Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit:
- V = I × R - Voltage equals Current times Resistance
- I = V ÷ R - Current equals Voltage divided by Resistance
- R = V ÷ I - Resistance equals Voltage divided by Current
In Solar Systems
Understanding Ohm's Law helps you:
- Size wiring correctly - Calculate voltage drop in long wire runs
- Troubleshoot problems - Identify high-resistance connections
- Design circuits - Choose appropriate components for your system
- Measure safely - Understand how meters work with your system
Voltage Drop in a 100-Foot Panel Run
A rooftop array produces 40V at 10A (400W). Using 10 AWG copper wire over 100 feet (200 ft round trip) with 1.02 ohms/1,000 ft resistance: total resistance is 0.204 ohms. Voltage drop = 10A × 0.204 = 2.04V (5.1%) — exceeding the recommended 3% maximum. Power lost as heat: 20.4W. Solution: upgrade to 6 AWG for a 2.0% drop.
Troubleshooting a Bad MC4 Connection
A solar string reads 142V at the combiner box under 8A load, but should be 150V. Missing voltage: 8V. Using R = V/I: 8/8 = 1 ohm of unexpected resistance. Normal wire resistance for the run should be ~0.2 ohms. The extra 0.8 ohms indicates a corroded or loose MC4 connector — wasting 51.2W as heat at the bad connection, a fire hazard.
Battery Fuse and Connection Resistance
A 48V battery bank powers a 3,000W inverter at 62.5A. A typical fuse has ~0.001 ohms (negligible drop). But a corroded fuse holder at 0.1 ohms causes V = 62.5 × 0.1 = 6.25V drop and P = 390W of heat — extremely dangerous. Regular connection inspections prevent this.
- Calculating wire size for panel runs — Using V = IR with known wire resistance per foot and expected current, determine minimum wire gauge. NEC requires voltage drop below 3% for branch circuits and 5% total.
- Diagnosing underperforming systems — By measuring voltage at different points (panel output, combiner box, inverter input), you can calculate resistance in each segment. Higher-than-expected resistance points to corroded connectors or damaged wire.
- Designing low-voltage battery systems — A 3,000W load on 12V draws 250A vs 62.5A on 48V. Using Ohm's Law, even 3 feet of cable at 250A produces significant voltage drop — this is why most modern solar systems use 48V.
