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An off grid solar inverter draws power from a battery bank, and this power is then used to run appliances and whatever else you want to load in the system. But what if you have a 24V inverter and a 12V battery, will they work together?
Most 24V inverters cannot run a 12V battery and even if it did, there is not enough power to keep the system going. The only way to do this is to connect two 12V batteries in a series, which will increase the voltage to 24 and make it acceptable for the inverter.
Why 24V Inverters Cannot Use a 12V Battery
The manufacturer will recommend the right voltage, but usually a 24V inverter requires 24V batteries, and a 12V inverter is designed for 12V batteries. However there is a bit more to it than that.
A 12V battery cannot generate enough power to run a 24V inverter. It is true that 12V batteries can reach 14.4V when charging, but even that is not enough.
Majority of inverters can only support 24V or 12V. Some inverters may provide separate connections for 24V and 12V, but they are the exception to the rule.
If you somehow get the inverter to run, it will not be able to carry any load. There are only two solutions, get a 12V inverter or combine two 12V batteries in a series.
How to Use 12V Batteries with a 24V Inverter
While you cannot use a 12V battery, you can combine two or more of these in a series. Doing so increases the voltage and provides enough power to run the inverter.
By joining two 12V batteries in a series, you overcome its voltage limitations. Another benefit of using a 24V system is you can use thinner wire gauges which cost less than the thick ones.
But what about parallel battery connections? A parallel connection increases power but not the voltage. In this case we want the voltage to go up so a series configuration is better.
It is also possible to set up a series parallel configuration. Take 6 x 12V 100ah batteries and connect three each in a series. You have two battery banks both in a series at a combined 36 volts. You can then connect the two battery banks together in parallel so the voltage remains at 36V, but the amps will go up to 49.8.
Which Should I Choose, 24V or 12V Inverter?
Most off grid inverters are 12V, 24V or 48V. If you are still deciding what to buy, base your decision on the battery bank voltage.
RVs and boats are designed to run on 12V, so in this case you will want a 12V inverter. For off grid homes, 24V is the norm. Even some tiny solar powered homes now run on this so a 24V inverter is preferable.
If your home is on the grid, the inverter size has to match the solar array voltage. So if you have 24V solar panels a 24V inverter is ideal.
Batteries are not required in grid tied homes because the power grid serves that purpose. If you decide to install a battery bank, the voltage must match the PV array and inverter.
One of the reasons most homes are grid tied is it eliminates the need to buy batteries. They do not come cheap especially if you want to run a lot of power loads.
But the benefit of batteries is you can have power during an outage. Grid tied solar systems automatically shut down during a power interruption, but with a battery bank you have a backup source.
If you do decide to get a battery bank, the voltage must match the inverter and PV array. Again you can connect 12V batteries in a series to match a 24V solar array or inverter.
Benefits of 24V Inverters and 24V Batteries
To keep it simple, if you are in an RV or any motorhome, use a 12V for the inverter and batteries. For homes, stick with 24V or 48V if you have really high power usage.
First we need to explain why 24V systems have become more popular. There are many reasons but they can be summarized as follows.
- Smaller wire gauge. All solar system setups recommend using thick wires and cables because they offer less resistance. With 24V systems, you can settle for thinner cables and not lose as much power.
- More watts generation. A 24V 100ah battery holds twice as many watts as a 12V 100ah battery. If you run a lot of solar appliances this makes a huge difference.
- Occupies less space. If you need 5000 watts of battery power you will need 2 x 300 12V batteries. Or you can buy a single 24V 300ah battery. This is important if space is an issue in your home.
When you pair a 24V inverter with a 24V battery bank, the risk of a solar fire or arc are reduced and it also minimizes energy losses. The input regulation is also better compared to a 12V system, a 4.6% drop compared to 1.05%.
A 24V system also does a better job converting DC to AC. When direct current is converted into alternating current, some of the power is lost. A 24V system is more capable of reducing these and ensures optimum performance.
What about 48V Inverters and Batteries?
While 12V is standard in RVs and 24V for homes, 48V systems are quickly becoming more popular. So is it time to switch now? Here are some simple guidelines.
- If you need less than 1000 watts, a 12V inverter will do.
- If you require between 1000 to 3000 watts, it is best to use a 24V inverter.
- For power requirements greater than 3000 watts, 48V inverters are recommended.
To put it another way, if the demand goes exceeds 140 amps you should opt for 48V. As the current goes up, the need for larger components increase. In other words, increasing the voltage doubles the watts at the same current.
- 83A x 12V = 1000W
- 83A x 24V = 2000W
- 83A x 48V = 4000W
Unless you have a very large shed, a large house or a solar powered office, 24V is probably enough. Just keep an eye on the amp usage and make sure it does not exceed 150.
More about Inverter and Battery Voltage and Sizes
Inverter size and voltage go hand in hand. Once you have decided what voltage to use, you can pick a size.
The inverter size will depend on how many instantaneous watts it must supply. Add the total watts plus 25% for surges and you have the inverter size.
You should also add a 20% or 25% buffer when sizing solar panels, batteries and charge controllers. Always round off the results to the nearest size. And remember, you cannot go wrong with a larger size.
Suppose you have a grid tied home. You want to install a battery bank so you can run essential appliances in case of a power outage.
First you have to list the appliances you want to run. Everyone has their own idea of what “essential appliances” are, so the following is just an example.
- Some LED lights (100W)
- Refrigerator (400W)
- Laptop (200W)
- Ceiling fan (100W)
- TV (100W)
The total is 900 watts, so your inverter must supply 900 watts continuously to run these appliances.
But this is only for the running watts. Inverters are also measured by their surge watt capacity. Some appliances like a refrigerator requires additional watts to start up. The surge requirement is usually twice the running watts, but with older appliances it might be triple.
So going with the example above, a 400W fridge might need 800 surge watts to start. This is only for a second or less though. With a 900W inverter there is enough power available. But if the surge watts is over 900 watts, the refrigerator will not run.
The rule of thumb is the inverter surge watt capacity is double the running watts. So a 1000W inverter has a 1000 running watt capacity and 2000 surge watts.
Setting up an off grid inverter involves more than just buying a battery bank. The voltage for the components have to match to run any load. By making sure the voltages match, a lot of problems can be avoided.