Sizing Battery Capacity for Your Solar Power System: Straightforward Guide to the Basics
School is back in session and today’s lesson is all about battery capacity. More specifically, we’re going to provide a no-nonsense breakdown of how you can choose the correct size and type of battery bank for your specific solar power system and storage needs.
It may sound a little dry, but learning these basics can help you avoid some costly errors down the line. So, grab a pen and paper, pour a cup of coffee, and get ready for a really simple overview of how to determine the correct battery capacity you’ll need for your system.
Why is Sizing Battery Capacity Important?
Sizing your solar power system starts with determining the size of your battery bank. This is the foundation of your entire setup. Without an appropriately sized battery bank, the power your solar panels generate will be wasted. On top of that, you’ll be left in the dark when you need to tap into your system for power.
There are three critical steps to consider when sizing your battery bank: understanding what you are going to be powering, determining how long you'll be powering these devices for on a daily basis, and deciding how many days of power storage you’ll need.
Let's break these down further.
1) Determine Your Power Needs – What Do You Plan on Powering?
First things first, you need to determine what exactly you plan to power with your solar system.
This includes every device and appliance you intend on using, so don't skimp on this step. Really make an effort to list everything you want to power. For instance, if you need to have enough stored power for lights, a small refrigerator, a heater, and even just the ability to charge your phone, make sure you list them all.
Now it’s time to add up how much power these appliances and devices are going to require.
Each electronic device or appliance’s power consumption is measured in watts, which you can find directly on the device, or by calculating it from volts and amps (Watts = Volts x Amps).
For example, a device might state 120V and 12.5A, which equals 1,500W.
Understanding this simple equation is really useful because not every device is going to have its power requirements listed in watts directly on it, but most will list voltage and amperage.
For devices that do not run continuously, like a refrigerator or space heater, estimate the average run time. For example, if a device runs about 35-40% of the time, it’s a good idea to roundup and calculate as if it runs 50% of the time to ensure you have enough power.
2) Calculating Daily Usage – What are You Actually Going to Use Each Day?
Here comes the boring part, so let’s just get through it.
Now that you’ve listed all of the devices and appliances, as well as their power requirements, this step involves calculating how long you will be running these devices for each day.
For example, if you plan to use lights for five hours a day, you need to multiply the wattage of the lights by the number of hours they will be in use. If you have 200 watts of lights and use them for five hours, that totals 1,000 watt-hours per day for the lights. Repeat this process for each device.
I know, it sounds like a pain in the ass, but it’s actually fairly straightforward. Just jot down each figure, then add them all together, it’s really that easy.
Now, let’s say you also use a 1,500-watt heater for one hour each day in addition to the lights - that adds another 1,500 watt-hours. Now if you add this together with the 1,000 watt-hours needed for your lights, you would get a total daily usage of 2,500 watt-hours.
3) Determining Storage Duration – How Many Days of Power Do You Need?
The final step in sizing your battery bank is deciding how many days you want your system to be capable of running without needing to be recharged.
This is particularly important in off-grid locations that have inconsistent sunlight exposure. For instance, if you determine you need 2,500 watt-hours per day and want your system to last for three days without needing to be recharged, you would need a battery bank that can store 7,500 watt-hours.
In some regions, you might go days without seeing enough direct sunlight to fully charge your batteries, so you might want to lean towards a larger battery bank. The idea here is you’d have more power to draw upon during periods where your solar panels were less productive.
We always recommend adding a little extra capacity to account for unexpected usage spikes or less-than-optimal charging conditions. In our example, you might consider a 10,000 watt-hour battery bank, rather than a 7,500 watt-hour battery, as this would ensure that you have a comfortable buffer period.
Not only do you want to give yourself a little bit of wiggle room for sh*t weather, there are also going to be days when your power usage spikes, like if you had guests over, or needed to run your heater longer than usual.
Practical Example
Let’s take another look at our practical example to drive this three-step process home.
Suppose you want to be able to power your garage with a 1,500-watt heater and five 40-watt lights for a few days at a time. Here’s how you would size your battery bank:
Step 1: Identify Power Needs:
Heater: 1,500 watts
Lights: 5 lights x 40 watts each = 200 watts
Step 2: Calculate Daily Usage:
Heater: 1,500 watts x 1 hour = 1,500 watt-hours
Lights: 200 watts x 5 hours = 1,000 watt-hours
Total Daily Usage: 1,500 + 1,000 = 2,500 watt-hours
Step 3: Determine Storage Duration:
Desired Duration: 3 days
Total Storage Required: 2,500 watt-hours x 3 days = 7,500 watt-hours
Given the usage variability and the need for a buffer, aiming for a 10,000 watt-hour battery bank or multiple batteries that could store a total of 10kWh would be a good move. This ensures that even with increased usage or less-than-ideal sunlight, you could still draw the power you need.
Final Words and Our Recommendation
You don’t have to be a mathematician, professional electrician, or solar expert to calculate your power storage needs, but it’s a skill that’s really worth knowing. Take the time to calculate things properly – you’ll be glad you did when you’re kicking back drawing free solar electricity, even when the sun isn’t out.
Anyways, you can learn more about how to size an entire solar power system for your needs by clicking the button below and watching our free Intro to Solar course on YouTube!