Understanding and Planning Your Energy Storage
Perhaps like many of you, I dream of one day being able to cut ties with the grid, but there are many things to learn about creating the ideal set up, and one question I had was how to exactly how many batteries I would need in my battery bank. I did a little looking around, and here is what I found.
How many batteries do I need for off grid solar? This will depend upon your daily kWh usage, and the type of battery you intend to use (lead acid vs. lithium). The formula for determining this is (kWh per day used) divided by (kWh per battery), rounded up for extra head room.
The task of choosing a type of battery, and how many you will need may seem daunting, but once you know some of the basics, it will be far less intimidating. Here is a quick overview that I’ve compiled in my research.
How Many Batteries Will I Need?
The main information you need to figure out the number of batteries you will need is how much energy you use on an average day, which is measure in kWh. This will be your minimum amount of energy storage. Mess up on this number, and you may find yourself out of power in the middle of the night!
Once you know that number, you can then compare it to the kWh ratings of various batteries. Different batteries have different ratings for this, as well as some other things to consider, which I will discuss below.
Whichever battery you choose, the kWhs of the total number of batteries will need to be at least the number of your kWh usage. That means that your usage divided by the battery’s kWh will give you the total number that you will need.
There’s a few things to remember here though. The rest of your solar system needs to be strong enough to fully charge the batteries, or you may not get the results you need. That means you need to have enough solar panels, which are positioned for optimal light, and a charge controller, which can handle the electric load. This will affect how much will actually be stored in the batteries. If any of these are less than ideal, you may end up not never fully charging your battery bank.
How Do I Determine My kWh Usage?
A kilowatt-hour (abbreviated kWh), is one thousand watts of usage per hour.
To determine you daily kilowatt-hour usage you need to add up the wattage of all things in your home, and multiply them by the number of hours used. Once you have each appliances individual usage, all you have to do is add them all together. That will give you your total daily kWh usage.
Most every device you own has an output wattage ratting printed right on the power plug, or the back of the device.
Here’s an example of electronics you might be using
- Small Refrigerator: 50w
- Window Air Conditioner: 900w
- LED Lighting (60w Equivalent): 13w
- Laptop Computer: 100w
- Cell Phone Charger: 15w
- Internet Router: 7w
- 19 Inch LCD TV: 16w
Many items will have a label that looks like this: “DC Rating: 19V – 2.15A” This is the voltage rating with the amperage of current. To convert this to wattage, all you have to do is multiply them together: 19V x 2.15A = 47.5W.
For smaller items, the amperage might be rated in milliamps (mA). One thousand milliamps equals one amp. So for example, if you have a camera battery charger that’s labeled with a rating reading “DC 8.4V – 500mA”, you would multiply 8.4 by .5 to find a wattage of 4.2, since 500mA is half an Amp.
Once you have the wattage, the next step is to estimate how long you use each one everyday, and multiply the time by the wattage. For example, lets say you have an LCD TV that is rated at 16 watts, and you typically watch 3 hours of TV a day: 16w x 3 hours = 48w of power every day.
Do that for every device, and ad the answers together. That is your total Watt Hours (or more likely kWh) per day. That wasn’t too bad huh? If you take the daily number and multiply that by 30, of course you’ll get your monthly wattage use. Multiply that by 12 and you’ll find your yearly use.
This maybe a bit of a time consuming process adding up all of your devices, but this number is crucial to determining not only your battery bank size, but your solar panel array size, and the size of your charge controller.
How Do I Decide What Battery is Right For Me?
Batteries are all the same right? Wrong. There are different types of batteries, different kWh, different life expectancy, and vastly different costs.
There are three main types of batteries that are currently used: Flooded Lead Acid, Sealed Lead Acid, and Lithium. There are several other types of batteries that are being developed today, but I’ll focus on these three, which are also the most available.
Flooded Lead Acid batteries are a more conventional type of battery. Chances are you have one of these in your car. They are the least expensive type of battery with a typical lifespan of around 5-7 years. Many off-girders start with this type of battery because of the low cost, which will help offset the initial investment of the solar array and other equipment necessary. The downfalls of this type of battery is that they require regular maintenance. You’ll need to check the water level in the cells monthly, and had distilled water. This type of battery also gives off hydrogen gas, meaning that if kept indoors, they will need to be vented outside.
