Tech

Choose the most suitable voltage for your off-grid system

Island solar panel systems are to be selected in their system voltage depending on the application. However, this must be fundamentally determined correctly, as it can become expensive afterwards. Here we take a closer look at the whole topic.

The system voltage is the first and most important step in planning an island photovoltaic system. The three typical nominal voltages are 12 volt lithium battery,24volt lithium battery and 48volt lithium battery. Although there is also 36V, the variety of components is not given here. We tend to find 36V systems in boats and other small vehicles. In this post, I leave them out.

A first comparison and key differences

Let’s first look at a few general differences before we take a closer look at the individual system voltages.The higher the voltage, the lower the price.Is that true?

Charge controller:

Most charge controllers(such as MPPT or PWM charge controllers) can handle 12V and 24V. Some can also use 48V. If you buy a charge controller and are not quite sure about your system voltage, you should choose a 12/24/48V charge controller.However, the price of the charge controller is not in the system voltage, but in the current (ampere).If you use a 12V system and a 10A charge controller, the maximum (with a lead-acid battery)

14.4V x 10A = 144W

charging power can be achieved. If you want more, a larger charge controller is required.

Let’s do the same calculation for a 24V system with two 12V lead-acid batteries connected in series:

28.8V x 10A = 288W

And at 48V it looks like this:

57.6V x 10A = 576W

Information: 14.4V, 28.8V or 57.6V stands for the end-of-charge voltage of a lead-acid battery. Depending on the battery type, this end-of-charge voltage is different.

As you can see, the charge controller can be smaller with a higher system voltage and is therefore cheaper.

Inverter:

There are almost exclusively inverters for one voltage range. 12V, 24V or 48V is the nominal voltage. For example, the voltage range of a 12V inverter can be between 10.8V and 15V.

If you subsequently change the system voltage of your photovoltaic system, e.g. from 12V to 24V, you need a new inverter. That can be very expensive.

The 12V component market is fairly well stocked with typical camping supplies. There are quite a few components here that can be cheaper than for 24V. For example, small coffee machines or small inverters. However, this does not apply to all inverters, since 24V and 48V is the typical market for photovoltaic systems.

Cable:

The longer the cable, the higher the losses. And the more current (in amperes) that flows through the cable, the more important it is either to have a short cable or a very thick one.

Please only use high-quality cables with a sufficient cross-section. CCA cables are copper-coated aluminum cables. These should be about 1.6 times as thick as copper cables. The savings factor is then almost canceled out again. Tinned copper cables are usually used for solar cables.

Specific resistance of:

Copper: 0.017 Ohm (x mm² / meter)

Aluminum: 0.026 ohms (x mm² / meter)

Thick copper cables are expensive. And the cable cross-section can be halved in a 24V system compared to a 12V system.

Example:

We need a cable from the battery to the inverter. The inverter is 2 meters away from the battery. We connected a load with a power of 1000W to the inverter.

With a 12V system, 83A (1000W : 12V) flow. According to the cable cross-section calculator, we need a copper cable with 16mm² or aluminum cable with 25mm².

With a 24V system, 41.5A (1000W : 24V) flow. According to the cable cross-section calculator, we need a copper cable with 4mm² or an aluminum cable with 6.3mm².

With a 48V system, 21A flow (1000W : 48V). According to the cable cross-section calculator, we need a copper cable with 1mm² or aluminum cable with 1.6mm².

Always use the next higher cross-section if the cable calculator outputs cross-sections that cannot be bought in this way! Always round up.

Storage batteries:

With batteries, it gets more difficult because there are so many different technologies. Lead acid/fleece/AGM, lithium ions, lithium phosphate and many more.

Typical 12V lead-acid batteries always have 6 cells permanently connected to each other. If you want to build a 24V system from it, two of the same batteries are connected in series.

Reading tip: Batteries in parallel vs series

The same applies to lithium batteries, which are interconnected in a housing. There is 24V or 48V in one housing, which can be cheaper.

If you want to tinker yourself, you can do this very well with lifepo4 battery(prismatic cells). For this you need exactly 4 individual cells for a 12V system, 8 cells for 24V and 16 cells for 48V.

If a cell fails, it can be replaced individually.

Important: you absolutely need a BMS (Battery Management System)

Let’s look at the cost differences. In another post, I show why a lithium battery is cheaper than a lead battery. It’s all about the tension here.

As you have already noticed in the example with the cable cross-section that the current halves when the voltage is doubled (and the cable cross-section roughly quartered), this is also the case with the capacity of batteries.

Example:

At 12V, a battery of at least 100Ah is recommended.

12V x 100Ah = 1200Wh / 1.2kWh

At 24V, only two 12V 50Ah batteries are required, which are connected in series.

24V x 50Ah = 1200Wh or 1.2kWh

At 48V only four 12V 25Ah batteries are required, which are connected in series.

48V x 25Ah = 1200Wh or 1.2kWh

However, we do not necessarily have a direct cost advantage here. The larger the individual 12v lithium battery, the lower the price per Ah (ampere hour). However, this can bring many cost advantages when replacing just one battery.

Which system voltage is recommended for which application?

Depending on the application, the system voltage can vary. There are cases where a 12V system makes more sense than a 24V or 48V system. Or there is no alternative to the 12V system.

In my opinion, 12V systems should be used up to a maximum of 500 W power. For everything above that, a 24V system often makes more sense.

If typical household consumers such as a washing machine, dishwasher or even an electric car have to be supplied, you should definitely take a look at the 48V system. However, it is better to consult an electrician, since the DC voltage of this magnitude is life-threatening.

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