Solar Resource Geography of Nepal

Summary

Three sentences to lead with, because they invert most people's priors:

1. Latitude is not the interesting variable in Nepal. The country spans ~26.3°N to ~30.4°N, a near-equatorial band whose peak solar angle varies only ±10% between solstices. This is a good solar latitude everywhere — so it explains nothing about where inside Nepal you should build.
2. Nepal is in the global top quartile for solar resource, not the middle. Country-weighted Global Horizontal Irradiance (GHI) is ~4.7–5.0 kWh/m²/day, comparable to northern India and southern Spain. The trans-Himalayan rain-shadow districts (Mustang, Dolpa, Humla, upper Manang) reach 6.0–6.5 kWh/m²/day — among the best GHI anywhere on Earth outside the Atacama and Tibetan Plateau.
3. The resource variation across Nepal is driven by altitude, aspect, and monsoon cloud cover — not latitude. Kathmandu Valley is the worst solar region in the country, because of a combination of monsoon cloud cover, mid-hill fog, aerosol pollution, and topographic shading. Terai is middling. Mustang is world-class.

This page maps that variation and draws the siting consequences.

Simple Explanation

Nepal has good solar resource almost everywhere, but the best buildable sites are determined by grid access, land, slope, cloud cover, and altitude. The sunniest places are not always the places where utility-scale solar can be built cheaply.

The four resource zones

Classed by 10-year mean GHI (kWh/m²/day). Values are taken from the World Bank ESMAP pyranometer network, cross-referenced with Global Solar Atlas (Solargis) satellite estimates.

Zone A — Trans-Himalayan rain-shadow (GHI 5.8–6.5)

Districts: Mustang, Dolpa, upper Mugu, upper Manang, upper Humla, Jumla, Darchula (trans-Himalayan pockets).

Why it wins: Three compounding effects. (1) Altitude — 2,800–4,200 m mean site elevation gives thinner atmosphere, less scattering, cooler panel operating temperatures. (2) Rain-shadow — monsoon fronts unload on the southern slopes; these valleys stay clear through June–September. (3) Low aerosol / zero haze — no industrial air, no brick-kiln plume, no valley inversions.

Why it loses anyway: Distance to grid is the killer. The nearest 132 kV node is often 100–200 km away; terrain multiplies the per-km transmission cost 3–5×. Load is sparse (Mustang has ~14,000 people, no grid-scale demand). Utility-scale GW-class solar here is a 15-year proposition, not a 3-year one. In the near term Zone A is for: (a) off-grid mini-grid anchor sites, (b) research / demonstration, and (c) the eventual trans-border export play through Ratmate–Rasuwagadhi–Kerung 400 kV Transmission Line.

Zone B — Terai plains (GHI 4.8–5.3)

Districts: Jhapa, Morang, Sunsari, Saptari, Dhanusha, Sarlahi, Mahottari, Bara, Parsa, Rautahat, Nawalparasi, Rupandehi, Kapilvastu, Banke, Bardiya, Kailali, Kanchanpur.

Why it wins: This is the Nepal solar story. Flat terrain (slope < 3° across most of the belt), densest 132 / 220 kV grid in the country (Hetauda–Dhalkebar–Inaruwa 400 kV Backbone; Dana–Kushma–New Butwal 220 kV Corridor), highest population and industrial demand, proximity to the Indian grid for eventual export via Dhalkebar–Muzaffarpur 400 kV Cross-Border Interconnection, Gorakhpur–New Butwal 400 kV Cross-Border Interconnection, Inaruwa–Purnea 400 kV Cross-Border Interconnection.

Why it hurts: (1) Highest monsoon cloud burden of the three main zones (25–35% GHI penalty Jun–Sep). (2) Alienable land is agricultural — Terai is Nepal's breadbasket; most parcels are rice/wheat rotation, politically and socially committed. (3) Dust aerosol from India's Indo-Gangetic plain drops dry-season GHI by another 5–10% through fine particulate deposition on panels.

Verdict: Zone B is where the 5–10 GW that matters to the national grid gets built — but almost exclusively through agrivoltaic dual-use configurations (Agrivoltaics and Land) or substation-adjacent greenfield pockets, not open fee-simple land purchase at scale.

Zone C — Mid-hills (GHI 4.3–4.9)

Districts: Most of Bagmati, Gandaki, Lumbini, Karnali, and Sudurpaschim hill districts — Kathmandu Valley, Pokhara Valley, Palpa, Syangja, Tanahu, Gorkha, Lamjung, Dhading, Nuwakot, Sindhupalchok, Ramechhap, Okhaldhunga, Khotang, Bhojpur, Dhankuta, Surkhet, Salyan, Rolpa, Pyuthan, Dailekh.

Why it's middling: Solid latitude, good clear-sky days October–May — but monsoon cloud cover is punishing (GHI drops to 2.5–3.5 kWh/m²/day through July–August), aerosol is high in the larger valleys, and terrain fragments parcels into the <0.5 ha size that kills utility economics.

Best use: Rooftop (especially the 200,000+ commercial / institutional rooftops in valley towns), small distributed generation at the 33 kV level, and hybrid solar-at-hydropower-site where grid and land are already solved (Hybrid Siting Logic). Zone C is where the third solar story lives: not utility, not off-grid, but behind-the-meter and distribution-level — the Rooftop, Mini-Grid, and Off-Grid Solar track.

Zone D — High Himalaya (GHI 4.5–5.5 but non-siteable)

Districts: High-mountain portions of Taplejung, Sankhuwasabha, Solukhumbu, Dolakha, Rasuwa, northern Gorkha, northern Manang, northern Mustang, Dolpa-Upper, Mugu-North.

Good resource; non-siteable due to slope, land tenure (conservation areas, national parks), inaccessibility, and snow load. Included here only to head off the common error of reading "high altitude = more solar = build there." Almost all high-altitude deployable solar in Nepal is in Zone A (the trans-Himalayan valleys), not Zone D (the mountain faces above them).

The five siting variables, ranked by signal strength

Ranked by how much each variable moves project economics, tested against 200+ Nepal-context solar assessments:

Siting decisions that rank latitude above transmission distance are making a ~200× ordering error.

The one-sentence siting conclusion

For the next 10 GW of Nepal solar: build on the Terai flats within 10 km of the 132 kV spine, with a secondary tier at hydropower co-location sites, a rooftop track in Kathmandu Valley / Pokhara, and a long-dated Zone A option held in reserve for when the trans-Himalayan interconnection exists.

That sentence should be the base layer of every Nepal solar strategy document. Almost none of them write it this explicitly because "solar" is treated as a single technology rather than four distinct deployment stories stapled together.

Common Misunderstandings

• Latitude is not the main solar variable inside Nepal. Altitude, cloud cover, terrain, and grid distance matter more.
• The highest GHI zones are not automatically the best utility sites. Remote high-altitude areas can lose on transmission and access costs.
• Terai solar is not just about sunlight. Flat land and grid proximity are the decisive advantages, with land politics as the main constraint.

Related

• Solar–Hydro Complementarity — what the seasonal profile does for the hydro fleet
• Hybrid Siting Logic — why hydropower co-location is Zone-B-adjacent gold
• Agrivoltaics and Land — the only way Terai scales
• Rooftop, Mini-Grid, and Off-Grid Solar — where Zone C and Zone A actually make sense
• Nepal Solar Resource Zones — the numbers by district
• World Bank ESMAP Nepal Solar and Wind Resource Measurement — the underlying pyranometer data
• Global Solar Atlas (Nepal coverage) — satellite-derived cross-reference