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A battery‘s battery c rating expresses current. It uses a multiple of its capacity. For a 100 Ah battery, 1C = 100 A, 0.5C = 50 A, and 0.2C = 20 A. This shows safe current for discharge or charge. It determines what size inverter it can run and how fast it can be charged.
Battery C-rate and power calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
The formulas
- Current (A) = C-rate × Capacity (Ah)
- C-rate = Current (A) ÷ Capacity (Ah)
- Time at that rate (hours) ≈ 1 ÷ C-rate (for an ideal full discharge)
So, 0.5C empties a battery in about 2 hours. Also, 1C empties it in about 1 hour. C/20 (0.05C) empties it in about 20 hours.
Converting C-rate to power
Power (W) = C-rate × Capacity (Ah) × Voltage
A 12.8 V 100 Ah LiFePO4 battery at 1C delivers about 100 A × 12.8 V = 1,280 W.
| Battery | 0.2C | 0.5C | 1C |
|---|---|---|---|
| 12.8 V 100 Ah LiFePO4 | 256 W | 640 W | 1,280 W |
| 12.8 V 200 Ah LiFePO4 | 512 W | 1,280 W | 2,560 W |
| 25.6 V 100 Ah LiFePO4 | 512 W | 1,280 W | 2,560 W |
| 51.2 V 100 Ah LiFePO4 | 1,024 W | 2,560 W | 5,120 W |
Typical limits
| Chemistry | Continuous discharge | Recommended charge |
|---|---|---|
| LiFePO4 (typical 100 Ah) | 0.5C – 1C (BMS-limited, e.g. 100 A) | 0.2C – 0.5C, max often 0.5C – 1C |
| AGM | up to about 0.2C – 0.3C for good capacity | 0.1C – 0.3C |
| Flooded lead-acid | about 0.1C – 0.2C for good capacity | 0.1C – 0.2C |
The BMS rating is usually the real limit for lithium batteries. For instance, a “100 Ah, 100 A BMS” battery is a 1C battery. A “100 Ah, 200 A BMS” battery can deliver 2C. Peak (surge) ratings for a few seconds are often higher.
Why C-rate matters for inverters
Your battery bank’s combined continuous current must exceed the inverter’s maximum battery current:
Inverter battery current = Inverter watts ÷ (Battery voltage × 0.90)
A 3,000 W inverter at 12 V needs about 278 A. Three 100 Ah batteries with 100 A BMS boards in parallel can supply 300 A, but two cannot. See how many batteries for a 3,000 W inverter.
Lead-acid and the Peukert effect
Lead-acid capacity is normally rated at the 20-hour rate (C/20). However, you get less if you draw current faster. For example, at a 1-hour rate, an AGM battery may deliver only 50% to 60% of its rated amp-hours. Lithium is affected far less. This is one reason a 100 Ah lithium battery outperforms a 100 Ah AGM under inverter loads. See LiFePO4 vs AGM.
Charging C-rate and solar
A 400 W solar array charges a 12 V 100 Ah battery. It delivers roughly 25 A, which is about 0.25C. This is fine for lithium batteries. However, it is on the high side for AGM batteries. Large arrays on small batteries may exceed safe charge rates. You should check your battery’s maximum charge current. See what size solar panel to charge a 12 V battery.
worked example: understanding battery c rating for a microwave
You have one 12 V 100 Ah LiFePO4 battery. Its data sheet lists a continuous discharge of 100 A (1C). Also, it has a peak discharge of 200 A for 5 seconds.
Your microwave draws 1,400 W from the outlet:
- Battery current = 1,400 ÷ (12.8 × 0.90) = 122 A
- That exceeds the 100 A continuous limit, so the BMS will shut off after a few seconds or minutes.
Solutions include a second battery in parallel. This provides 200 A total. You can also use a battery with a 150 to 200 A BMS. Another option is a smaller microwave. See microwave on an inverter.
How C-rate affects usable capacity
| Discharge rate | LiFePO4 capacity delivered | AGM capacity delivered (approx.) |
|---|---|---|
| C/20 (5 A on 100 Ah) | about 100% | 100% (rated) |
| C/5 (20 A) | about 100% | about 80 – 85% |
| C/2 (50 A) | about 98% | about 65 – 70% |
| 1C (100 A) | about 95 – 97% | about 50 – 60% |
This is one of the biggest practical advantages of lithium for inverter loads.
Charging C-rate from solar, alternator and generator
| Source | Typical charge current into a 12 V 200 Ah bank | C-rate |
|---|---|---|
| 400 W solar via MPPT | about 25 A | 0.13C |
| 40 A DC-DC charger | 40 A | 0.2C |
| 100 A inverter-charger on generator | 100 A | 0.5C |
All of these are within typical LiFePO4 limits. However, for an AGM battery, the 100 A generator charge is at the high end. Therefore, check the maximum charge current on the data sheet.
Common mistakes
- Reading only the Ah rating and ignoring the BMS current limit.
- Assuming peak ratings are continuous. Peak ratings often last seconds.
- Charging too fast in the cold. Many lithium batteries allow only 0.1 to 0.2C between 0 and 10 °C.
- Undersized cables that limit current anyway.
Worked example: sizing a bank by current, not just capacity
An RV owner plans a 2,000 W inverter at 12 V. Also, they only need about 1.5 kWh per day.
- Inverter current at full load: 2,000 ÷ (12 × 0.90) = 185 A
- Energy need: 1.5 kWh ÷ 0.85 = 1.76 kWh, so one 12 V 150 Ah battery would be enough for energy
- But a 150 Ah battery with a 150 A BMS cannot supply 185 A
Solutions: one 12 V 200 Ah battery with a 200 A BMS, two 100 Ah batteries in parallel (200 A combined), or a smaller 1,500 W inverter if the loads allow.
C-rate quick reference
| Capacity | 0.5C | 1C | 2C |
|---|---|---|---|
| 50 Ah | 25 A | 50 A | 100 A |
| 100 Ah | 50 A | 100 A | 200 A |
| 200 Ah | 100 A | 200 A | 400 A |
| 280 Ah | 140 A | 280 A | 560 A |
FAQ
What does 0.5C mean?
A current equal to half the capacity: 50 A for a 100 Ah battery, discharging it in about 2 hours.
Is a higher C-rating better?
Yes, a high C-rating is important for high-power loads. However, for energy storage, capacity matters more. So, choose a C-rating that comfortably covers your inverter.
Does C-rate affect battery life?
Yes, continuous operation near the maximum C-rate creates heat and wear. As a result, spreading the load across more batteries extends their life.
What is 2C on a 100 Ah battery?
200 A, which would empty it in about 30 minutes.
Is a 0.5C battery good for solar?
For most home and RV systems, yes. Size your bank so the inverter’s maximum current is within the combined continuous rating.
Related guides
- What Is a Battery Management System (BMS) and Why It Matters
- Depth of Discharge (DoD) Explained for Solar Batteries
- What Size Battery Backup Do I Need for My Home?
- What Size Battery Do I Need for a 1,000W Inverter?
Sources and further reading
- Battery University: BU-402 What Is C-rate? This article covers charge and discharge rates. It also discusses capacity and runtime.
- Battery University: BU-205 Types of Lithium-ion: how lithium chemistries, including LiFePO4, compare.
- Battery University: BU-409b Charging Lithium Iron Phosphate details LiFePO4 charging voltages. It also describes their behavior.