Ah (amp-hours) is a battery capacity rating. It tells you how much current a battery can deliver over time under stated conditions.
For most people, the most useful “mental model” is to think in energy (watt-hours, Wh), because it automatically includes voltage:
- Energy (Wh) = Voltage (V) × Capacity (Ah)
- Runtime (hours) ≈ Battery energy (Wh) ÷ Load power (W)
You can still estimate runtime with amps if you know current draw:
- Runtime (hours) ≈ Ah ÷ A
Real-world runtime is usually lower than ideal math because of usable depth of discharge (DoD), inverter/system losses, temperature, battery aging, and sometimes BMS current limits (common on lithium batteries).
Ah, A, V, W, Wh: the cheat sheet
| Term | What it measures | Unit | Simple relationship |
|---|---|---|---|
| Current | How much current is flowing right now | A (amps) | — |
| Capacity | Current over time (how much “total current”) | Ah | Ah = A × hours |
| Voltage | Electrical “pressure” | V | — |
| Power | How fast energy is used | W | W = V × A |
| Energy | Total energy stored/used | Wh | Wh = V × Ah |
Why this matters: Ah is helpful, but Wh is the most comparable number across different battery voltages (12V vs 24V vs 48V).
Amp-hour meaning (Ah) in plain English
An amp-hour is simply amps × hours.
- 1Ah means the battery can theoretically supply 1 amp for 1 hour
- Or 2 amps for 0.5 hours
- Or 0.5 amps for 2 hours
So a 100Ah battery means it stores about 100 amp-hours of capacity under its rated test condition.
Ah vs mAh
Smaller batteries may list mAh (milliamp-hours):
- 1000mAh = 1Ah
- Example: 5000mAh = 5Ah (at the cell’s nominal voltage)
What does Ah mean on a battery label?
On a label/spec sheet, Ah is a capacity rating, not a direct “power output” rating.
Two practical notes help avoid confusion:
- Rated capacity vs usable capacity
The printed Ah is often a rated value. What you can actually use depends on your battery chemistry, how deeply you discharge it, system losses, and conditions like temperature. - Capacity can depend on discharge rate (especially for lead-acid)
Some chemistries deliver less usable capacity when discharged quickly. Lithium (LiFePO4) usually holds voltage better under moderate loads, but it still has limits (current limits, temperature behavior).
Takeaway: Treat the label Ah as a starting point, then plan around usable Wh for real-world runtime.
Ah vs amps: what’s the difference?
These two are related but not interchangeable:
- Amps (A) = current draw right now
- Amp-hours (Ah) = capacity available in total
A quick way to remember:
- A = speed (how fast you’re draining the battery)
- Ah = tank size (how much you have available)
Does higher Ah mean more power?
Not necessarily.
- Higher Ah generally means longer runtime
- “More power” depends on voltage and how much current the battery can safely deliver (BMS limits, internal resistance, C-rate)
Amp-hours vs watt-hours: why voltage matters
When people compare batteries, the most common mistake is comparing Ah alone across different voltages.
Use this conversion:
Wh = V × Ah
Quick examples
- 12V 100Ah → 12 × 100 = 1200Wh
- 24V 100Ah → 24 × 100 = 2400Wh
- 48V 100Ah → 48 × 100 = 4800Wh
That’s why a 24V 100Ah battery can run the same watt load roughly twice as long as a 12V 100Ah battery (all else equal).
How to calculate battery runtime (the practical way)
Step 1: Convert your battery to watt-hours (Wh)
Use nominal voltage for a quick estimate:
- Battery Wh ≈ V × Ah
If you’re using an inverter, you’ll also want to account for inverter efficiency (covered below).
Step 2: Estimate runtime from your load power (W)
- Runtime (hours) ≈ Battery Wh ÷ Load W
This method works well because many appliances list watts directly.
Step 3: Adjust for real-world usable energy
In real systems, you rarely get 100% of rated Wh as usable runtime. A practical approach is to plan with a “usable fraction” to cover:
- usable DoD (how much you actually discharge)
- inverter/controller losses
- wiring losses
- temperature/aging
So you can think like this:
Real runtime ≈ (Battery Wh × usable fraction) ÷ Load W
Where the usable fraction might be a rough planning range like 0.75–0.90 depending on system type and conditions.
Worked examples (so it clicks)
Example A: DC load (watts known)
- Battery: 12V 100Ah
- Battery energy: 12 × 100 = 1200Wh
- Load: 120W (DC)
- Ideal runtime: 1200 ÷ 120 = 10 hours
- If you plan for 80–90% usable energy: ~8–9 hours
Example B: AC load through an inverter
- Battery: 12V 100Ah → 1200Wh
- Load: 300W (AC)
- Inverter efficiency: assume ~90%
- Battery must supply more than 300W: 300 ÷ 0.9 ≈ 333W
- Ideal runtime: 1200 ÷ 333 ≈ 3.6 hours
- With usable energy planning (e.g., 0.8–0.9): ~2.9–3.2 hours
Equivalent “amps-based” method (optional)
If you know current draw instead of watts:
- Runtime (hours) ≈ Ah ÷ A
And if you only know watts, you can estimate amps:
- A ≈ W ÷ V (for DC loads)
Simple runtime table (12V 100Ah)
12V 100Ah ≈ 1200Wh (rated)
The “realistic range” below uses a planning factor (usable fraction) to reflect typical losses and usable DoD.
| Load (W) | Ideal runtime (hours) | Realistic range (hours) |
|---|---|---|
| 50W | 1200 ÷ 50 = 24.0 | 18–22 |
| 100W | 1200 ÷ 100 = 12.0 | 9–11 |
| 200W | 1200 ÷ 200 = 6.0 | 4.5–5.5 |
| 500W | 1200 ÷ 500 = 2.4 | 1.8–2.2 |
Tip: For 24V 100Ah (≈ 2400Wh), runtime is roughly double for the same load wattage, assuming similar losses.
