Battery Storage Calculator Formula
Understand the math behind the battery storage calculator. Each variable explained with a worked example.
Formulas Used
Required Battery Capacity
battery_kwh = total_capacityUsable Capacity
usable = usable_kwhEstimated System Cost
system_cost = total_capacity * cost_per_kwhTesla Powerwalls Equivalent
num_powerwalls = ceil(total_capacity / 13.5)Variables
| Variable | Description | Default |
|---|---|---|
daily_kwh | Daily Energy Requirement(kWh) | 30 |
backup_days | Backup Duration(days) | 1 |
dod | Depth of Discharge(%) | 80 |
cost_per_kwh | Battery Cost(USD/kWh) | 500 |
usable_kwh | Derived value= daily_kwh * backup_days | calculated |
total_capacity | Derived value= usable_kwh / (dod / 100) | calculated |
How It Works
How to Size Battery Storage
Battery systems are sized based on energy needs and desired backup duration.
Formula
Usable Capacity = Daily kWh * Backup Days
Total Capacity = Usable / Depth of Discharge
Depth of discharge (DoD) limits how much battery capacity you actually use. Most lithium batteries recommend 80-90% DoD to maximize lifespan.
Worked Example
A home needing 30 kWh/day with 1 day backup at 80% DoD and $500/kWh battery cost.
- 01Usable capacity: 30 * 1 = 30 kWh
- 02Total capacity: 30 / 0.80 = 37.5 kWh
- 03System cost: 37.5 * $500 = $18,750
- 04Powerwalls needed: ceil(37.5 / 13.5) = 3 units
Frequently Asked Questions
What is depth of discharge?
Depth of discharge (DoD) is how much of the battery capacity you use. An 80% DoD on a 10 kWh battery means you use 8 kWh. Lower DoD extends battery life.
How long do home batteries last?
Most lithium-ion home batteries are warranted for 10-15 years or 3,000-5,000 cycles. After that, they retain about 70-80% of original capacity.
Can batteries pay for themselves?
In areas with time-of-use rates, batteries can save $50-200/month by storing cheap off-peak electricity. Combined with solar, payback can be 7-12 years.
Ready to run the numbers?
Open Battery Storage Calculator