System voltage - 24V or 12V, which is best?

Posted Wednesday 10th of August 2016 at 14:08

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12V or 24V - which is best?We often get asked what’s the benefit of configuring an off-grid power system for 24V rather than 12V, especially as a 12V system can seem cheaper at first glance?  

Most deep-cycle industrial grade batteries are 12V so assuming one battery can store the required power, it is cheaper to have just one battery than doubling up for 24V – it’s practically impossible to source a 24V battery.  

DC solar panels are more frequently available as 12V panels so again, assuming one panel can generate enough power, you only need to buy one panel. That’s not to say there aren’t 24V panels, they are but only available in a limited number of power outputs.  

The decision to move to 24V comes into play with medium to large sized systems. Let’s look at the impact of each component in turn.

Solar controller

Suppose I’m looking to install 400W of PV, as 4 x 100W solar modules, on a 12V system you would need a 30A MPPT controller costing around £150 +VAT.  As soon as you change to 24V, the same amount of PV can be run through a 15A MPPT controller at half the price.

Cable size

Doubling voltage, halves the size of the cable required. Keeping cable size down, keeps cost down too. In longer cable runs, losses come into play so cable size is increased to bring losses down to an acceptable level. 

For 200W of PV, a cable run of 5m requires 2.5mm2 with losses of 3%. A longer run of 10m using 2.5mm2 cable results in a 7-10% loss so the cable size is increased to 4mm2 to bring losses back down to 3%.  By comparison, we can use 2.5mm2 cable for a 10m run in a 24V system.  

For even longer distances such as 30m, the 24V system requires 10mm2 cable and this jumps to 25mm2 cable costing £4/m in a 12V system (a 48V system would only need 2.5mm2 cable).

Batteries

It is inadvisable to have more than 6 batteries in parallel otherwise the battery bank won’t balance properly.  Balancing ensures all of a battery banks’ energy is available for use and it increases the battery's longevity.  As batteries are heavy items, there’s a tendency to choose smaller batteries that are easier to lift – sizes such as 120Ah or 145Ah are popular as the safe limit for one person to lift is 40Kg.

The largest battery bank using 120Ah batteries is 720Ah on a 12V system, holding 8640Wh.  Changing to 24V means we can now have 12 x 120Ah of batteries storing twice as much power or on a 48V system, twice as much power again.   

Inverter

When taking power from the battery bank and converting to 230V, it’s advisable not to draw more than 250A, otherwise you’ll need more expensive cables to cope with the high number of amps, larger fuses and a large enough battery bank to not become fried!

For a 12V system, this is a maximum of 3kW so if you need to draw more than this, consider a 24V system and if you need to draw more than 5kW consider moving to 48V.