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Module 2: Sizing Your Battery Bank

In Module 1, you learned some basic electrical terminology and concepts, including volts, amps, watts, and the difference between AC and DC power. You also you gained an understanding of how they fit into solar power systems.

This lesson will show you how to calculate the battery storage you actually need, so you don’t overspend on batteries you’ll never use, or undersize your battery bank in a way that leaves you without power when you need it the most. 

If you want to keep these formulas handy, remember you can download the full course as a PDF.

Why Sizing Battery Capacity Is Important

When most people think about solar power systems, they focus on the panels. In reality, your batteries are going to be the foundation of your entire solar power system.

Without the right sized battery bank, the power your solar panels generate will be wasted. Your battery capacity also determines:

• How long you can go without grid power
• The types of appliances and devices you can run
• How big you can make your solar panel array

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.

The Three-Step Battery Sizing Process

To size your batteries the right way, you’ll need to start by answering three basic questions: 

1. What are you going to be powering?
2. How much power will you use each day?
3. How many hours or days of backup power do you need?

Let’s break this down step-by-step.

Step 1: Determine What You Want to Power

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.

Really make an effort to list everything you want to power. Don’t skip anything, even small devices and loads will add up. This means writing down everything you’ll need, like the following:

• Lights
• Refrigerator
• Heater, fans, small air conditioner
• TV
• CPAP
• Laptop and phone chargers
• Wi-Fi router

Once you have listed everything you want to run with your solar power system or power station, you have to calculate their power consumption in watts.

Finding Power Consumption Totals (Watts)

Now it’s time to add up how much power these appliances and devices are going to require.

The power consumption for each electronic device or appliance is measured in watts, which you can normally find directly on the device itself. If watts aren’t listed, but the voltage and amperage are, use the formula we discussed in Module 1:

       Volts x Amps = Watts

For example, a device might state 120V and 12.5A, which equals 1,500W (120 x 12.5 = 1,500)

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 almost everything will list voltage and amperage for safety reasons.  

For devices that do not run continuously, like a refrigerator or space heater, estimate the average run time. 

⚡ Solar Lab Tip: Overestimate Your Runtimes

Let’s say a device runs about 35-40% of the time, like a refrigerator, which will only pull power while the compressor is on. We recommend rounding up and calculating as if it runs 50% of the time. 

This will ensure you have enough power. In the real world, it’s always better to have more battery capacity than you need, rather than not enough.

Step 2: Calculate Your Daily Power Use (Watt-Hours)

Now that you’ve listed all of the devices and appliances, as well as their power requirements, the next step involves calculating how long you will be running these devices for each day.

How much energy you use in a day will be measured in watt-hours or kilowatt-hours, which we also explained in Module 1. 

Daily Power Requirements Formula

When you’re sizing batteries, knowing how much power an appliance or portable device requires is pretty meaningless if you don’t know how long you’ll need to power it. 

To calculate your watt-hours, you can use the following formula:

       Watts x Hours Used Per Day = Watt-Hours Per 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 all of your lights have a total wattage of 200W, and you plan to leave them on for five hours, that totals 1,000 watt-hours per day for the lights (200W x 5h = 1,000Wh/day). 

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 to your daily power use. 

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. In this example, this would be your total daily usage, which is one of the most important numbers you’ll need for sizing your entire system or power station. 

⚡ Solar Lab Tip: 1 Kilowatt-Hour = 1,000 Watt-Hours

Don’t get confused if you see kilowatt-hours instead of watt-hours. Remember, it’s just a bigger measurement for the same thing. One kilowatt-hour (kWh) is just the equivalent of 1,000 watt-hours (Wh). Bigger batteries and power stations usually list kWh to make things cleaner.

Repeat this calculation for each device and appliances. We have a free Power Consumption Calculator you can use to add up your energy usage. It has typical power requirements for a wide range of appliances and devices, as well as an option for customer loads.

It sounds tedious, but adding up your total power requirements in watt-hours is going to allow you to accurately size a battery bank. Just jot down each figure, then add them all together, it’s really that easy.

Step 3: Decide How Many Days of Power 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 being recharged.

Do you just want a few hours of backup power for a short-term blackout, a full day of power for longer outages and emergencies, or do you want to be able to live fully off-grid for multiple days at a time?

This is particularly important in off-grid locations that have inconsistent sunlight exposure. 

For example, let’s say you need 2,500 watt-hours per day and want your system to last for three days without needing to be recharged. That would mean you’d need a battery bank that can store 7,500 watt-hours.

       2,500Wh x 3 Days = 7,500Wh Total Power Storage

In some regions, you might go days without seeing enough direct sunlight to allow your solar panels 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 are less productive. 

Why You Should Oversize Your Battery Bank

We always recommend adding a little extra battery capacity to account for unexpected usage spikes or less-than-optimal charging conditions. 

Using the example above where we calculated 7,500Wh of total storage requirements, we’d recommend aiming for a 10,000 watt-hour battery bank, rather than a 7,500Wh battery, as this would ensure that you would have a comfortable buffer. 

Not only do you want to give yourself a little bit of wiggle room for bad weather that would prevent your solar panels from generating enough power to recharge the batteries, there are also going to be days when your power usage spikes, like days you need to run your heater longer than usual, or maybe a situation where you just had more people in the house than expected.

Having buffer battery capacity ensures:

• You’re not risking running out of stored power
• You can handle longer-than-expected outages
• Your batteries can last longer, as full discharges tend to shorten their lifespan

Tying it All Together: Garage Solar Power System Example

Let’s take another look at a practical example to drive this three-step process home.  

Suppose you want to be able to supply power to your garage, which has a 1,500-watt heater and five 40-watt lights. For this example, we’ll say you want to be able to run those loads without grid power for three full days. 

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 = 7,500Wh or 7.5kWh

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. 

You could then break this into a single 10kWh battery, two 5kWh batteries, or any other combination of smaller batteries that added up to a total capacity near that 10kWh total.

Doing so would ensure that even in situations where your usage increased beyond 7.5kWh, or when you needed more than three days of power, you would still have the extra power you needed. 

⚡ Solar Lab Tip: Batteries Are the Most Expensive Part of Solar

For most solar power systems, batteries account for roughly 50% to 70% of the total cost. Sizing them correctly can be the biggest money-saving step you can take, so it’s worth taking the time to accurately calculate your needs. 

Putting it All Together

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. Buying batteries without calculating your usage requirements could result in wasted money on more expensive batteries than you need, or undersized batteries that won’t last. 

If you’re looking for backup power for your entire home, we recommend reading the following guide:

Up Next: Module 3 – Types of Solar Power Systems Explained

In the next lesson, we’ll break down the four main types of solar power systems and explain when each one actually makes sense for you.

By the end of Module 3, you’ll know which system category fits your goals, budget and situation. Again, the aim here is to avoid getting upsold on something you don’t actually need.