Solar Voltage Drop Calculator
Already have a wire size in mind? Enter it to see the exact voltage drop over your run — and whether it stays within a safe limit.
1 Your wire & circuit
Estimates for copper conductor. Aim for ≤3% on most runs, ≤2% on critical panel-to-controller runs.
How this is calculated
(2 × ρ × L × I) ÷ A — copper resistivity ρ, one-way length L, current I, cross-section A. The 2 covers the round-trip conductor.2. Percentage =
drop ÷ system voltage × 100.3. Power lost as heat =
drop × current.A drop under 3% is generally fine; above 5% you lose noticeable power and the wire runs warm. If your result fails, use a thicker wire or shorten the run.
Understanding voltage drop in solar systems
Voltage drop is the voltage lost as current travels along a cable. Every conductor has some resistance, and pushing current through that resistance turns a little of your power into heat in the wire instead of delivering it to your battery, controller or inverter. Over short runs it's negligible; over long runs, or at low system voltages, it can quietly rob you of a meaningful share of the energy your panels worked to produce. Because you've already paid for that generation with expensive panels, controlling voltage drop is one of the cheapest ways to protect your return.
How much voltage drop is acceptable?
The widely used guideline (from NEC Article 690 informational notes) is no more than 3% per circuit section and 5% total from source to load. In practice:
- Panel to charge controller / inverter: aim for 1–2%. This is where lost voltage directly reduces harvest, and on an MPPT system excessive drop can pull the input below the controller's operating window.
- Battery to inverter: keep it very low (often under 2%) and the cable very short. These runs carry huge currents at low voltage, so even a small resistance causes a large drop.
- Battery bank wiring: up to 3% is generally tolerated.
The formula, and what each factor does
Voltage drop on a DC circuit is VD = (2 × L × I × R) ÷ 1,000, where L is the one-way length in feet, I is the current in amps, R is the wire's resistance per 1,000 feet, and the 2 accounts for the current's round trip. Three levers reduce it:
- Shorter runs. Drop is directly proportional to distance — site equipment close together where you can.
- Thicker wire. Lower AWG means lower resistance; each step down roughly halves it.
- Higher voltage. The most powerful lever. Because the same power at higher voltage means lower current, raising the system or string voltage slashes the drop for free.
Why higher-voltage strings win
Consider moving 1,000 W over 75 feet. Wired as low-voltage parallel panels at, say, 40V and 25A, you'd need heavy 8 AWG cable just to stay near 3%. Wired as a higher-voltage series string into an MPPT controller, the current drops sharply and the same job is done with far thinner, cheaper wire and a smaller drop. This is the practical reason most home and off-grid systems favour series strings and 24V or 48V architectures — and the wire savings alone often offset the price premium of an MPPT controller over PWM.
What happens if voltage drop is too high?
- Lost energy. The dropped voltage becomes waste heat, reducing the power that reaches your battery or inverter.
- Undercharged batteries. On charging circuits, the battery never sees the full voltage, so it may not charge fully.
- Inverter nuisance trips. If input voltage sags below the inverter's minimum, it can shut down.
- Heat. Excessive drop means a hot cable, which is both inefficient and a safety concern.
A worked example
Suppose a 400 W panel array produces about 22 A at 18V, sited 25 feet from the charge controller, and you're considering 10 AWG wire (resistance ≈ 0.999 ohms per 1,000 feet). The drop is (2 × 25 × 22 × 0.999) ÷ 1,000 ≈ 1.10V, which against 18V is about 6.1% — too high, wasting harvest. Now wire those same panels as a higher-voltage series string: the current falls sharply, and the same 25-foot run on even modest wire drops a fraction of a percent. Nothing changed but the wiring configuration, yet the loss went from unacceptable to negligible. That is the practical power of voltage in DC design, and why checking the percentage — not just picking a "thick enough" wire — matters.
The takeaway: always calculate the actual percentage for your specific current, distance and voltage rather than guessing. A cable that looks generous at 48V can be badly undersized for the same power at 12V.
Where voltage drop matters most
Not every circuit in a solar system is equally sensitive. The battery-to-inverter cable is the most critical: it carries the highest current in the whole system at low voltage, often briefly surging far above the steady figure when the inverter handles a heavy load, so even a short run of slightly undersized cable can drop significant voltage and run hot. That's why these cables are kept very short and very thick. The panel-to-controller run matters next, because losses there come straight off your harvest. Battery-interconnect cables in a bank should be identical in length and gauge so the cells share current evenly. By contrast, low-current sense or communication wiring barely matters for drop. Spending your wire budget where the current is highest gives the best return.
Frequently asked questions
Target 3% or less per circuit section and 5% or less overall. On the panel-to-controller run, 1–2% is best practice because losses there directly cut your energy harvest.
Far more at 12V. A 3% allowance is only 0.36V at 12V but 1.44V at 48V, so low-voltage systems need much thicker wire over the same distance. Higher-voltage systems tolerate longer runs with thinner cable.
Shorten the cable run, or raise the voltage — for example by wiring panels in series rather than parallel, or moving to a 24V/48V system. Both reduce the current for the same power, which reduces the drop.
They're linked: the voltage dropped across the wire's resistance, multiplied by the current, is the power lost as heat. So reducing voltage drop directly reduces wasted power.