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 need to 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 (kWh) over a period of time.
For example: 1,000 watts x 10 hours per day = 10 kWh per day
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.
PICKING THE RIGHT BATTERY
there are a few different battery types commonly used for off-grid storage systems:
FLOODED LEAD ACID
- Typical lifespan: 5-7 years
- Requires maintenance – add distilled water and equalize charge on a monthly basis
SEALED LEAD ACID
- Typical lifespan: 3-5 years
- No maintenance
- Typical lifespan: 10+ years
- No maintenance, no venting
The two main battery chemistries for off-grid are lead acid (flooded or sealed) and lithium. These two chemistries have unique characteristics. Lithium batteries are more efficient which means less power is wasted in the charge/discharge process. They also have a greater depth of discharge (DOD) allowing you to fully utilize all of your battery capacity.
Lead acid batteries are sensitive and need to be fully recharged every day where lithium batteries can stay at a partial charge without any adverse effect. Lead acid batteries also have more limited amount of usable capacity and can be typically discharged to 50%.
Because of the better efficiency and deeper discharge depth, lithium battery banks tend to be half of the size of a comparable lead acid bank!
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 the usage of 10 kWh per day, here are some examples:
LEAD ACID SIZING
10 kWh x 2 (for 50% depth of discharge) x 1.2 (inefficiency factor) = 24 kWh
10 kWh x 1.2 (for 80% depth 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 Ah at 48 V → 500 Ah x 48 V = 24 kWh
It’s usually a good idea to round up to help cover inverter inefficiencies, voltage drop and other losses. Think of this as the minimum battery bank size based on your typical usage. You may want to consider 600-800 amp hours of capacity, based on this example, depending on your budget and other factors.
Battery banks are typically wired for either 12, 24 or 48 volts depending on the size of the system. Here are examples of battery banks for both lead acid and lithium, based on an off-grid home using 10 kWh per day.
FOR LEAD ACID, 24 kWh IS EQUAL TO:
- 2,000 Ah at 12 V
- 1,000 Ah at 24 V
- 500 Ah at 48 V
FOR LITHIUM, 12 kWh IS EQUAL TO:
- 1,050 Ah at 12 V
- 525 Ah at 24 V
- 262.5 Ah at 48V
Other factors that influence battery sizing:
- Ambient Temperature – head 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’l need to run your backup generator.
This is not intended to be a comprehensive guide on off-grid design.
Use this information, based on your energy usage, to get an idea of the minimum battery bank size, and then call us at 818-280-3091 for help pricing the best battery for your needs.