You’ve probably heard of solar batteries, the energy storage units that allow you to capture solar energy that you can’t use immediately. By linking more than one battery, you can store more energy in what’s known as a battery bank.
You could use either lead-acid or lithium ion batteries in your battery bank. Lead-acid batteries are cheaper and more common than lithium ion batteries, but lithium ion batteries have longer lifespans, higher efficiencies, and higher energy density than their lead-acid counterparts.
Buying or building a battery bank will both add cost to your home solar energy system, but you’ll probably pay more for your battery bank if you hire an installer to set it up for you. On the other hand, putting a battery bank together yourself can take time. If you doubt your ability to accurately calculate your energy needs or work safely with electrical wiring and equipment, or if you just want to save yourself some time, it’s best to have a solar installer set up your battery bank.
Many solar installers offer the option of including a solar battery with your system’s installation. If you’re linked to the grid and have reliable electricity service, you probably don’t need to add a solar battery. But if you’re in an off-grid location, a battery or battery bank can be crucial for ensuring your home gets enough energy at night, during overcast weather, or when the sun isn’t shining.
If you change your mind and later want a solar battery, it’s possible to have one (or more) added to your system, though the cost will likely be greater than it would be if you got it installed at the same time as the rest of your system. The best way to add a battery is to contact the installer who originally set up your system. They’ll have a good sense of which batteries would work well with your system, and will be able to install or recommend a battery that matches your needs. If you installed the system yourself, contact the manufacturer of your solar panels and ask them for battery recommendations.
If you can’t find an installer to provide you with a battery bank, or if you just like the challenge of a hands-on home project, you can start building a battery bank of your own with just a few easy steps.
The first step is to determine the amount of energy your solar panels produce per day. To calculate this amount, you could either use a daily average or the amount of energy necessary to meet your needs during the winter peak, when you get the least sunlight. If you’re off-grid, your best bet is probably to use the energy you need to power your home during the winter peak. If you choose to use your daily average, look over your past energy bills and add up your annual kilowatt-hour total. Divide this number by 365 to determine the number of kilowatt-hours you use per day.
If you’re in an off-grid location and don’t receive a home electric bill, you’ll need to calculate the kilowatt-hour usage of your home the long way. This means tabulating the total wattage of each of your devices and appliances, then multiplying that number by the number of hours you use it per day. For instance, if you have 10 LED light bulbs, each of which is rated at eight watts, and you usually use each one six hours daily, you’ll need to develop a battery bank capable of sustaining at least 480 watt-hours (10 times eight times six) for lighting per day.
When using this wattage calculation method, create a digital spreadsheet to keep track of all your appliances, their wattage, and the number of hours you use each one.
After figuring out the watt-hour expenditures for everything in your house, add all the totals together to arrive at your home’s daily energy total. For instance, if you have a TV that uses 100 watt-hours, a fan that requires 300 watt-hours, and a computer that uses 400 watt-hours, plus the 480 watt-hours of lighting mentioned above, your home’s total daily energy use would be 1,280 watt-hours (480 + 400 + 100 + 300) per day.
You can use the daily watt-hour total to determine the amount of power your battery bank needs to sustain your home for one day. Just divide your daily watt-hours by the voltage of your battery system. For instance, if you’re developing a battery bank for the 1,280 watt-hour system described above and your battery system has a voltage of 24, you’d divide 1,280 by 24, yielding a quotient of 53.3 amp-hours. In other words, your battery bank needs at least 53.3 amp-hours of storage to provide power for one day.
You’ll also need to determine how deeply you expect to discharge your batteries. Totally discharging your batteries can shorten their lifespans. The recommended depth of discharge for lead-acid batteries is usually around 50 percent. In other words, manufacturers recommend that you only use 50 percent of the battery’s total energy. Lithium ion batteries, on the other hand, can be discharged down to around 20 percent of their total storage capacity.
Since discharge rates require you to always leave at least some portion of the battery energy in reserve, you’ll need to upsize your battery bank by an appropriate amount to ensure you have enough usable energy. For instance, suppose you need 100 amp -hours and your batteries can be discharged to a depth of 20 percent. In this case, you’ll divide 100 amp-hours by 0.8 (80 percent, or 100 percent minus your maximum discharge depth of 20 percent), leaving you with a final tally of 125 amp-hours.
Once you know how much battery energy you need to sustain your appliances and electronics each day, determine the amount of backup power you want. For instance, you could install a battery bank that would provide you with enough energy for one day (equal to your amp-hour total), half a day (by dividing your basic amp-hour total in half), two days (by doubling your basic amp-hour total), and so on. Just remember: the larger your battery bank, the more expensive it will be, and the more space it will require.
There are two ways to connect your batteries: in parallel or in series. Connecting your batteries in series means that each battery’s positive terminal is placed next to another battery’s negative terminal, then the two are linked. Linking your batteries in this way adds the batteries’ combined voltages together. If your battery system is 12 volts, for instance, you could link two six-volt batteries in series instead of using one 12-volt battery.
When connecting your batteries in parallel, each battery’s negative terminal is linked to the next battery’s negative terminal, and each battery’s positive terminal is linked to the next battery’s positive terminal. This adds all the batteries’ amp-hours together.
In addition to batteries, you’ll need an inverter to convert the battery bank’s DC energy into usable AC energy. If you want to charge your battery from the grid as well as your solar panels, you’ll need an additional inverter or a bi-directional inverter. Additionally, some batteries are only compatible with certain types of inverters, so check your battery’s user guide and warranty before selecting an inverter.
To determine the size of the inverter you’ll need, add up the maximum possible wattage of all your appliances and devices that could run simultaneously. For instance, if your 10 lights require 60 watts each, your TV requires 100 watts, and your computer requires 150 watts (and you have no other electronic devices in your house), you’ll need an inverter rated at 850 watts (600 + 100 + 150 watts) or slightly more.
Yes, you’ll probably need a charge controller (sometimes called a charge regulator) to regulate the charge from your solar system to your battery. Without a charge controller, your battery could become damaged due to overcharging.
There are two types of charge controllers: maximum power point tracking (MPPT) and pulse width modulated (PWM). Most charge controllers are of the PWM variety, but they charge batteries less efficiently than MPPT controllers. On the other hand, PWM controllers also cost less than MPPT controllers.
The easiest way to choose the right charge controller for your battery bank and solar array is to use the sizing tools offered on manufacturer websites. But if you want to size your charge controller yourself, you’ll need to account for both the voltage of your battery bank and the wattage of your the solar array. Dividing the array’s wattage into the voltage of your battery bank will yield an amperage figure that you can use to size the charge controller.
For instance, if you have a four-kilowatt (4,000-watt) array and your battery bank is 48 volts, you’d divide 4,000 by 48, yielding a quotient of 83.3 amps. Since most charge controllers should have an amperage 20 to 25 percent larger than the actual system you’re planning around, you’d want a charge controller rated at approximately 100 amps.