Line Losses

Answer: Line losses are power dissipated in electrical conductors, principally as resistive heat, and the corresponding energy accumulated over an operating period. In a solar system they can occur in DC and AC collection circuits. Their magnitude depends on circuit resistance and current over time. Define the circuit and measurement boundaries before calculating losses or applying a percentage in an energy model.

Power loss and voltage drop are different quantities

For a simple resistive conductor, P_loss = I² × R, with current I in amperes, resistance R in ohms and power loss in watts. In a simple DC resistive circuit, the conductor voltage drop is ΔV = I × R, in volts. Neither expression alone provides annual energy loss.

PVsyst’s current ohmic-loss documentation identifies resistance as the underlying parameter and accumulates losses at each simulation step using current. It distinguishes a loss fraction at a reference operating condition from the resulting period’s energy loss. This documented model behavior is not a native SurgePV capability claim or a universal annual loss percentage.

If current is constant for a stated interval, energy loss is power loss multiplied by time. For changing current, accumulate I² × R × Δt across the intervals with consistent units. A loss at rated power cannot simply be assumed to persist during every operating hour.

A hypothetical DC circuit calculation

Assume a complete two-conductor loop resistance of 0.20 Ω at the declared operating temperature and constant current of 10 A. The loop voltage drop is 10 × 0.20 = 2 V; its heat dissipation is 10² × 0.20 = 20 W.

At a declared source voltage of 400 V, source power is 400 × 10 = 4,000 W. The loss fraction at that condition is 20 / 4,000 = 0.5%. Over five hours at that same current and resistance, lost energy is 20 × 5 / 1,000 = 0.10 kWh.

If current doubles to 20 A with resistance unchanged, loss becomes 20² × 0.20 = 80 W, four times the original power loss. These are instructional inputs, not an approved conductor size or measured system. The complete loop resistance already includes both outgoing and return conductors; do not double it again.

Specify resistance and circuit boundaries

For a uniform conductor, R = ρ × L / A, using compatible resistivity, length and cross-sectional-area units. If two identical DC conductors each have one-way length L, their loop resistance is 2 × ρ × L / A. Real circuits may have multiple branches, different sections and connection resistances.

Resistance depends on material and temperature. AC calculations also require the applicable RMS current, frequency/circuit conditions and effective resistance; voltage-drop work may involve reactance and power factor. Do not apply a two-wire DC loop formula indiscriminately to a three-phase circuit.

Keep inverter conversion, transformer and other system losses distinct in the accounting model to avoid double counting. A percentage entered at one DC boundary is not automatically interchangeable with an AC meter loss or a utility settlement loss factor.

Reduce losses within an approved design

Shorter routes and appropriate conductor sections can reduce resistance, but routing, inverter location and equipment selection also have protection, installation and operational constraints. Ampacity, insulation ratings, termination compatibility and applicable requirements must be checked separately; low calculated loss does not establish safe current carrying capacity.

Do not infer a universal allowable voltage drop from this definition. The adopted rules, equipment instructions and project requirements determine the applicable design checks. Optimizers retain DC circuits, and microinverters move conversion to module level but still require AC collection; neither eliminates conductor losses.

Handover and verification

Designers should retain resistance calculations and loss boundaries in loss analysis. Site teams should report route, conductor or connection changes and follow equipment-specific assembly instructions. Qualified personnel should plan commissioning or diagnostic measurements under the applicable safety process; no universal deviation threshold alone proves a wiring fault.

For sales or financial evaluation, compare the modeled energy change and actual upgrade cost under declared tariff and time assumptions. There is no guaranteed conductor-upgrade payback. Verify what the actual software calculates before promising automatic circuit-level results.

Frequently asked questions

Are voltage drop and line loss the same thing?

No. Voltage drop is in volts; resistive power loss is in watts, and period energy loss is in Wh or kWh.

Can a rated-condition loss percentage be used as annual loss?

Not automatically. Annual loss depends on the operating current and resistance over time and the chosen energy boundary.

Do optimizers eliminate long DC cable losses?

No. Their systems still have DC conductors; evaluate the actual circuit architecture.

Does low calculated loss prove a conductor is compliant?

No. Ampacity, protection, equipment compatibility and applicable installation requirements need separate review.