Short answerWire panels in series until the string's operating voltage sits well inside the pump's input window, and check that cold-morning open-circuit voltage, about 10% above the rated figure, stays under the pump or controller maximum. Add parallel strings for more current.
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Key takeaways
- Series adds voltage; parallel adds current.
- Cold mornings raise open-circuit voltage by about 10%: check it against the pump's absolute maximum.
- Hot afternoons lower voltage: keep the operating voltage well above the pump's start voltage.
Safety. Panels and batteries can deliver dangerous current. Follow NEC Article 690 and local code, and use a licensed electrician or well contractor where your state requires one.
Series raises voltage, parallel raises current
Two panels in series add their voltages and keep the current of one. Two in parallel keep the voltage and add the currents. A typical 100 W 12V-class panel produces about 18 to 20 V at its maximum power point and about 22 to 25 V open-circuit. The Eco-Worthy 100 W panel in the 400 W kit we link lists 20.2 V at maximum power and 24.5 V open-circuit.
| Arrangement | Watts | Vmp | Voc at 25C | Suits |
|---|---|---|---|---|
| 1 panel | 100 | 20.2 | 24.5 | 12V pumps (check the maximum input) |
| 2 in series | 200 | 40.4 | 49.0 | 24 to 42V pumps such as the VEVOR 24V |
| 2 series x 2 parallel | 400 | 40.4 | 49.0 | Same voltage, twice the current |
| 4 in series | 400 | 80.8 | 98.0 | 48 to 82V pumps, but Voc nears the limit in cold |
Cold raises voltage
Panel voltage rises as cells get colder. A panel with a voltage temperature coefficient of about -0.38% per degree C (the figure Eco-Worthy lists) produces roughly 10% more open-circuit voltage at -2C than at 25C. On a frosty, clear morning a string that reads 98 V at 25C can briefly exceed 105 V. Always check the coldest-morning Voc against the pump's or controller's absolute maximum input, not its nominal voltage.
Heat lowers voltage
In summer, panels run 25 to 35C hotter than the air and lose voltage. A string that barely clears a pump's start voltage on a mild day may stall at noon in July. Leave margin at the bottom of the window too: aim for an operating voltage nearer the top of the pump's range than the bottom.
Pump controllers change the rules
Many solar pumps ship with, or need, a pump controller with maximum power point tracking. It accepts a wider voltage range and converts it for the motor, often letting you use fewer, higher-voltage panels. Read the controller's input limits, not the motor's. See controllers, MPPT and float switches.
Put your own gallons, lift and pipe in and see every step of the working.