You've got a battery spec sheet in front of you that says "100 Ah" — but how much energy is that, really? And how long will it actually power your setup? The answer depends on voltage, discharge rate, and a few other factors that aren't always obvious from a label.
This battery capacity calculator takes those raw numbers and turns them into something useful. Enter your battery's voltage and amp-hour rating, and you'll instantly see the stored energy in watt-hours, along with C-rate, discharge current, and estimated runtime. Whether you're building a solar battery bank, picking a battery for a DIY e-bike, or figuring out if your UPS will survive a two-hour power outage, this tool does the math so you don't have to.
Understanding Battery Capacity: Ah vs. Wh
Battery capacity gets confusing fast because manufacturers use two different units — and they measure different things.
Amp-hours (Ah) tell you how much current a battery can deliver over time. A 100 Ah battery can supply 1 amp for 100 hours, 10 amps for 10 hours, or 100 amps for 1 hour. Think of it like the size of a water tank — it tells you the volume, but not the pressure.
Watt-hours (Wh) tell you the actual energy stored. They factor in both current and voltage, which gives you the full picture. This is like knowing both the volume and the pressure of that water tank — now you know how much work it can actually do.
Here's why this matters in practice: a 100 Ah battery at 12V stores 1,200 Wh. The same 100 Ah rating at 48V stores 4,800 Wh — four times the energy. If you're shopping for batteries and only comparing amp-hours, you're missing the real story.
Battery Voltage | Capacity (Ah) | Stored Energy (Wh) | Real-World Equivalent |
|---|---|---|---|
3.7V | 5 Ah | 18.5 Wh | Typical phone battery |
12V | 100 Ah | 1,200 Wh | Car/RV battery |
36V | 14 Ah | 504 Wh | E-bike battery |
48V | 100 Ah | 4,800 Wh | Home solar storage |
The Formulas Behind the Calculator
The math here is refreshingly simple.
Amp-hours to watt-hours:
Wh = Ah × V
Multiply your battery's amp-hour rating by its voltage. A 20V power tool battery rated at 5 Ah stores 100 Wh of energy. Double the voltage to 40V with the same 5 Ah, and you've got 200 Wh.
Watt-hours to amp-hours:
Ah = Wh ÷ V
This one is handy when you know how much energy you need and want to figure out the battery capacity. Need 600 Wh from a 12V system? That's 50 Ah (600 ÷ 12).
Runtime:
Runtime (hours) = Battery Capacity (Ah) ÷ Load Current (A)
A 100 Ah battery feeding a 25A load runs for about 4 hours on paper. Real-world runtime will be shorter — more on that below.
C-rate:
C-rate = Discharge Current (A) ÷ Battery Capacity (Ah)
C-rate tells you how fast you're draining the battery relative to its size. A 1C rate empties it in one hour. A 0.5C rate takes two. This number matters more than most people realize.
How to Use This Calculator
- Enter your battery voltage. This is the nominal voltage — 3.7V for a single lithium cell, 12V for a standard lead-acid battery, 48V for a typical e-bike pack. Use the dropdown to switch between volts, millivolts, or kilovolts.
- Enter battery capacity. Type in the amp-hour rating from your battery's label or datasheet. The dropdown lets you switch to milliamp-hours if you're working with smaller batteries like phone or drone cells.
- Check your stored energy. The calculator instantly shows total energy in watt-hours. Switch to kilowatt-hours using the dropdown if you're working with larger systems — a 4,800 Wh solar bank reads more cleanly as 4.8 kWh.
- Explore the advanced parameters. Click "Other battery parameters" to reveal C-rate, discharge current, and runtime. Try adjusting the C-rate to see how different load levels change your runtime — this is where the calculator really shines for system planning.
Understanding C-Rate (and Why It Matters More Than You Think)
C-rate trips up a lot of people, but the concept is simpler than it sounds. It's just a way of expressing how fast you're using (or charging) a battery relative to its total capacity.
Picture a 10 Ah battery. If you draw 10 amps from it, that's a 1C discharge — you'll empty it in about one hour. Pull 5 amps, and you're at 0.5C — two hours of runtime. Crank it up to 20 amps (2C), and the battery lasts roughly 30 minutes.
But here's what most people miss: C-rate doesn't just affect how fast the battery drains — it affects how much usable energy you actually get.
At higher C-rates, internal resistance causes more energy to be lost as heat. A battery rated at 100 Ah might only deliver 85-90 Ah at a 1C discharge rate because the voltage sags under heavy load. At a gentle 0.1C rate, you'll get closer to the full rated capacity.
This has real consequences for system design:
C-Rate | Discharge Time | Typical Usable Capacity | Best For |
|---|---|---|---|
0.05C | 20 hours | 100% of rated | Standby, IoT sensors |
0.2C | 5 hours | ~95% of rated | Solar storage, overnight backup |
0.5C | 2 hours | ~90% of rated | Power tools, e-bikes |
1C | 1 hour | ~85% of rated | High-performance applications |
2C+ | 30 min or less | ~75-80% of rated | Drones, RC vehicles, burst loads |
Bottom line: if you're sizing a battery for a specific runtime, don't just divide amp-hours by current draw and call it done. Factor in the capacity loss from your actual C-rate, especially for high-drain applications.
