How Much Energy Storage Do You Need?
Figuring out how many batteries you need can be daunting.
If you don’t have enough battery capacity, you run out of power and you'll need to add solar battery backup and fire up the backup generator.
On the other hand, if you buy too many batteries, you add unnecessary expense to your system, with extra components, complexity and maintenance.
Sizing solar batteries is one of the first steps in designing your off-grid system.
The amount of battery storage you need is based on your energy usage. Energy usage is measured in kilowatt hours over a period of time.
For example: 1,000 watts x 10 hours per day = 10 kWh per day
Check out our off-grid load evaluation calculator.
After estimating daily usage we need to consider which type of battery will work best, as they have unique performance characteristics and are sized differently.
Sizing Your Battery Bank
The exact math for sizing your battery system is based on your daily power usage and the battery type. Based on usage of 10kWh per day, here are some examples:
Lead Acid Sizing
10kWh x 2 (for 50% depth of discharge) x 1.2 (inefficiency factor) = 24 kWh
Lithium Sizing
10kWh x 1.2 (for 80% depth of discharge) x 1.05 (inefficiency factor) = 12.6 kWh
Battery capacity is specified either in kilowatt hours, or amp hours.
For example, 24 kWh = 500 amp hours at 48 volts → 500 Ah x 48V = 24 kWh
Other factors influence battery sizing:
- Ambient Temperature - Heat or cold has a big impact on battery performance and capacity.
- Seasonal Factors - People use more power at different times of the year. The sun produces more power in the summer than in the winter.
- Budget - Battery bank size is often a compromise between what you want to spend on batteries and how often you'll need to run your backup generator.
How To Calculate Solar Battery Bank Size
Our solar battery bank calculator helps you determine the ideal battery bank size, watts per solar panel, and the suitable solar charge controller. If you choose to build an off-grid system, it's important to size your system based on the month with the least amount of sunlight. That way, you will have access to a sufficient amount of energy at all times. There are several steps to follow when calculating your solar battery bank. Let's review them below:
Step 1: Determine Your Daily Energy Usage
You'll want to look to your electric bill for this information. It will usually be printed as your monthly kilowatt-hour output. To calculate your daily kilowatt-hour output, you will need to divide that number by 30, then multiply by 1000 to convert the number into watt-hours. Which translates to one watt of power sustained for one hour. This is the first step in determining your solar battery bank size.
Step 2: Estimate How Many Days Your Solar System Will Be Without Sun
If you don't know this information off-hand, you can look up the annual average of cloudy days for your area online. This step is crucial in ensuring you'll have access to your solar energy year-round. A large solar battery bank size will be best utilized in areas with more cloudy days, while a smaller solar battery bank should be sufficient in areas with prevalent sunlight. However, it's always recommended to size up rather than down.
Step 3: Estimate the Lowest Temperature Your Battery Bank Will Experience
Again, you can search the average low temperature for your area online. This step will help accurately predict the adequate capacity of your battery bank.
