12, 24, or 48V?
When entering into the off-grid and renewable energy industry, one of the first questions that will need to be answered is “what capacity and voltage configuration do I need for my battery bank?”. Most inverters will fall into 3 categories for their input requirements: 12VDC, 24VDC, and 48VDC. This is referring to the nominal DC voltage that the inverter will invert to AC voltage (i.e. 120VAC or 240VAC). **Though there are multiple other AC supply voltages and configurations, we will be generally referring 120VAC as it is the most widely available.
Deciding which one of these to go with can be extremely tedious and sometimes time consuming as the answers to this question can be quite ambiguous. The reason being is each system has its own set of unique variables that makes it impossible to provide a single answer. Therefore, we find it is much more efficient to provide the answer to “why would one choose 12VDC, 24VDC, or 48VDC?” to which there is only one answer, electrical resistance. Electrical resistance is a measure of ease or difficulty electrical current flows through a conductive material, fundamental property inherit to all electrical devices. For all intents and purposes, resistance is equivalent to heat caused by friction on a particle scale. Having more resistance will cause a rise in heat. Heat in electric circuits is bad. To reduce this amount of resistance (or heat) in a circuit we sometimes can use a bigger conductor (a.k.a. cable) or even different material compositions. However, there is a limit to the size of cable you would be able to find/purchase that would still be capable of handling the amperage that comes with larger power draws. For example, a fuse rated for 10A is around $1, whereas a 1000A fuse is roughly $396.00. This is obviously just one example of pricing as a factor of higher amperage but size issues and ease of installation are very close behind in terms of impact. This is where we look to higher nominal DCV inputs. *****
Ohms law tells us that Power is the product of Voltage and Current in a given circuit (Watts = Volts*Amps). With this, we can use some basic algebra to work out exactly what nominal voltage we should be using for your system.
First, I would like to breakdown this formula into portions relative to our conversation. Watts, or power, should be calculated based on the actual power ratings for the electronic devices you wish to power using an inverter. Usually, this number will be located on the power cord or the nameplate of the device. Once you have these numbers you will want to add together the values for any devices that would be powered at the same time, this will be the number you use to pick the size of inverter you will need (e.g. if you had a 50W fan, 250W TV, and 200W DVD player that would be running at the same time, you would need at least a 500W inverter).
Once you have calculated your total wattage need, you will want to pick a nominal voltage for your battery bank. This can be done arbitrarily at first to simply check what the amperage would be in different configurations or based on external variables such as only having one battery available. Regardless, once you have the power and voltage values, we can then move on to calculating the amperage which by proxy will tell us how large our conductors should be and the ratings for components such as fuses, breakers, lugs and the composition thereof.
To calculate the amperage of your chosen DC configuration, you will need to modify Ohm’s Law to fit our needs. Skipping the mathematical instruction, the resulting equation is W÷V = A. For example, if we had a need of 6000W and we want to use 12V as our DC source, this would give us an Amperage rating of 500A. This at first doesn’t sound terrible, until you attempt to find a cable size capable of supplying that amount of current and see the price per foot. There are obviously ways to accommodate this set up; connecting multiple cables to each lug or finding cables and fuses that are capable of carrying the necessary amperage, but the easiest and most efficient solution is to simply raise the DC voltage. Again referencing our equation W÷V = A, we can see that raising the voltage by a factor of 2 reduces the amperage by half. 6000W÷12V=500A vs. 6000W÷24V=250A vs. 6000W÷48V=125A
By decreasing the amount of amperage in the DC circuit, there will be a wider range of products such as cables and fuses, available for your specific configuration.