Inverters & equipment

DC cable loss calculator

Enter your DC circuit's current, run length, wire gauge, and system voltage. Get the power lost in watts, as a percent of array output, and projected as annual kWh and dollars.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 DC circuit
Combined string current from the parallel strings current calculator.
Operating Vmp for the string, from the string length voltage calculator.
3 Wire run
Measure one-way along the actual routed path.
Use the gauge from the wire gauge sizing calculator.
4 Production estimate
Peak sun hours for your location; 4-6 is typical across most of the US.
Your utility's retail rate, used to translate lost energy into a dollar figure.

How it works

Three steps to your cable loss

Step 1

Confirm current, voltage, and gauge

Pull circuit current from the parallel strings current calculator and the wire gauge from the wire gauge sizing calculator so the loss figure matches your actual planned run.

Step 2

Add your site's production assumptions

Enter average daily sun hours for your location and your utility's per-kWh rate, so the watt figure converts into a real annual energy and dollar estimate rather than staying abstract.

Step 3

Compare against an upsized gauge

Re-run the calculator with the next gauge up to see the dollar savings, then weigh that against the extra cost of the larger conductor over the length of the run.

Understanding DC cable loss

Why watts, not just volts, is the number that matters

The voltage drop calculator reports how many volts and what percentage of system voltage a run loses. That's useful for confirming code compliance, but it doesn't directly answer the question a homeowner or system owner actually cares about: how much energy, and how much money, does that loss cost over a year. This calculator converts the same underlying resistive loss into power, using P = I squared times R, based on the ampacity assumptions in NEC Table 310.16, then projects it forward using your site's production assumptions.

Why current matters more than voltage for this calculation

Because power loss scales with the square of current, a circuit carrying twice the current loses four times as much power in the same wire, all else equal. This is why combined string circuits, sized with the parallel strings current calculator, deserve a closer look at cable loss than a single string would, since combining strings multiplies the current running through the shared conductor.

Why small percentages still add up over 25 years

A 1-2% power loss sounds negligible on a single day, but solar systems are commonly warrantied for 25 years, and a consistent daily loss compounds into a real number over that span. A 100W constant loss adds up to hundreds of kilowatt-hours a year, and multiplying that by a couple of decades of operation and a realistic electricity rate turns a rounding error into a meaningful line item worth weighing against the upfront cost of a larger conductor; the cost per kWh produced calculator can help put that specific dollar figure together.

Why this differs from the wire gauge sizing check

The wire gauge sizing calculator finds the minimum gauge that won't overheat. That minimum gauge is not necessarily the most cost-effective one over the system's life. A slightly larger conductor than the bare code minimum often pays for itself in avoided energy losses well within the system's warranty period, particularly on longer DC home runs, and NREL's PVWatts calculator can help confirm whether that tradeoff makes sense for your specific production numbers.

What this tool doesn't cover

This calculator estimates loss from a single wire run's resistance using standard 75C resistivity constants. It doesn't account for connector losses, combiner box resistance, or panel-level degradation from mechanisms like PID, which the output loss from PID degradation estimator covers separately, and the annual figures here are estimates based on your stated sun hours, not an hour-by-hour production model. For a full system energy loss picture, use this alongside the AC cable loss calculator for the inverter-to-panel segment.

Questions

Frequently asked

Find the wire's round-trip resistance from its gauge and length, then multiply by current squared to get power loss in watts. A 25A DC circuit on 10 AWG copper over a 75 foot one-way run loses about 116W, roughly 1.2% of a 9,500W array.

Voltage drop reports the volts and percentage lost along the wire. This calculator converts that same resistive loss into watts, then projects it into kilowatt-hours and dollars lost per year based on your site's sun hours and electricity rate, which is the number that actually shows up on a production estimate or utility bill.

A modest 100W constant loss adds up fast: at 5 sun hours a day it's roughly 182 kWh a year, and over a 25-year system life that's more than 4,500 kWh, worth several hundred dollars at typical residential electricity rates. Losses that look small in percentage terms compound meaningfully over a system's expected lifetime.

Not always. Power loss falls roughly in proportion to the wire's cross-sectional area, so doubling the circular mil area roughly halves the loss, but larger copper conductors cost more per foot. For short runs the extra wire cost often outweighs decades of energy savings; for long DC home runs with real annual dollar losses, the upsize can pay for itself well within the system's warranty period.

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Keep sizing your equipment

Voltage drop calculator Wire gauge sizing calculator AC cable loss calculator Parallel strings current calculator Conduit fill calculator String length voltage calculator Combiner box sizing calculator All inverters and equipment tools