12 Volt, 24 Volt or 48 Volt
Question: Should I choose a 12 Volt, a 24 Volt or a 48 Volt stand-alone power system?
Reply: In short, your energy consumption should determine the voltage of your power system. You should not have continuous currents greater than 100 Amp.
Power - Current - Voltage
- 1,000 Watt = 83 Amp @ 12 Volt
- 2,000 Watt = 83 Amp @ 24 Volt
- 4,000 Watt = 83 Amp @ 48 Volt
- 20,000 Watt = 83 Amp @ 240 Volt
The higher the current (measured in Ampere or Amps) the bigger the components need to be. High currents require large diameter cables and fuses, both of which are expensive. By doubling the voltage you get double the power (Watt) at the same current.
Dealing with currents over 100A is costly (and therefore inefficient) and potentially dangerous. A perspective: a standard household extension cord is rated at 10A max. current. 100A would probably melt it and could start a fire!
12 Volt used to be a standard for extra low voltage power systems. Today, most systems are 24V or 48V and include a 240V AC inverter. This means the wiring of the house does not have to be different from any other grid-connected household and cabling cost is greatly reduced.
We advise that you get an electrician to wire your house for 240V AC. This way you can use standard AC appliances and lighting, most of which are a lot cheaper to buy and many are becoming quite efficient.
In the past we tried to reduce the cost of an off-grid system by limiting its size. This was achieved by using 12V or 24V appliances & lighting that do not require an inverter. In recent years, inverters and solar panels have become more efficient and a lot more affordable. In addition, most customers seem to want more power over the years. A 12V DC system with a tiny inverter is difficult if not impossible to upgrade/upsize. Not to mention that only very few companies sell extra low voltage appliances or lighting.
To summarise: Most systems we design are 24V or 48V with a 240V inverter. The criteria we use is power consumption and scalability. We would only suggest a 12V DC power system (like the Rainbow Power Cube) if you need a little light in a shed or caravan and wish to wire it yourself.
Battery Bank Size
With solar panels as the primary energy source, it is usually recommended to have a minimum of 5 days battery storage with the battery bank still retaining a minimum of 50% charge after the end of those 5 days. The largest single battery bank available will provide 550 amp-hours over a 100 hour period to be 50% discharged at the end of that period. It is not recommended to increase storage capacity by connecting two or more battery banks side by side (in parallel). By doubling the battery voltage, the current (amps) from the loads is effectively halved, so doubling the voltage has the same effect as doubling the amp-hour storage capacity of the battery bank without having the battery bank connected in parallel.
The battery voltages generally used for stand alone power systems are 12V, 24V, 48V, 110V and 240V DC.
- Batteries may be placed in parallel with a battery isolator between the charging source and the batteries. You would then use one battery bank for some of the loads and the other battery bank for the rest of the loads. You may, for example connect all DC loads to one battery bank and inverter loads to the other.
- Batteries may be placed in series with separate charging sources, regulators and loads. With this technique you can also have the advantage of being able to use both the individual and the combined voltages. You may, for instance, have 12VDC and 24VDC loads and/or use a 24V to 240VAC inverter. You may also have solar panels to charge either or both 12V banks and a 24V wind turbine to charge both banks.
- Less battery storage and more reliance on generator back-up.
For any particular battery voltage there is a limit as to how large an inverter is available. With higher battery voltages larger inverters are available. So if you expect big 240VAC loads choose a higher voltage for your stand-alone system
If your demands increased over time, and a higher voltage for your system is not a feasible option, you may be able to overcome the inverter shortcoming by having several inverters, or having inverters that can operate in tandem.
Cable Length & Size
The lower the battery voltage, the higher the current draw from the battery bank to supply a given load (measured in watts). There is an acceptable limit in the voltage drop in the cable before the voltage drop becomes excessive with the resultant output voltage becoming too low. A more serious limitation of the cable is its "current carrying capacity" (ccc). If the ccc is exceeded the cable will melt and/or catch fire.
Doubling the voltage effectively halves the DC loads and halves the voltage drop. Because the battery voltage is doubled, the percentage of the voltage drop in relation to the battery voltage is only a quarter of the percentage drop with the lower battery voltage. Hence, with a 24 Volt system the cable need only be one quarter of the diameter as it does with a 12 volt system. Unless the cable runs are exceptionally long or the power draw (amps) of the loads is exceptionally high this consideration would not be an issue.
Instead of opting for a higher voltage, an increase in cable size could also have solved the problem. Both the battery voltage and the Amp-Hour storage capacity of your battery bank should be appropriate to your needs. Avoid placing many small batteries in parallel. Battery cells connected in series is OK.
Please see our Cabling/Wiring Chart.
Number of Required Solar Panels
Solar regulators are generally limited to 30 amps maximum. With a large 12 volt system you may require twice as much cabling and twice as many regulators as with an equivalent 24 volt system.
This limitation can be overcome by having several solar arrays separately wired through separate regulators. It must be remembered that maximum charging rate of most battery banks is 10% of their amp-hour capacity (see Charge Rate).
Maximum Charging Rate
The maximum charging rate for a battery bank is usually 10% of its amp-hour capacity measured at the 10 hour rate. A 600 Ah battery should therefore not be charged at more than 60 amps.
This limitation can be partially overcome by placing batteries in parallel with a battery isolator between the charging source and the batteries (see Battery Bank - Solution 1). If one battery bank is full and the other is not, you would still have to throttle down the charging rate to 10% of the capacity of the one battery bank.
Voltage of Charging Source
If a large wind turbine or large DC generator is incorporated into the system then the system voltage will be dictated by the availability and voltage of these charging sources.
Place your batteries in series with separate charging sources, regulators and loads (see Battery Bank - Solution 2).