Volume 2 - SOLAR OFF-GRID BASICS
Introduction
In this installment of “Going Off-Grid with Solar” we are going to learn how to size our off-grid system’s PV array & battery bank size using a fictitious example to show how to calculate your own system.
This will not be too granular in detail to keep it very simple and easy to replicate for your own needs. Some simple calculations and you’re well on your way to determining just how much PV you will need as well as how much storage is going to be needed. So, let’s just dive in.
Calculating AC Loads
The very first thing that we want to evaluate are the loads that we wish to run off of our off-grid system. This can be a challenge because it is here that we discover just how much energy our appliances, lighting and our gadgets consume. It can be a tough pill to swallow that we have to be a little bit more energy conscious and maybe let go of some of our creature comforts in order to minimize the PV and battery requirement of our off-grid home.
With that in mind, let’s continue on with a made up off-grid system to learn how to calculate the weekly AC load. In this example I want to include some lighting, a couple of ceiling fans, a fridge, TV, microwave, coffee pot and a toaster. These are my must have loads. I cannot live without my coffee pot!
I tally up the wattage that each of these loads draw:
- Lighting (CFL’s) = 98 Watts / 6 hours per day
- Ceiling Fans = 128 Watts / 8 hours per day
- TV = 125 Watts / 3 hours per day
- Microwave = 1,100 Watts / 6 minutes per day
- Coffee Pot = 700 Watts / 6 minutes per day (Keurig mini)
- Toaster = 1,000 Watts /6 minutes per day
At this point you want to multiply the wattage of each by the runtime and days per week that those appliances run. I determined that I use all of these everyday and came up with a total of 22,939 Watt hours per week. I want to add a little bit of padding to this to account for AC efficiency so I wanted to add an extra 25% to this number for a total of 28,673.75 Wh/week.
Now that we know what the weekly AC load is going to be we can determine just how much PV is going to be needed to supply this off-grid system. The next step is to calculate the daily PV requirement. In our example we know that we have 28,673.75 Wh/week. Divide this number by 7 days to get 4096.25 Wh/day. Then we can divide this value by the systems voltage. This could be 12, 24 or 48 volts. I’m going to use 48V as this systems voltage. So, I would need to divide 4,096.25 Wh/day by 48V to determine the daily Amp hour requirement of the system. This turns out to be 85.34Ah.
Knowing that our requirement is going to be 85.34Ah we divide this by the total average annual sun hours for the area. In this example we consult a solar irradiance map to obtain the area’s sun hour value. Ours is going to be 3.4 hours. To get the array’s current requirement we divide 85.34Ah/3.4hrs. The array must be 25.1A or greater.
The next step in calculating the PV array size is to select a module and find its ampere rating in the data sheet. I selected one with an ampere rating of 8.63 and a voltage rating of 36.8. The next step in calculating our array size is to determine how many parallel modules will be required. Divide the array’s amperage requirement of 25.1 by the modules ampere rating. This ends up not being a whole number (2.91) so you want to round up to the next whole number. We end up requiring 3 modules in parallel to obtain the necessary amperage.
Next we want to figure out how many series connected modules we will need. Divide the system voltage (48V) by the module’s voltage (36.8V). This will not be a whole number, but don’t fret just yet. We calculated that we need 1.3 series connected modules. **Note, that if you round down to 1 your recovery time to recharge batteries is going to be greater. You may option to round up to 2 instead.** To calculate how many modules total for the array we multiply the number in parallel by the number in series. In our example will will be using 6 modules total to complete the array.
Battery Bank Sizing
We now know what our AC load requirement and subsequently what the PV array size will need to be, so we are now down to the last piece of the puzzle. To determine the size of your off-grid system’s battery bank you want to take the Daily amp hour requirement (85,34Ah/day) and multiply this by the total number of autonomous days that you want your system to have. Please note that the more autonomous days you want to have, the larger the battery bank is going to become. Batteries are the most expensive component of many off-grid projects.
For the area in this example I determined that I will need to have 3.5 days of autonomy. This would account for a few consecutive cloudy days and the system would still function without PV charging the battery bank. Our new daily amp hour requirement is now calculated to be 298.68Ah.
I selected from a list of available batteries a 12V - 342Ah wet battery. With this in mind, I can calculate the total number of batteries that will be required in this system. First we need to decided how far down the battery will be drawn. Typically you never want to exceed 50%. This can impact how long the life of the battery bank will be. I would urge to sway in the way of safety. In this system, I’m only going to draw them down to 70% capacity.
The calculation for this is to take the new daily amp hour requirement of 298.68 and divide that by 30%. We come up with 995.6Ah of required capacity for the system. To find the number of parallel batteries we need to have we divide 995.6Ah by the capacity of the battery selected (342Ah). This calculation also does not come out to be a whole number. So we round to the next whole number and come up with 3 parallel batteries.
Now we want to know how many series connected batteries we will need. This is the easiest calculation by far. Take the system voltage (48V) and divide by the battery voltage (12) to come up with 4 series batteries for a total of 12 batteries for the entire system.
Congrats! You have now calculated your first off-grid system. The are more granulated calculations that come later, but this will get you on the ground and running. With these basic calculations you can easily adjust your system’s equipment by swapping out the batteries and modules to see which favor your system best.