more.

For our example, we selected Baltimore, MD, which has 3.1 sun hours in the worst month. It calculated that I need at least 73.8W of solar panels. Well, thatís an odd size, so we rounded up to an 80W solar panel. We are almost there!

Step 4 - How to Size a Solar Charge Controller

The calculator says we need at least a 7 amp (A) solar charge controller to manage putting the solar power into the battery. We rounded up to a 10 amp PWM solar charge controller. That was quick.

Step 5 - How to Size an Off-grid Inverter

Finally, to power the AC appliances, we need an off-grid inverter to convert the DC from the battery to AC for the appliance. To determine what size, we add up the wattage of everything that may be plugged into it and turned on at the same time. Thatís the 40W laptop + (2 x 6W light bulbs = 12W) = 52W. I am not counting the cell phone charger, because that is being plugged into DC, and will not go through the inverter. I can get a nice 350 watt, 12 volt inverter that will do the job nicely.

Donít Forget the Safety Protection...

You still need to select breakers or fuses to protect between all of the solar devices, but this should give you a good starting point to determine what size off-grid solar system you need for your project.

Written by: Amy Beaudet, altE Store


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