Getting off the grid with a solar power system is going to take some calculations on your part, especially when it comes to the amount of batteries you’ll need and the size of the inverter. This means becoming very meticulous about how much energy you use and when you use it. In this article we’ll be looking at the steps you need to take if you want to calculate the amount of batteries you’ll be needing.
The Size of the Inverter
When it comes to choosing the right inverter you want to take into account the peak amount of wattage your home uses throughout the day. This means counting up everything that might possibly run at the same time, even appliances you don’t think take up that much electricity. For example, in a single room you might have a computer running at 300 watt, along with a light of 60 watt. The total sum of watts used inside the room comes to 360 watt, so the inverter has to be at least 360 watt.
The next step is to calculate the amount of watt hours, or the amount of time these appliances are going to work throughout the day. This is another simple equation of multiplying the watt of the appliance with it’s running time. Building on the previous example it should look something like this:
If the bulb runs for 6 hours then you multiply 6 with 60 watt, which equals 360 watt hours. If the computer runs for 3 hours you multiply 300 watt with 3, leaving you with 900 watt hours. Now, the combined watt hours for this room is 1260 watt hours. Keep in mind that you have to account for efficiency losses as well, so just to be safe multiply the total with 1.5. This brings your watt hours to a safe estimation of 1890.
Now that you have a rough estimation of the electricity you’ll need on a daily basis, you have to decide how many days should the batteries be able to provide power without recharging. It typically varies between 2 and 5 days, but the choice is up to you. Things get a little more complicated here, because you have to convert the watt into voltage capacity (AH). Here is an example:
Let’s assume you want the battery to last for 3 days and you need 1.6kwh per day, you take your daily need and multiply it by the amount of days.
3 x 1.6kwh = 4.8khw
This amount represents about 50% of discharge depth, which is why you want to multiply the answer with 2.
2 x 4.8khw = 9.6khw
Now comes the conversion to voltage (AH) part, but first you have to decide on the voltage system you want to use. Homes voltage systems usually run on 12v or 24v, whereas commercial buildings use a 48v system. Let’s assume you want a 24v system, then your conversion should look something like this:
9 600 / 24 = 400AH
This is the minimum amount of energy your home needs to run. All that is left for you to do is divide the 400AH according to the ratings of your battery, which is the number of batteries you’ll need to sustain this energy for 3 days.