I travel around 2000Km per year with my bicycle. And what can be better than 2000Km? 4000Km 🙂
So, I will attempt to attach a solar panel to my bicycle to replenish the battery (at least partially).
Solar panels efficiency
Typically ranges from 150-250 W/m² under peak sunlight.
High-end panels hitting 220-300W/m².
Factors Influencing Output
- Solar irradiance: The intensity of sunlight (around 1000 W/m² in peak conditions).
- Panel efficiency
- Time, location & weather
California/Africa= 5 kWh per day
Northern Europe= 1.5–2 kWh. - Temperature
High temperatures decrease efficiency by 0.3% to 0.5% for every degree above 25°C - Angle & tilt
Angle of Deviation from Sun
| Remaining Efficiency | Energy Loss | |
|---|---|---|
| 0° (Directly facing sun) | 100% | 0% |
| 15° | 96.6% | 3.4% |
| 30° | 86.6% | 13.4% |
| 45° | 70.7% | 29.3% |
| 60° | 50.0% | 50.0% |
| 75° | 25.9% | 74.1% |
| 90° | 0% | 100% |
Solar panels round-up
What is expected from a solar panel to be suitable for bicycle?
- Endurance – This is obviously the most important factor. A broken panel is a useless panel 🙂
- Size – Must fit on the bicycle trailer
- Power / Efficiency
- Weight
- Voltage – Must be as a bit higher than battery’s charging voltage
- Price
Here is a color-coded table of suitable solar panels, sold on Amazon.
50W Budget Solar – Real-world review
I purchased a “50W” mono-crystalline solar panel from Amazon.
Size: 360 x 330 x 3 mm (0.12m² )
Power: 50 W?
Open circuit voltage: 22 V max.
Module efficiency: 19.9% ?
Nominal Operating Cell Temperature: 45°C
680 gram
Power: 50 W?
Open circuit voltage: 22 V max.
Module efficiency: 19.9% ?
Nominal Operating Cell Temperature: 45°C
680 gram
30 euro
The 50W Claim vs. Reality
The panel’s stated 50W power rating is a laboratory figure STC (Standard Test Conditions).
The advertised 19.9% efficiency is theoretical; expect an actual operating efficiency closer to 14-16% under normal use.
Measurement
In December I measured in full sun (11AM): 0.83A x 21V = 17.4W (V in open circuit and I in short-circuit) which means 13-15W in Vmpp.
In real-world summer conditions in Germany (Berlin and similar latitudes), I expect a peak output of around 30–35 Watts on a clear, sunny summer day around noon, so 200-210 Watt-hours per day on a good, sunny day in June or July. This assumes the panel is placed in direct sun for the entire day (tilted slightly around 15-20 degrees from horizontal).
In real-world summer conditions in Germany (Berlin and similar latitudes), I expect a peak output of around 30–35 Watts on a clear, sunny summer day around noon, so 200-210 Watt-hours per day on a good, sunny day in June or July. This assumes the panel is placed in direct sun for the entire day (tilted slightly around 15-20 degrees from horizontal).
Warning: The power calculated by simply multiplying V×I will be an overestimation of the panel’s actual, usable output power. The actual power will typically be 75% to 80% of that theoretical maximum. The actual number we need is MPP (Maximum Power Point).
Vmpp
To measure a solar panel’s maximum power (Pmax), you need its Voltage at Maximum Power (𝑉𝑚𝑝𝑝) and Current at Maximum Power (𝐼𝑚𝑝𝑝) under bright, direct sunlight and multiply them 𝑃𝑚𝑎𝑥=𝑉𝑚𝑝𝑝×𝐼𝑚𝑝𝑝), typically done with a solar power meter or watt meter, but you can approximate with a multimeter by finding the sweet spot where 𝑉𝑚𝑝𝑝 and 𝐼𝑚𝑝𝑝 yield the highest power reading using a variable resistor load.
In other words, use a bunch of high power resistors in the 0.1-5 Ohm range and measure the power for each resistor. Highest power found is your P𝑚𝑝𝑝. This means that the efficiency of your solar panel depends on the impedance of your load!
Charging a Bicycle Battery (415Wh Example)
For my typical 415Wh e-bike battery, the charging time will be substantial: a full sunny day in summer will replenish roughly 50% – 55%. If driving in shade and considering some clouds, the efficiency will probably be 30%.