Free Engineering Calculator

Battery Capacity & Sizing Calculators
for Solar and UPS Systems

Whether you are designing a solar power system, specifying a UPS for your facility, or planning an off-grid energy setup — these two free calculators tell you exactly how many batteries you need and what AH rating they must have.

1
Battery Bank VAH Capacity

Calculates the total energy storage your battery bank must hold, expressed in Volt-Ampere Hours. This is the "fuel tank size" your system requires — accounting for load, backup time, inverter losses, and discharge limits.

Result: VAH capacity required

2
Battery AH Capacity

Takes the VAH result from Calculator 1 and converts it into the AH rating each individual battery must have — based on your chosen battery bank voltage. Use the two calculators in sequence for a complete sizing result.

Result: AH rating per battery
Calculator inputs

What You Need to Enter

Load (kVA)
The total power demand of the equipment you want to run on battery backup. Your electrician or equipment nameplate will show this. If you only have the load in watts, divide by 1000 to get kW, then divide by the power factor to get kVA.
Tip: For mixed loads, add up all individual loads to get the total
Power Factor
An efficiency measure for electrical equipment. Motors, air conditioners, and fluorescent lights typically have a power factor between 0.7 and 0.9. Pure resistive loads like heaters and incandescent bulbs have a power factor of 1.0.
Tip: If unsure, use 0.8 as a safe default
Backup Time (Hours)
How long the batteries must keep your load running. Enter 2 for two hours of backup, 0.5 for 30 minutes, and so on. Longer backup times require proportionally larger battery banks.
Inverter Efficiency
Your inverter converts DC battery power into usable AC power, losing a small amount as heat in the process. A good quality inverter is typically 85–92% efficient. Enter as a decimal.
Tip: Enter 0.9 for 90% efficiency, 0.85 for 85%
Utilization Factor
A correction factor that accounts for reduced battery output when discharging quickly. Shorter backup times require a lower utilization factor. The calculator provides preset values matched to common backup durations — simply select the one that matches your backup time.
End Cell Voltage (ECV)
The minimum safe voltage per cell before the battery is considered fully discharged. Going below this causes permanent damage. Each ECV option in the calculator shows its corresponding Depth of Discharge so you can see exactly what level of discharge you are selecting.
Tip: 1.75V per cell (DoD 87.5%) is the standard recommended value for lead-acid batteries
Terminology

Key Terms Explained Simply

VAH — Volt-Ampere Hours

The total energy capacity your battery bank must hold. It is calculated from your load, backup time, inverter efficiency, utilization factor, and depth of discharge. Think of it as the total "fuel tank size" for your system.

AH — Ampere-Hours

The capacity rating printed on individual batteries. A 100 AH battery can deliver 5 amps for 20 hours, or 10 amps for 10 hours. The AH figure tells you what to specify when purchasing batteries.

Depth of Discharge (DoD)

How much of a battery's total capacity you use before recharging. Using 80% and keeping 20% in reserve significantly extends battery life compared to fully draining it every cycle. Lead-acid batteries should not regularly go below 50% DoD.

End Cell Voltage (ECV)

The minimum safe discharge voltage per cell. Once a battery reaches this point it must be recharged. Discharging below ECV causes irreversible damage to the plates and reduces the battery's rated capacity permanently.

Utilization Factor

Batteries deliver less total energy when discharged rapidly. The utilization factor corrects for this — a 30-minute backup uses a factor of 0.52 while a 4-hour backup uses 0.85, reflecting the greater efficiency at slower discharge rates.

Inverter Efficiency

The percentage of battery energy that actually reaches your load as usable AC power. The remainder is lost as heat inside the inverter. A higher efficiency inverter means a smaller battery bank is needed for the same backup capability.

Technical reference

Battery Discharge Rate — The C-Rate Explained

Every battery has a maximum recommended discharge rate set by the manufacturer, expressed as a C-Rate. This tells you how quickly a battery can safely deliver its stored energy. Exceeding the rated C-rate reduces usable capacity and shortens battery life.

C-Rate Discharge Time Typical Application
5C 12 minutes High-rate industrial applications
2C 30 minutes Short backup UPS systems
1C 1 hour Standard UPS backup
0.5C 2 hours Extended UPS / small solar
0.1C 10 hours Solar energy storage
0.05C 20 hours Lead-acid standard rating

Important: Lead-acid batteries are rated at 0.05C — they are designed to discharge slowly over 20 hours. Discharging faster than the rated C-rate reduces effective capacity and shortens battery service life significantly.

Battery type comparison

⚡ Lead-Acid Batteries

  • Most common type in UPS and solar systems
  • Do not regularly discharge below 50% DoD
  • Standard C-rate: 0.05C (20-hour discharge)
  • Lower upfront cost, heavier and bulkier
  • Require regular maintenance checks

⚡ Lithium-Ion Batteries

  • Lighter and more compact for same capacity
  • Can safely discharge to 20% remaining
  • Handles faster discharge rates better
  • Higher upfront cost, longer cycle life
  • Maintenance-free sealed units
Use the calculators below — start with Calculator 1

Calculator 1 for Battery Bank Capacity in VAH

Battery Capacity Calculator in VAH

Please fill in all fields with valid numbers before calculating.
Battery Bank VAH Capacity Required
—
Volt-Ampere Hours (VAH)

Disclaimer This tool provides an engineering estimate only. Actual battery capacity may vary due to temperature, discharge rate, battery quality, inverter efficiency, and aging. Always consult a qualified engineer for final system design.

End Cell Voltage (ECV) The minimum safe discharge voltage per cell. Discharging below this damages the battery. 1.75V per cell is the standard recommended value for lead-acid batteries.

Depth of Discharge (DoD) The percentage of battery capacity used before recharging. An 80% DoD means 20% charge is kept in reserve to protect battery life.

Calculator 2 for Battery Capacity in AH

AH Capacity Calculator

Step 2 — Use the VAH result from the Battery Bank Calculator above to find the required AH rating of each battery.

How to use: First run the Battery Bank VAH Calculator above. Take the VAH result and enter it here along with your chosen battery bank voltage to get the required AH per battery.
Enter the VAH result from the Battery Bank Capacity Calculator above.
Please fill in all fields with valid numbers before calculating.
Required AH Rating per Battery
—
Ampere-Hours (AH)

What does this result mean? The AH figure is the minimum rated capacity each battery in your bank must have. When purchasing batteries, always select the next standard size up from the calculated value — common ratings are 50, 65, 100, 150, 200 AH.

Disclaimer This tool provides an engineering estimate only. Actual requirements may vary due to battery quality, manufacturer specifications, temperature, and system design. Always consult a qualified engineer for final system sizing.