System configurations – How does all this stuff go together?

Let’s start with the most basic – a 12 or 24V system used to power DC loads such as a 12/24V fridge, lighting, compressors etc. As a case study (and because it’s around 90% of people who walk through the door) let’s say this is going on a vehicle.

You will need either:

• A solar panel, solar controller, and battery (or several) or
• A DC-DC charger and battery or
• Both.

For a small solar array, panel plugs into solar controller, battery plugs into solar controller. Make sure you put the cables in the correct port and make good connections, and away you go. The controller regulates the voltage going to your battery to avoid overcharging.

There are two main classes of solar controller -MPPT and PWM (There’s a whole other post about the difference between these) – but the takeaway is that an MPPT is more efficient, takes better care of your battery, and will take a much wider range of panels. An MPPT controller will work with 12V panels, but it will also work with a higher voltage panel, which will perform better when conditions are poor.

For a DC-DC system, you run your cable from the starter battery to the DC-DC charger, then from the DC-DC to the second battery. In a vehicle without a smart alternator, earthing the DC-DC to the body of the vehicle is an option, if you have a smart alternator you will need to get an isolated DC-DC and run both a positive and a negative (earth) cable. (A setup using a VSR in stead of a DC-DC charger is a possibility for deep cycle batteries if you don’t have a smart alternator, however you will likely never get a 100% state of charge with just a VSR)

In both cases, you want to fuse everything (including loads) that is connected to the battery. Anywhere the positive and negative cable can physically come into contact (and in a vehicle where the entire body of the vehicle is earthed – where the positive can come into contact with metal) there is a potential for a short circuit, which could cause a fire.

What if you want to run 240V appliances (anything you’d plug into the wall at home) – Get a 240V battery inverter (make sure your battery is capable of handling the load). Bear in mind that if it doesn’t have plugs, you will need an electrician to hard wire the AC side.

Surprisingly, this configuration holds true for much larger systems. A larger array of solar panels and a much larger MPPT controller going to a much larger battery bank, and from there, to a battery inverter. These systems are more complex to setup and require more planning, and as permanent installations, there is more red tape, but where grid power isn’t available yet or has yet to be connected, it can work out far cheaper than paying for a connection and running underground cables.

What about PV inverters? The ones used on grid – tie solar systems? Can those be used?

Yes and no.

PV inverters need to ‘see’ the grid in order to startup, meaning it’s impossile to get a pv inverter started without first having a battery inverter as well. On top of that, the two inverters need to be able to communicate. Once you have a battery inverter and a PV inverter that can talk to each other, you can run them in parallel.

Say you have a 5kVA multiplus and a 5kW Fronius inverter. The AC output of the Fronius connects to the AC output of the multiplus. This gives the Fronius it’s ‘grid’ frequency and lets it start up. On a good day, where the fronius is producing 5kW, you will have up to 10kW available during the day.

If you’re not using that, the excess power is still used to charge your batteries – by going ‘backwards’ through the Victron inverter.

This is generally slightly more expensive than going the MPPT route, but in situations where daytime power use is higher (very popular for irrigation, cool rooms and large aircons) that extra inverter power can make a big difference in system performance