Calculator Inputs
Example Data Table
| Example input | Value | Example output | Value |
|---|---|---|---|
| Daily load | 8.50 kWh/day | Required nominal bank energy | 29,513.89 Wh |
| Autonomy | 2.00 days | Required bank capacity | 614.87 Ah |
| Bank voltage | 48 V | Series batteries | 4 |
| Allowed discharge | 80% | Parallel strings | 4 |
| Inverter efficiency | 92% | Total batteries | 16 |
| Battery rating | 12 V, 200 Ah | Installed bank capacity | 800 Ah |
| Temperature derate | 0.90 | Installed nominal storage | 38,400.00 Wh |
| Design margin | 15% | Estimated backup | 2.60 days |
Formula Used
This calculator sizes a practical battery bank in several steps.
- Daily load in watt-hours
Daily Load Wh = Daily Load kWh × 1000 - Corrected usable storage needed
Required Usable Wh = (Daily Load Wh × Autonomy Days × Margin Factor) ÷ (Inverter Efficiency × Temperature Derate) - Required nominal battery storage
Required Nominal Wh = Required Usable Wh ÷ DoD Fraction - Required bank capacity in amp-hours
Required Bank Ah = Required Nominal Wh ÷ System Voltage - Series battery count
Series Count = ceil(System Voltage ÷ Battery Voltage) - Parallel string count
Parallel Strings = ceil(Required Bank Ah ÷ Battery Ah) - Total batteries
Total Batteries = Series Count × Parallel Strings
Margin factor = 1 + design margin percentage. DoD fraction = depth of discharge percentage ÷ 100.
How to Use This Calculator
- Enter the total site energy demand per day.
- Choose how many days of autonomy you need.
- Select the planned DC bank voltage.
- Set the allowable depth of discharge.
- Enter inverter efficiency from equipment data.
- Add battery voltage and rated amp-hour capacity.
- Apply a temperature derate for cold conditions.
- Add a design margin for aging and uncertainty.
- Submit the form and review the bank sizing.
- Use the CSV or PDF buttons to save results.
Frequently Asked Questions
1. What does autonomy mean here?
Autonomy means how many days the battery bank must support the load without useful solar charging. Remote sites often use one to three days, depending on weather risk and service expectations.
2. Why is depth of discharge important?
Depth of discharge affects usable energy and battery life. A higher value reduces the required bank size, but repeated deep cycling can shorten service life for many battery chemistries.
3. Why does the calculator include inverter efficiency?
Loads on the AC side need extra battery energy because conversion losses occur. Lower inverter efficiency increases the storage required to deliver the same useful site energy.
4. What is the temperature derate factor?
Cold weather can reduce available battery capacity. The derate factor models that reduction. For example, 0.90 assumes only ninety percent of rated capacity is practically available.
5. Why add a design margin?
The design margin helps cover aging, cable losses, real-world load growth, and operating uncertainty. It makes the final recommendation more practical for construction and field deployments.
6. What do series batteries and parallel strings mean?
Series batteries raise voltage. Parallel strings raise capacity. A bank needs enough series units to match voltage and enough parallel strings to meet the required amp-hour capacity.
7. Can I use this for lithium and lead-acid batteries?
Yes. The math works for both, but you should adjust depth of discharge, temperature assumptions, and design margin to match the chosen chemistry and manufacturer guidance.
8. Does this replace a full electrical design?
No. It is a planning tool. Final designs should also verify current limits, charge controller sizing, cable losses, protections, enclosure conditions, and applicable site codes.