When you experience a blackout or load shedding in South Africa, you can have a backup power system. It works like a UPS. I chose the Victron Multiplus 2 48V 3000VA because it has a fast transfer switch.
This system uses a 48V server rack battery. This battery is stackable and is easily expandable later. Because of the 48V, the current will be lower than a 24 or 12V system. This makes it possible to save money on wiring costs and charge controllers.
Since we have to recharge the battery in one day and the average sunhours is 3 per day, we need the following amount of solar panels:
Total battery power: 100Ah ×51.2V=5,120Wh
5,120Wh⁄(3 sunhours=1,700W of solar)
We are going to use the same solar panels as the previous example. We need:
1,700W⁄365W=4.6 solar panels
We can round down to 4 panels because it’s a backup system that is not used daily.
The solar panel specs are:
- Power: 365W
- Open Circuit Voltage (VOC): 48 V
- Short Circuit Current (ISC): 9.83 A
- Maximum Power Voltage (VMP): 39.10 V
- Maximum Power Current (IMP): 9.33 A
We need to wire the panels the same way (2S2P) we did in the previous chapter. Refer to those for calculations. This is the calculation for the charge controller. Max input voltage:
48Voc ×2 panels in series ×1.25=120V DC
Current to the batteries:
(365W ×4 panels)/57.6V=25.3A
We can use Victron’s 150V/35A charge controller. Notice that if we use a higher-voltage battery, the current is reduced? In the previous example, we used a 24V battery, which required a 60A charge controller for the same amount of panels.
If the distance is shorter than 70 feet or 20 meters, we need an 8AWG or 10mm² solar PV cable. I calculated this in the previous system, so take a look at how I calculated it there.