Here’s a Crash Course in Battery System Sizing
Get your calculator ready.
Depth of discharge
As discussed a few days ago on the Fourth Day of Storage, depth of discharge plays an important role when sizing batteries because battery banks must be calculated according to the actual amount of usable energy storage. Check your battery’s warranty for the most accurate statement of its depth of discharge. For example:
- 80% DoD = 3.5 kWh x .8 = 2.8 kWh
- 90% DoD = 3.5 kWh x .9 = 3.1 kWh
- 100% DoD = 3.5 kWh x .1 = 3.5 kWh
Sizing based on loads
The most important step when sizing a battery system is to determine the required or desired amount of energy storage — most often using a measure of kWh-per-day. The minimum kWh-per-day value can be calculated based on the wattage and runtime of all potential loads to be supported by the system. From there, the battery size may be adjusted depending on whether the system is intended for daily cycling or backup power. The load profile and desired duration of backup power should also be considered.
There are a number of online resources available to calculate loads and determine the appropriate kWh-per-day value from utility billing information.
Consider this example:
- 2.8 kWh at 80% DoD
- Load calculations: 10 kWh per day
- Customer requests: 1.5 days of backup power
- 10 kWh x 1.5 days = 15 kWh of desired storage
- 15 kWh/2.8 kWh (battery size) = 5.3 batteries
In this example, based on the actual usable amount of energy for 5.3 of the batteries selected, you may choose to size up to 6 batteries or round down to 5 batteries based on customer preference, either to create extra cushion for unforeseen electrical loads or to be cost conscious and design a more conservative system.
The battery bank at California Governor Jerry Brown’s off-grid residence.
When retrofitting an existing PV installation to add storage, battery bank size is most often computed based on the size of the solar array. It is important to consider peak sun hours, PV Watts data (realistic energy production based on location), and PV size (kW) as part of the calculation. In addition, it’s critical that the system cannot exceed the maximum continuous charge rate of the battery bank to prevent damage and ensure a long life. For example:
Battery for system: 3.5 kWh battery with maximum charge of 1.7 kW continuous
PV array size: 4 kW
Average PV daily production: 20 kWh per day
4 kW (PV) / 1.7 kW (Max. battery charge) = 2.3 batteries
Round up the 2.3 battery units to determine that the minimum number of batteries would be three of the 3.5 kWh batteries.
Based on daily PV production: 20 kWh (PV per day) / 3.1 kWh (battery at 90% DoD) = 6.4 (3.5 kWh batteries)
From this calculation, you can round up to 7 or round down to 6 batteries based on customer preference (such as mode of operation).
Every battery has a maximum charge and discharge capacity. Those rates must be adhered to for adequately charging the battery, like PV system size (charge rate) and the continuous load value to be supported by the battery (discharge rate).
Consider this example:
Battery for system: 3.5 kWh with a maximum continuous discharge of 1.7 kW
Home maximum continuous discharge: 6 kW
6 kW (continuous load) / 1.7 kW (battery maximum discharge) = 3.5 batteries
When it comes to power requirements, you always round up to determine the minimum battery bank size. In this example, the system requires 4 of the 3.5 batteries.
For additional guidance, SimpliPhi Power offers a simple battery bank sizing estimator tool right here.
This blog post first appeared as part of Solar Builder's 12-Days of Storage Series.