Why real-world runtime is lower than the math
Even if your formula is correct, runtime can be reduced by several practical factors:
- Depth of discharge (DoD): Many users don’t (and shouldn’t) drain to 0% every cycle.
- Inverter and conversion losses: AC loads through an inverter reduce usable energy.
- High current increases losses: Higher current means more heat and voltage drop in cables/connections.
- Temperature: Cold reduces available capacity and power output.
- Aging: Battery capacity declines over time.
- BMS current limits (lithium): A battery can have high Ah but still limit discharge current for protection.
If your device “should” run longer but doesn’t, the fastest troubleshooting checks are:
- Verify real load watts (many loads consume more than expected)
- Check inverter efficiency and idle draw
- Confirm wiring size/connection heat
- Check battery SOC reading accuracy and battery health
Series vs parallel: do amp-hours add up?
This is where many buyers get confused, so here are the rules.
Batteries in series
- Voltage adds
- Ah stays the same
Example:
- Two 12V 100Ah in series → 24V 100Ah
- Energy doubles: 12V×100Ah = 1200Wh → 24V×100Ah = 2400Wh
Batteries in parallel
- Voltage stays the same
- Ah adds
Example:
- Two 12V 100Ah in parallel → 12V 200Ah
- Energy doubles: 12V×200Ah = 2400Wh
Quick rule: Convert to Wh and you’ll never get lost in series/parallel comparisons.
Why “amps per hour” is usually the wrong term
People often say “amps per hour” when they actually mean:
- amps (A): current draw right now, or
- amp-hours (Ah): capacity over time
Technically, “amps per hour” describes how current changes per hour (a rate of change), which is rarely what battery shoppers mean.
If you’re discussing batteries clearly:
- Use A for current draw
- Use Ah for capacity
- Use Wh for total stored energy
What is a “good” Ah rating?
There isn’t one universal “good Ah.” The right capacity depends on:
- your load watts (W)
- desired runtime hours
- system voltage V
- how much reserve you want
- acceptable weight/cost/space
A practical selection workflow:
- List devices and watts
- Estimate energy per day: Wh = W × hours
- Choose a battery (or battery bank) with enough usable Wh (not just rated Ah)
Common mistakes when comparing batteries
- Comparing batteries by Ah only (ignoring voltage)
- Assuming rated Ah equals usable capacity in every situation
- Forgetting inverter losses and startup surges (motors/compressors)
- Ignoring temperature effects
- Assuming higher Ah automatically means higher power output (current limits matter)
FAQ
1) Is Ah the same as “battery capacity” or “battery energy”?
Ah is a capacity (charge) rating, not energy by itself. To compare “how much energy” two batteries store, convert to watt-hours: Wh = V × Ah. This is the most reliable apples-to-apples comparison across 12V/24V/48V systems.
2) What does “100Ah at C/20” mean in real use?
“100Ah at C/20” means the battery was rated over a 20-hour discharge test. In theory, that’s about 5A for 20 hours under that standard. At higher currents, some chemistries (especially lead-acid) can deliver less usable capacity than the label suggests.
3) What does 5Ah mean on a battery (like tool batteries)?
5Ah means the pack can theoretically deliver 5A for 1 hour, or 1A for 5 hours (same idea, different current). In practice, tool runtime also depends heavily on the tool’s load, the pack voltage platform, and temperature.
4) Why do two “100Ah” batteries sometimes give very different runtimes?
Three common reasons:
- Different voltage (Ah same, Wh different): Wh = V × Ah.
- Different test rate/chemistry behavior (especially lead-acid vs lithium): rated capacity can be sensitive to discharge rate.
- System losses and limits: inverter efficiency, wiring loss, and lithium BMS current limits can reduce usable runtime even when capacity is high.
5) Is “amps per hour” correct when talking about batteries?
Usually no. Most people mean amps (A) or amp-hours (Ah). “Amps per hour” is a rate-of-change term (how current increases/decreases over time), and it’s rarely what shoppers intend. If you want to be precise: 1Ah = 3600 coulombs of charge (A×time).
6) How many amps is a 100Ah battery?
This is a trick question: Ah doesn’t tell you max amps.
Max continuous amps depends on battery design (chemistry, internal resistance) and protection limits (for lithium, the BMS). A 100Ah battery might be able to supply 50A, 100A, or more—the spec you need is “max continuous discharge current”, not Ah. (Many consumer “Ah explainers” also highlight that runtime depends on condition/age and discharge behavior, not Ah alone.)
7) Quick reality check: how long will a 12V 100Ah battery run a 100W load?
A 12V 100Ah battery is about 1200Wh (rated). Ideal math says ~12 hours at 100W, but many real systems land closer to a practical range because of usable DoD + losses (especially if an inverter is involved). The exact number depends on your setup, but the Wh method is the clean way to estimate.
8) Series vs parallel: what changes first—Ah, voltage, or both?
- Series: voltage adds, Ah stays the same (energy increases because V increases).
- Parallel: Ah adds, voltage stays the same (energy increases because Ah increases).
This “rule of wiring” is why Wh is the safest number to compare packs and predict runtime.
