Battery Size Calculator
Enter your load power, system voltage, required runtime, and battery chemistry. The calculator works out the minimum amp-hour (Ah) and watt-hour (Wh) capacity you need, accounting for depth of discharge and efficiency losses. Switch between watts and amps for the load input, and see a charge-over-time chart for how the battery depletes under your load.
How to size a battery correctly
Choosing the right battery is a two-step process. First, work out how much energy your load will consume over the time it runs (watt-hours = watts x hours). Second, account for the fact that a battery cannot be fully drained without damage (depth of discharge) and that some energy is lost to heat in the cell chemistry (round-trip efficiency). The formula is: Required Ah = (Load current x Runtime) / (Depth of discharge x Efficiency). The result is the minimum nameplate capacity you need. Adding a 10-25% safety margin protects against cell aging, unexpected loads, and cold temperatures reducing capacity.
Depth of discharge and why it matters
Every battery chemistry has a safe maximum depth of discharge (DoD) -- the fraction of its total capacity that can be removed before the chemistry is damaged. Lead-acid batteries (AGM, gel, flooded) should typically not be drawn below 50% state-of-charge, meaning their usable DoD is 50%. Lithium-ion cells can go to 80% DoD and LiFePO4 cells even to 90% without shortening life significantly. Regularly exceeding these limits accelerates plate sulfation in lead-acid or lithium plating in lithium cells, which permanently reduces capacity. If your application requires discharging deeply and often, choose a lithium chemistry.
Understanding the C-rate
The C-rate tells you how fast a battery is being discharged relative to its total capacity. A 100 Ah battery discharged at 10 A has a C-rate of 0.1C (often written C/10). Lead-acid batteries are particularly sensitive to discharge rate: a battery rated at 100 Ah at C/20 (20-hour rate) might only deliver 75 Ah at C/5 and 55 Ah at C/2. Lithium chemistries are far less affected. If your C-rate works out to above C/5, especially for a lead-acid system, you should use a larger battery than the minimum this calculator suggests, or switch to a lithium chemistry.
Comparing lithium and lead-acid batteries
Lithium-ion and LiFePO4 batteries cost more upfront but deliver more usable energy per kilogram, survive more charge cycles, tolerate deeper discharge, and lose less energy in round-trip inefficiency. Over a 5-10 year horizon they are often cheaper on a cost-per-cycle basis than lead-acid. Lead-acid batteries -- AGM, gel, and flooded -- are less expensive to buy, widely available, and perform acceptably in temperatures down to about -20 degC (-4 degF) with appropriate derating. Flooded lead-acid is the lowest cost option but requires regular electrolyte checks and a vented enclosure. AGM and gel are sealed and maintenance-free.
Battery chemistry comparison
| Chemistry | Max DoD | Efficiency | Typical cycles | Notes |
|---|---|---|---|---|
| Lithium-ion (Li-ion) | 80% | 95-99% | 500-1000 | High energy density, no memory effect |
| LiFePO4 | 90% | 95-99% | 2000-5000 | Safest lithium, long life, wide temp range |
| AGM (lead-acid) | 50% | 80-90% | 300-600 | Maintenance-free, good cold-weather rating |
| Gel (lead-acid) | 50% | 80-90% | 300-500 | Vibration-resistant, slow charge required |
| Flooded lead-acid | 50% | 75-85% | 200-400 | Lowest cost, requires venting and watering |
Typical depth of discharge, round-trip efficiency, and expected cycle life by chemistry. Values are general guidelines; consult your battery datasheet for exact figures.
Frequently asked questions
What is the difference between Ah and Wh?
Amp-hours (Ah) measure how much charge a battery holds -- specifically how many amps it can deliver for how many hours. Watt-hours (Wh) measure energy, combining both voltage and charge. The relationship is: Wh = Ah x Voltage. A 100 Ah battery at 12 V holds 1200 Wh (1.2 kWh) of energy. Wh is the more useful unit when comparing batteries of different voltages, because two 50 Ah batteries at different voltages hold completely different amounts of energy.
Why does lead-acid lose capacity at high discharge rates?
In a lead-acid cell, the chemical reaction that converts lead sulfate back to lead and sulfuric acid happens at the surface of the plates. At high discharge rates there is not enough time for ions to diffuse into the plate material, so only the outer surface participates and you get less total energy out. At slow rates (C/20 or lower) the reaction proceeds throughout the plate and you approach the rated capacity. This is known as the Peukert effect. Lithium-ion cells have faster ion transport and are much less affected.
Can I connect batteries in parallel to increase capacity?
Yes. Batteries of the same chemistry, voltage, capacity, and ideally age can be connected positive-to-positive and negative-to-negative to add their amp-hour ratings. Two 100 Ah batteries in parallel give 200 Ah at the same voltage. Use cables of equal length to ensure balanced current sharing. Connecting batteries of significantly different states of charge or age in parallel can cause large equalizing currents that damage cells, so always start with fully charged, well-matched batteries.
How does temperature affect battery capacity?
Cold temperatures slow the electrochemical reactions inside a battery, reducing available capacity. Lead-acid batteries can lose 20-40% of their rated capacity at 0 degC (32 degF) and up to 50% at -20 degC (-4 degF). Lithium cells lose less -- typically 10-25% at 0 degC -- and perform better at moderately low temperatures. High temperatures above 40 degC (104 degF) temporarily increase capacity but accelerate aging significantly. The safety margin in this calculator is partly intended to cover cold-weather losses.
What safety margin should I add to the minimum battery size?
A 20% safety margin is a common starting point for most applications. Increase it to 30-40% if the battery will be used in cold environments (below 10 degC / 50 degF), if the battery is already a year or more old, if you want extra redundancy, or if the exact load is uncertain. For critical backup power applications such as medical equipment or security systems, a 50% margin is conservative but appropriate. The margin compensates for manufacturing tolerances, aging, temperature derating and any unaccounted loads.