Unless you’re planning on going so off-grid that you’re not going to use any electronic devices at all, you’re probably planning on putting in some sort of power generating capability, either solar or wind. Both are good choices, but a combination of the two is the best.
No matter what you decide to do for power generation, there are always going to be limits. One of those limits is that your power generators won’t provide power 24/7. Solar power only works when there is sunlight and wind power only works when there is more than 10 mph of wind. That means that there will undoubtedly be times when you can’t produce power with either of them.
This is the same problem that the nation as a whole faces with these technologies. While solar and wind power are both great systems, they have to be backed up by more conventional power generation systems, simply because they can’t always provide power. To make that problem worse, there is no known electrical power storage system, which is capable of storing large amounts of power from these systems. Basically, the overall power generation of the electrical grid has to match the power being consumed at all times.
That doesn’t mean it has to match when you’re talking about your home power generation though. Storing electrical power on a small scale isn’t much of a problem, although it can be a bit expensive. Of course, the more you want to store, the more expensive it will be.
Explaining the Technical Stuff
Electricity generated by solar panels and wind generators ranges from 14 to 24 volts DC (direct current). That’s because the designers of these systems are assuming that they will be used in conjunction with a battery backup system. Since the batteries used are 12 volts DC, the charging voltage needs to be at least that high.
The reason why these power generation systems are built to a higher voltage than needed for charging the batteries in a battery backup system is to accommodate fluctuations in their output. If a day is partially overcast, a solar panel won’t put out as much voltage. If it was designed for producing only 13 or 14 volts, then it would probably not provide enough voltage to charge the batteries.
The same happens as solar panels age. The expected life of a solar panel is 20 to 25 years. But during the later years the output begins to drop slightly, perhaps even as much as 15%. By making their output higher than needed, they still produce enough power to charge the batteries.
The batteries are storing electricity at 12 VDC (volts DC), but there are few devices that you will use, which will actually need 12 VDC. Most portable devices today operate off of 5 VDC, while most home appliances and electronics are designed to work off of 120 VAC (volts, alternating current). Clearly, the output of the batteries has to be adapted for those uses.
Components of a Battery Backup System
A battery backup system is actually fairly simple, consisting of three main components. The first component is a battery charger, which connects the power generation to the batteries. From the batteries, there is a voltage inverter, which makes the voltage usable to your electronics.
The battery charger needed for a battery backup system is not the same as what is needed for charging a car battery, even though they are both charging 12 VDC lead-acid batteries. The main difference between the two is that the battery charger needed for the battery backup system must be designed for use with a 24 VDC input, rather than a 120 VAC input that a car battery charger normally uses.
When selecting a battery charger, or solar charge controller, make sure that the current carrying capacity of the charger, as measured in watts, is higher than the wattage output of all the solar panels in your system; otherwise, the battery charger will burn out.
The batteries used for a battery backup system are 12 VDC lead-acid batteries, similar to what are used in automotive use. However, they need to be “deep cycle” batteries. These deep cycle batteries are commonly used in boats and recreational vehicle. They are designed so that the charge dropping below 50% (known as deep cycling the battery) won’t damage the battery. Deep cycling will damage normal automotive batteries.
The more batteries you have in your system, the more you can run off of it, or the longer you can run your equipment off of it. Additional batteries can be connected in parallel, without having to make any other changes to the system.
The purpose of a voltage inverter is to take the 12 VDC stored in the batteries and convert it to 120 VAC for use with home electronics and appliances. If you have home electronics which will also run off of 12 VDC or 5 VDC, then you are better connecting them to the battery, without the inverter. Inverters always have some power loss, so connecting directly, through a normal power supply will reduce waste and increase your battery life.
When looking at voltage inverters, you need to think about how much output you need. Voltage inverters come in a variety of sizes, based upon their output current. Be careful as you look at these figures, as there is a running output current and a peak output current. You want to select your inverter based upon the running output current. This will be stated in watts.
The equipment you want to connect to your inverter probably won’t have its power requirement stated in watts, but rather amps. To convert from amps to watts, multiply the volts by the amps. So, a device that draws 7 amps of power at 120 VAC, needs 840 watts of power (7 x 120 = 840).
Building Your Voltage Inverter
Once you have your components, building a battery backup is extremely easy. Simply connect the solar panel to the battery charger, the battery charger to the battery and the battery to the voltage inverter.
In this diagram, the red lines represent positive wires and the blue lines represent negative wires. It’s very important with DC circuits to connect the positive and negative terminals correctly. The yellow line is the output of the voltage inverter, 120 VAC, which can be connected to home appliances.
If you look at the batteries in the diagram, you’ll see that I’ve put in three batteries. They don’t all have to be the same brand or capacity, as long as they are 12 volt deep cycle batteries. The important part is that I’ve connected all the positive terminals together and all the negative terminals together. This is called wiring them in parallel. They must be wired in this way.