Battery Size Calculator - Required Capacity Sizing
Calculate required battery capacity in Ah for UPS, solar, and backup power systems. Enter load, runtime, voltage, battery type, and DoD for accurate sizing.
About This Calculator
The Battery Size Calculator helps you determine the required battery capacity in amp-hours (Ah) for any DC-powered system including UPS backups, solar power systems, inverters, RVs, marine applications, and off-grid installations. By entering your load power, desired runtime, system voltage, battery chemistry, and depth of discharge (DoD), you get accurate sizing recommendations that prevent undersizing or overspending.
The calculator uses industry-standard formulas: load current = load power / voltage, then base capacity = load current x runtime. For lead-acid batteries, it applies the Peukert correction factor (0.02t + 0.6) which accounts for capacity loss at higher discharge rates. For lithium-ion batteries, the simpler 100 x I x t / (100 - Q) formula applies. A 15% safety margin is added to arrive at the recommended capacity.
Proper battery sizing is critical for system reliability. An undersized battery will discharge too deeply, reducing cycle life and potentially damaging the cells. Oversizing adds unnecessary cost and weight. This calculator accounts for both your actual energy needs and the safe depth of discharge limits specific to each battery chemistry.
Regional Notes
India: Home UPS systems commonly use 12V lead-acid batteries in the 100-200Ah range. Solar installations typically run at 24V or 48V. Voltage fluctuations and frequent power cuts make proper sizing especially important. Tubular lead-acid batteries are popular for their longevity in Indian conditions.
United States: Residential solar systems typically use 48V battery banks with lithium-ion (Powerwall-style) or lead-acid batteries. NEC compliance for wire sizing based on load current is important — higher voltage systems reduce current and allow thinner conductors. 120V/240V AC loads are converted via inverter.
United Kingdom: 230V AC household supply means inverters convert from battery DC to 230V. British homes increasingly adopt solar-plus-storage with lithium-ion batteries. The 50% DoD rule for lead-acid and 80-90% DoD for lithium remain the same, and battery sizing follows identical electrical principles.
Frequently Asked Questions
How do I calculate the battery size I need?
Divide the load power in watts by the battery voltage to get load current in amps, then multiply by desired runtime in hours. Then divide by the depth of discharge (DoD) factor. For lead-acid batteries, also apply a Peukert correction factor of 0.02t + 0.6. A 500W load at 12V for 2 hours with lead-acid battery at 50% remaining charge needs approximately 55Ah.
What size battery for a 1000W inverter?
At 12V, a 1000W load draws 83.3A. For 1 hour of runtime with a lead-acid battery (50% remaining charge), you need approximately 92Ah. For 2 hours, you need about 155Ah. Higher voltage systems need smaller Ah for the same energy. A 48V system at 1000W only draws 20.8A.
How to size a battery for a solar system?
Calculate daily watt-hour usage, divide by battery voltage, and multiply by autonomy days. Apply depth-of-discharge factor. A home using 5kWh per day at 48V with 2-day autonomy and lithium battery (80% DoD) needs approximately 260Ah.
What is the difference between CCA and Ah?
CCA (Cold Cranking Amps) measures a battery's ability to start an engine in cold temperatures. Ah (Amp-Hours) measures total energy storage capacity. Car starter batteries use CCA ratings, while deep-cycle batteries for solar, marine, and UPS systems use Ah ratings.
Should I oversize my battery?
Yes, oversize by 15-30% to account for aging, temperature effects, and unexpected loads. Lead-acid batteries should not be discharged below 50% remaining charge. Lithium-ion batteries can safely discharge to 80% depth of discharge (20% remaining charge), requiring less oversizing.
What battery voltage should I choose?
Higher voltage systems require less current for the same power, reducing required Ah and allowing thinner wiring. A 500W load at 12V draws 41.7A, at 24V draws 20.8A, and at 48V draws only 10.4A. Common voltages are 12V (small systems), 24V (medium), and 48V (large solar systems).
How does battery chemistry affect sizing?
Lithium-ion batteries can be discharged deeper (80-90% DoD) than lead-acid (50% DoD), so you need less Ah capacity with lithium for the same usable energy. Lithium also has a flatter discharge curve, higher energy density, and longer cycle life but costs more upfront.
What is the Peukert effect in lead-acid batteries?
The Peukert effect describes how lead-acid batteries deliver less usable capacity at higher discharge rates. The correction factor 0.02t + 0.6 accounts for this, where t is the discharge time in hours. For short discharges under 1 hour, the correction is significant and requires a larger battery.