Solar Charge Time
t = (Cap × V) / (P × η)
Quick answer
This calculator solves t = (Capacity × V) / (Ppanel × η) for Charge Time. With Battery Capacity 20 Ah, Battery Voltage 12 V, Panel Power 10 W, System Efficiency 50 %, the result is 48 h. It solves in 5 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
This estimates the time needed to fully charge a battery from a solar panel, accounting for system efficiency losses. The numerator is the total energy needed in watt-hours, and the denominator is the effective charging power. Real-world solar charging is complicated by variable sunlight, panel angle, temperature derating, and charge controller behavior. The panel wattage is its rated peak power under standard test conditions (1000 W/m² irradiance, 25°C cell temperature), which is rarely achieved in practice. Actual average power is typically 4-6 peak sun hours per day depending on location.
Variables
- t — Charge time in hours (h)
- Capacity — Battery capacity (Ah)
- V — Battery nominal voltage (V)
- Ppanel — Solar panel rated power (W)
- η — System efficiency including charge controller losses (0-1)
Practical Notes
For a realistic estimate, divide the panel rating by 2-3 to account for non-ideal conditions. MPPT charge controllers are 95-98% efficient, while PWM controllers are 70-80% efficient. Battery charge acceptance also decreases above 80% state of charge, extending total charge time.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| t | Charge Time | h | ps, ns, µs, ms, s, min, h |
| Cap | Battery Capacity | Ah | mAh, Ah |
| V | Battery Voltage | V | pV, nV, µV, mV, V, kV |
| P | Panel Power | W | µW, mW, W, kW |
| eta | System Efficiency | — | —, % |
Step by step
- 1. What you know: Battery Capacity 20 Ah, Battery Voltage 12 V, Panel Power 10 W, System Efficiency 50 %
- 2. The relation: t = (Capacity × V) / (Ppanel × η)
- 3. Substitute: Cap = 20 Ah, V = 12 V, P = 10 W, eta = 50 %
- 4. Result: Charge Time = 48 h
Worked examples
Minimum range
Given: Battery Capacity 0.2 Ah, Battery Voltage 0.33 V, Panel Power 10 mW, System Efficiency 2.5 %
Result: Charge Time = 264 h
Low range
Given: Battery Capacity 2 Ah, Battery Voltage 3.3 V, Panel Power 100 mW, System Efficiency 25 %
Result: Charge Time = 264 h
Typical
Given: Battery Capacity 20 Ah, Battery Voltage 12 V, Panel Power 10 W, System Efficiency 50 %
Result: Charge Time = 48 h
High range
Given: Battery Capacity 200 Ah, Battery Voltage 0.23 kV, Panel Power 1 kW, System Efficiency 0.8
Result: Charge Time = 57.5 h
Maximum range
Given: Battery Capacity 2000 Ah, Battery Voltage 2.3 kV, Panel Power 10 kW, System Efficiency 8
Result: Charge Time = 57.5 h
Solving for each variable
Solve for Charge Time
Battery Capacity 0.2 Ah, Battery Voltage 0.33 V, Panel Power 10 mW, System Efficiency 2.5 % → 264 h
Solve for Battery Capacity
Charge Time 6 min, Battery Voltage 0.33 V, Panel Power 10 mW, System Efficiency 2.5 % → 0.07576 mAh
Solve for Battery Voltage
Charge Time 6 min, Battery Capacity 0.2 Ah, Panel Power 10 mW, System Efficiency 2.5 % → 125 µV
Solve for Panel Power
Charge Time 6 min, Battery Capacity 0.2 Ah, Battery Voltage 0.33 V, System Efficiency 2.5 % → 26.4 W
Solve for System Efficiency
Charge Time 6 min, Battery Capacity 0.2 Ah, Battery Voltage 0.33 V, Panel Power 10 mW → 66
How Charge Time changes with Battery Capacity
| Battery Capacity (input) | Charge Time (result) |
|---|---|
| 2 Ah | 4.8 h |
| 5 Ah | 12 h |
| 10 Ah | 24 h |
| 15 Ah | 36 h |
| 20 Ah | 48 h |
| 30 Ah | 72 h |
| 40 Ah | 96 h |
| 80 Ah | 192 h |
| 140 Ah | 336 h |
| 200 Ah | 480 h |
The same result in other units
| Unit | Value |
|---|---|
| s | 172800 |
| min | 2880 |
| h | 48 |
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<p><a href="https://vbatpower.com/formulas/solar-charging/">Solar Charge Time</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate Solar Charge Time?
Use t = (Capacity × V) / (Ppanel × η). With Battery Capacity 20 Ah, Battery Voltage 12 V, Panel Power 10 W, System Efficiency 50 %, Charge Time works out to 48 h.
What is the formula for Solar Charge Time?
The formula is t = (Capacity × V) / (Ppanel × η). This estimates the time needed to fully charge a battery from a solar panel, accounting for system efficiency losses.
How do you find Battery Capacity when you know the other values?
Rearrange for Battery Capacity: with Charge Time 6 min, Battery Voltage 0.33 V, Panel Power 10 mW, System Efficiency 2.5 %, Battery Capacity is 0.07576 mAh.
How do you find Battery Voltage when you know the other values?
Rearrange for Battery Voltage: with Charge Time 6 min, Battery Capacity 0.2 Ah, Panel Power 10 mW, System Efficiency 2.5 %, Battery Voltage is 125 µV.
What units does Solar Charge Time use?
Charge Time is shown in s, min, h. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
Related Concepts
All Power & Efficiency formulas →Need more features?
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