Practical Examples
Sizing a solar battery bank for overnight use
Your home uses about 1,500 Wh between sunset and sunrise. With a 48V battery system: 1,500 Wh ÷ 48V = 31.25 Ah minimum. But you should never plan to use 100% of your battery. With LiFePO4 batteries (safe to 80% depth of discharge), you need: 31.25 ÷ 0.8 = ~39 Ah. Add 15% for inverter losses: ~45 Ah at 48V. In practice, a 50 Ah 48V LiFePO4 battery bank would handle this nicely with some margin to spare.
Comparing two e-bike batteries
Battery A: 36V, 14 Ah (504 Wh, $300). Battery B: 48V, 10.5 Ah (504 Wh, $350). These store identical energy — same range potential. But the 48V battery delivers that energy at lower current, meaning less heat, slightly better efficiency, and often better hill-climbing performance. Whether that's worth $50 more depends on your terrain.
How long will a UPS keep your home office running?
Your setup draws 350W (monitor, laptop, router, lamp). Your UPS has a 24V, 9 Ah battery. Stored energy: 24 × 9 = 216 Wh. At 350W: 216 ÷ 350 = 0.62 hours. Factor in ~85% inverter efficiency: about 31 minutes of real runtime. Enough to save your work and shut down cleanly, but not enough to power through a long outage.
Estimating drone flight time
Your racing drone runs a 22.2V 6S LiPo at 1,300 mAh (1.3 Ah). Stored energy: 22.2 × 1.3 = 28.9 Wh. Average current draw in aggressive flying: about 30A — that's a 23C discharge rate. At that rate, usable capacity drops to maybe 70-75% of rated, giving you around 2-2.5 minutes of hard flying. In a more moderate cruise, drawing 15A (11.5C), you'd get roughly 4-5 minutes. Race LiPos are a different beast than solar batteries.
Common Mistakes When Calculating Battery Capacity
Comparing batteries by Ah alone across different voltages. A 200 Ah 12V battery (2,400 Wh) stores less energy than a 100 Ah 24V battery (2,400 Wh) — wait, actually those are equal. But a 50 Ah 48V battery (2,400 Wh) stores the same energy in a much smaller, lighter package. Always convert to Wh before comparing.
Assuming you can use 100% of rated capacity. Lead-acid batteries should only be discharged to 50% regularly. Even lithium batteries perform best when kept between 20-80%. If a calculation says you need exactly 100 Ah, buying a 100 Ah battery means you'll routinely over-discharge it and kill it prematurely.
Ignoring temperature effects. Cold weather can cut battery capacity by 10-30%, depending on chemistry. If you're sizing batteries for an outdoor installation in a cold climate, you need extra headroom. A 100 Ah lithium battery at -10°C might only deliver 75-80 Ah.
Using rated capacity at the wrong C-rate. That 100 Ah rating on your battery was measured at a specific discharge rate — usually the 20-hour rate (0.05C) for lead-acid. If you're pulling power at 1C, you won't get anywhere near 100 Ah. Check your battery's datasheet for capacity at different discharge rates.
Forgetting inverter losses. When powering AC devices from a DC battery through an inverter, you lose 10-15% of the energy as heat in the conversion process. Always add this to your calculations.
Battery Types at a Glance
Battery Type | Energy Density | Typical Cycle Life | Safe Depth of Discharge | Max Continuous C-Rate | Common Uses |
|---|---|---|---|---|---|
Li-ion | High | 500–1,000 cycles | 80–90% | 1–2C | Phones, laptops, EVs |
LiFePO4 | Medium | 2,000–5,000 cycles | 80–100% | 1–3C | Solar storage, RVs, marine |
Lead-acid (AGM) | Low | 200–500 cycles | 50% | 0.2C | Cars, budget backup power |
LiPo | High | 200–500 cycles | 80% | 10–50C burst | Drones, RC, high-drain |
NiMH | Medium | 500–1,000 cycles | 100% | 1–2C | AA/AAA rechargeables, hybrids |
Quick guidance: For most home solar and backup applications, LiFePO4 is the best long-term value despite higher upfront cost. The 2,000-5,000 cycle lifespan means you'll replace lead-acid batteries 4-5 times before a LiFePO4 pack wears out. For weight-sensitive portable applications, Li-ion or LiPo are the way to go.
Tips for Getting More Life Out of Your Batteries
Charge to 80%, not 100%. This one tip alone can nearly double the cycle life of most lithium batteries. The last 20% of charge puts the most stress on the cells. If you don't need the full range every time, stopping at 80% is the single best thing you can do.
Avoid deep discharges. Running lithium batteries below 20% regularly accelerates degradation. For lead-acid, the threshold is even higher — keeping them above 50% dramatically extends their life. Think of it as a rule of thumb: the less of your battery's range you use per cycle, the more cycles you'll get.
Store batteries at 40-60% charge. If you're putting batteries away for more than a month, charge them to about half. Storing fully charged or fully depleted causes faster capacity loss. Most lithium batteries lose 1-2% capacity per month in storage at room temperature.
Keep batteries cool. Every 10°C above 25°C roughly doubles the rate of calendar aging in lithium batteries. Don't leave battery packs in hot cars, and if your system generates heat, make sure there's adequate ventilation.
Match your batteries. In multi-cell packs or parallel battery banks, use batteries of the same age, chemistry, and capacity. Mixing old and new batteries forces the weaker cells to work harder, which drags down performance and shortens the life of the entire pack.