Sealed Lead Acid batteries are a little more expensive than Flooded Lead Acid batteries, but require no maintenance, and give off less gas. The downside to this type of battery is that it only lasts 3-5 years.
Lithium batteries so far reign supreme, and they will likewise cost you supremely, sometimes multiples of times more than the other types. The upside is, there is no maintenance, venting, and they last much longer. Lithium batteries will last ten or more years. They also charge faster, and have a more usable capacity, meaning you can run the voltage down on them further between charges, without harming their lifespan.
You may hear Marine Batteries mentioned for use in off-grid energy. Marine batteries are similar to car batteries, they are designed to put off a strong burst of energy to crank an engine, but unlike car batteries, they are designed to give off a smaller amount of energy over a long period of time (as to power appliances). This may work for smaller systems, but they are designed mainly for short term use (boating). Over a longer period of time of being constantly drained and recharged, they will loose a lot of their storage capacity.
Deep-Cycle batteries are a types of batteries that are designed to be discharged to a lower percentage on a regular basis without causing any overall loss of battery life. While continually draining and recharging non-deep-cycle batteries is possible, it will overtime decrease how long the battery lasts. Deep-cycle batteries can be drained down to 20% without causing damage (although some manufacturers recommend not going below 45%). That makes this type of battery is the best suited for off-grid energy.
You’ll also need to know a little about Amp Hours (Ah) of the different batteries. An Ah is what they use to designate how long the battery life will last, and at how many amps. It’s determined by multiplying the amps being used by the number of hours it is used. So for example, a 10A battery running for 10 hours before full discharge would be 100Ah, since 10×10=100.
You can use the Amp Hours to determine how many Kilowatt-Hours you have per battery. To find this, multiply the Voltage of the battery by the Amp Hours. So if you have a 12V battery with a rating of 100Ah, 12 x 100 = 1200. This battery has 1200 Watt Hours of power. Since there are 1000 Watts in a Kw, you have 1.2 kWh in that one battery
Now that you know some of the basics on batteries, you can shop around and see what best suits your needs, and budget.
How Do I Wire My Battery Bank?
Before purchasing the batteries for your bank, you may also want to consider how to wire them, as this will affect the overall storage (kWh/Ah) and voltage. Lets discuss the basics.
Batteries joined in a Series combine the voltage of the total number of batteries connected, but maintains the same Amp Hours (Ah). For example, connecting two 6 Volt (V), 10 Ah batteries, will create at 12V 10Ah bank. This is accomplished by jumping the negative terminal of one battery to the positive terminal of the second battery, while connecting the two remaining terminals to the system (such as to an inverter). This works best with batteries with the same voltage and capacity ratings.
Joining batteries in Parallel does the opposite, increasing the capacity (Ah), while the Voltage stays the same. Connecting both positive terminals together with one cable, and both negative terminals together with another does this. To connect the bank to the system, run a second set of cables from only one of the batteries positive and negative terminals. Keeping with the example above, the same two 6V 10 Ah batteries will put out a voltage that remains at 6V, but with a 20Ah capacity.
You can also combine the last two methods to create a bank that is in Series and Parallel, which both raises the Voltage and Capacity. This is done by connecting two sets of Parallel batteries in a Series, by running one cable from the positive terminal of one set of Parallel, to one negative terminal of another set in Parallel. With four 6V 10 Ah batteries, this would produce a bank that is 12 V and 20 Ah.
Related Questions
What temperatures should my batteries be stored at? Battery manufacturers have their own recommendations for their products, but usually 68-77°F (20-25°C)
Are there any other additional ways to charge my battery bank? In addition to solar panels, some may choose to back up their systems with wind or water turbines, or backup generators.
What else will I need besides solar panels and batteries? You will also need a charge controller to maintain your batteries, and a power inverter to run your electronics.What Happens if My Batteries Fully Charge in the Middle of the Day? If your battery bank fills completely while there is still sunlight, the charge controller will shut off charging to your batteries. In some areas you can sell the access power you your local power company if you have a grid tied backup.
