Skip Cleaning or Accept a Worse Roof Face? Dust on an Ahvaz Array

Method note

Modeled 8 kW fixed arrays at Ahvaz (31.3203°N, 48.6692°E). Clean monthly AC comes from the JRC PVGIS PVcalc API v5_2 (run 2026-07-31): tilt 31°, aspects 0° (south), 45° (southwest), and 90° (west), with 10% non-soiling system loss so dust can be layered separately. Soiling uses a linear sawtooth between washes: daily rate × days until clean, average loss ≈ peak/2 (with a 40% peak cap for extreme dry stretches). Daily rates are literature bands for dusty arid climates, not Ahvaz field meters: 0.10%/day (moderate, within NREL soiling-station dry-period ranges commonly ~0.03–0.15%/day) and 0.25%/day (high arid band within published field rates up to ~0.4–0.5%/day). Background: soiling and monitoring and panel maintenance.

Direct answer

Clean south on this Ahvaz run yields 13,301 kWh/year. Rotating the same clean array 45° toward southwest costs about 397 kWh/year (−3.0%). At a moderate dusty rate (0.10%/day), cleaning every 90 days instead of every 7 days costs about 552 kWh/year more. That extra soiling loss already exceeds the mild orientation penalty. At a high arid rate (0.25%/day), a 90-day interval alone loses about 1,496 kWh/year versus clean south (~11%). Thesis: in a dusty climate, wash discipline is often a larger annual lever than fretting over a modest compass compromise on an otherwise clean roof.

Hypothesis

On an 8 kW Ahvaz south rooftop, skipping washes for about 90 days at a dusty-climate soiling rate costs more annual energy than a mild 45° southwest orientation compromise on a regularly cleaned array at the same site.

Inputs

Clean PVGIS baseline for Ahvaz

Figure: monthly AC for the clean south Ahvaz baseline (PVGIS PVcalc, 2026-07-31).

A full west face is a large design hit. A 45° southwest compromise is real but modest next to what prolonged dry soiling can do.

How much does cleaning interval cost on south-facing Ahvaz?

Average annual soiling loss for a linear sawtooth is half the peak loss between washes (until the 40% cap bites).

Figure: annual AC for clean orientations and selected cleaning intervals.

Does skipping washes beat a mild orientation compromise?

Figure: annual energy cost of orientation compromise versus delayed cleaning.

At the moderate dusty rate, the extra loss from 90-day versus weekly cleaning (552 kWh) exceeds the SW45 penalty (397 kWh). Hypothesis holds. At the high arid rate, even absolute 90-day soiling (1,496 kWh) dwarfs that mild compass hit and approaches the scale of a full west-facing roof.

NREL soiling briefings note Middle East annual losses can reach 20% or more when cleaning is weak, with peak losses after dust events far higher. This Ahvaz model is a controlled comparison, not a claim that every Khuzestan roof hits those extremes. See also what affects production and tilt and orientation.

What this means

Homeowners often treat orientation as destiny and cleaning as optional housekeeping. On this dusty-climate run, the opposite ranking shows up for a common trade-off: a regularly cleaned array that is 45° off south still beats a south array left dirty for a full season at a moderate soiling rate. Principle: quantify soiling with a stated rate and wash plan, then compare that kilowatt-hour cost to design compromises you cannot change. Use weather-normalized output and clear-day curve checks so you wash on evidence, not a rigid calendar alone. For proposal reading, pair this with the proposal checklist and ROI method.

Uncertainty and limits

- PVGIS PVcalc is a typical-year model, not a forecast. Interannual weather can move annual yield by several percent (plan around a P50-style central estimate; financing often uses a more conservative P90-style band). - Daily soiling rates are literature bands applied to Ahvaz yield. They are not measured on this coordinate. Dust storms, sticky films, and bird droppings can break a simple linear sawtooth. - Rain can reset soiling earlier than your planned wash. A dry Ahvaz stretch without rain is the stressful case this model stresses. - The 10% non-soiling loss is deliberate so dust is visible as a separate lever. Installer proposals that already embed average soiling in a 14% loss will look different on paper. - No currency, cleaning-labor cost, or water cost in this study. Energy recovered still has to clear the cost and safety of roof access. - No shade, clipping, or module temperature sweep here.

What to do with this

1. Ask what soiling assumption (percent or wash plan) sits inside any production estimate. 2. Log dry streaks and clear-day peak power; wash when the curve shifts down and rain did not reset it. 3. If the roof must face southwest, prioritize cleaning discipline before chasing another kilowatt of nameplate. 4. Treat west-facing yield hits and multi-month dirty south arrays as different problems: one is geometry, the other is O&M.

Closing

For this Ahvaz 8 kW south baseline (13,301 kWh clean), a mild 45° southwest compromise costs about 397 kWh/year. Skipping from weekly to 90-day cleaning at 0.10%/day costs about 552 kWh/year more. At 0.25%/day, a 90-day interval alone loses about 1,496 kWh/year versus clean. In dusty climates, the wash plan is not a footnote next to compass direction.