Introduction #

Solar system losses (and the matching PV derating factors) describe how much of ideal STC or POA energy you do not deliver as AC kWh—soiling, temperature, mismatch, wiring, inverter efficiency, shading, and availability. In CalcPanel tools, these show up as system loss % or system efficiency η. This guide explains what to put in those fields for commercial/industrial screens.

Best for: engineers using the PV Energy Production Calculator or Solar Panel Sizing Calculator who need defensible derate assumptions.

Not ideal for: full PVsyst / SAM bid models, module lab characterization, or camping kit affiliate reviews.

Hub workflow: Solar calculator.

What solar system losses mean (and how they map to tools) #

Conclusion: One combined derate is fine for screening—as long as you know which effects you folded in.

Tool field Typical meaning
System loss % (pv-energy-production) Fraction of ideal energy removed → multiply by (1 − loss)
System efficiency η (solar-panel-sizing) Remaining fraction after losses → Array kW ≈ daily kWh ÷ (PSH × η)

If η = 0.80, that is roughly a 20% combined loss screen (1 − 0.80). Prefer site data when you have it; otherwise use the band tables below.

CTA: After you pick a loss band, run annual kWh in the PV Energy Production Calculator.

Typical derating factors (commercial screens) #

Conclusion: Build a transparent stack—then collapse to one number for the calculator.

Factor Typical planning band Notes
Inverter / clipping 2–6% Higher if DC/AC ratio is aggressive
DC & AC wiring 1–3% Long DC runs and undersized AC feeders hurt more
Soiling 1–5% Industrial dust / low rain → upper end
Mismatch / nameplate 1–3% Binning and age
Shading / snow 0–10%+ Site-specific; worst month for autonomy
Availability / downtime 0–2% O&M assumptions
Temperature (module) See next section Often the largest single weather effect

Screening defaults used on CalcPanel: production tool default 15% loss; sizing tool default η = 0.80. Tighten only when you have measured soiling or OEM loss reports.

Solar panel temperature derating (merged) #

Conclusion: Module power falls as cell temperature rises; cold weather raises Voc (string design)—do not confuse the two.

Approximate power temperature effect (planning):

P ≈ P_STC × [1 + γ × (T_cell − 25 °C)]

where γ is the power temperature coefficient (often about −0.3 to −0.4 %/°C for c-Si—use the datasheet).

Example: γ = −0.35 %/°C, T_cell ≈ 55 °C → ΔT = 30 °C → relative power ≈ 1 − 0.0035 × 30 = 0.895 (~10.5% temperature loss vs STC).

Fold that into your combined loss or η. For cold Voc string limits, use the Solar Panel Series Parallel Calculator and MPPT Sizing Calculator—that is voltage compliance, not annual kWh derate.

Worked example (numbers) #

Given: 10 kW array, 5.0 peak sun hours, combined system loss 15% (η = 0.85 for production).

Annual energy screen:

10 × 5.0 × 365 × (1 − 0.15) ≈ 15,513 kWh/yr

Same site with dusty industrial soiling raising loss to 22%:

10 × 5.0 × 365 × 0.78 ≈ 14,235 kWh/yr (~8% less energy).

For array sizing: 40 kWh/day need, PSH 5.0, η = 0.80 → Array kW = 40 ÷ (5 × 0.80) = 10 kW—use the Solar Panel Sizing Calculator. Resource first: Peak Sun Hours Calculator.

How to choose a single loss % for screening #

Conclusion: Prefer conservative combined losses for budget talks; tighten after O&M data.

  1. Start at 12–18% combined for clean commercial roofs with modern inverters.
  2. Add soiling / shading if the site is industrial or partially obstructed.
  3. Keep temperature inside the stack (or raise loss 3–8 points in hot climates).
  4. Enter the result in PV Energy Production; mirror η ≈ 1 − loss in Solar Panel Sizing.

Bill-offset screens can continue in the Industrial Energy Estimator.

Common mistakes #

  1. Using η = 1.0 “because the inverter is 98%” — inverter efficiency is only one slice.
  2. Double-counting temperature in both PSH and loss %.
  3. Using summer-only derates for winter autonomy — autonomy needs worst-month resource.
  4. Treating Voc cold rise as an energy loss — it is a string-voltage limit.
  5. Copying residential DIY loss tables for dusty industrial roofs without adjustment.

Next steps #

  1. Pick PSH: Peak Sun Hours Calculator.
  2. Size array with honest η: Solar Panel Sizing Calculator.
  3. Estimate annual kWh with loss %: PV Energy Production Calculator.
  4. Continue BOS: MPPT Sizing · Solar Inverter Sizing on the Solar calculator hub.
  5. For interconnection after yield screens: Commercial Solar Interconnection Requirements.

Assumptions and disclaimer #

Bands above are planning estimates. Real projects use measured soiling, thermal models, and OEM loss reports. This guide does not replace PVsyst/SAM or contractual performance guarantees.

FAQ #

What are solar system losses?

Solar system losses are the combined reductions from ideal resource-to-AC energy—soiling, temperature, wiring, inverter, mismatch, shading, and downtime. Tools often express them as a single loss % or efficiency η.

What derating factor should I use?

For clean commercial screens, 12–18% combined loss (η ≈ 0.82–0.88) is a common starting band. Raise losses for dust, shade, or hot rooftops. Confirm with site data before procurement.

How does temperature derating work?

Module power typically falls roughly 0.3–0.4%/°C above 25 °C cell temperature (datasheet γ). Example: 30 °C rise at −0.35%/°C ≈ 10.5% power reduction vs STC. Cold weather mainly raises Voc for string design.

How do I enter losses in CalcPanel tools?

Use system loss % in the PV Energy Production Calculator, or system efficiency η in the Solar Panel Sizing Calculator (η ≈ 1 − loss).

Is peak sun hours already derated?

PSH is a resource metric. You can apply a small site derate on the Peak Sun Hours Calculator, but still apply equipment/soiling losses separately in production or sizing—avoid double-counting the same effect.