Solar Irradiance Calculator (GHI / DNI / DHI → POA)

What this calculator does

Converts the three irradiance components — Global Horizontal (GHI), Direct Normal (DNI) and Diffuse Horizontal (DHI), in kWh/m²/day — into Plane-of-Array (POA) irradiance for any module tilt and azimuth at a UK site. POA is the single most important input in every PV yield estimate; everything downstream (annual kWh, MCS commissioning yield, ROI) flows from it.

It also reports annual kWh/m², annual specific yield (kWh per kWp installed), single-module daily and annual energy, and the value of one module per year at the local retail tariff. A consistency check flags inputs where GHI ≠ DNI · cos(zenith) + DHI — the most common manual-entry error when reading TMY values out of a PVGIS CSV.

How to use it

1. Pull GHI, DNI and DHI for your site from PVGIS 5.2 (re.jrc.ec.europa.eu/pvg_tools) — pick “TMY generator”, enter your postcode, and download the hourly CSV. Daily averages are in the JSON-summary endpoint.
2. Enter your tilt (typical UK pitched roof is 30–40°) and azimuth (180° = true south; for an Ordnance Survey grid bearing, add the local grid declination from the Met Office).
3. Set albedo to 0.18 for typical British roofing (slate, tile, asphalt-shingle) or 0.55 for a fresh frost.
4. The calculator returns POA in kWh/m²/day plus annual specific yield and per-module economics in pounds.

The math

The Liu–Jordan (1960) decomposition splits POA into three terms, all required by IEC 61724-1 and the MCS PV calculation methodology:

• Beam: POA_beam = DNI × cos(AOI) — what hits the panel directly. AOI is the angle between the sun and the panel’s normal vector.
• Sky diffuse: POA_diffuse = DHI × (1 + cos β) / 2 — scattered light from the sky dome. A flat panel sees the full dome; a vertical panel sees only half.
• Ground reflected: POA_ground = GHI × ρ × (1 − cos β) / 2 — light bouncing off the ground. Higher tilt and brighter ground (snow, fresh paving) increase this term.

Total POA × 365 gives annual kWh/m². Multiplied by module efficiency × PR × area, you get per-module annual energy.

UK irradiance, PVGIS 5.2 typical year

PVGIS 5.2 uses the SARAH-2 CMSAF satellite reanalysis; the typical year is built from the most representative month from the 2005–2020 baseline. UK GHI varies by ~30 % from the Highlands to the south coast.

Source: PVGIS 5.2, accessed 2024 Q4. UK MCS MIS 3002 commissioning yield uses these values directly.

What POA tells you about UK system sizing

Once annual POA is known, the MCS / Solar Energy UK design chain is:

1. Annual specific yield = annual POA × PR. A south-facing 35° London array with PR 0.78 gives ≈ 3.41 × 365 × 0.78 ≈ 971 kWh/kWp, matching the MCS irradiance tables for SE England within 1 %.
2. System size for a target annual kWh: kWp = annual_kWh / specific_yield. A 4000 kWh South-East household needs ≈ 4.1 kWp.
3. Module count = kWp / panel_kWp. At 425 W panels (the JA Solar / LONGi Hi-MO baseline in 2026), that is 10 modules. Cross-check against the solar panel count calculator which folds in roof-area constraints.

UK-specific accuracy tips

• Use TMY, not a single year. UK irradiance varies ±8 % year-on-year (Met Office MIDAS, 2010–2023). A single low-irradiance year (heavy summer cloud) can underpredict 25-year fleet output by 6 %; PVGIS TMY averages it out.
• Update albedo for snow days. Above 55°N (Glasgow, Edinburgh, Aberdeen) winter snow cover lifts effective albedo to 0.55 for 5–15 days/year, lifting POA on a 60°-tilt panel by 2–4 % during the heating-load months. The solar panel snow loss calculator handles the soiling side of the same coin.
• For east-west split arrays, run two cases. Many UK terrace and semi-detached homes split panels across an east and a west pitch. Run the calculator twice (azimuth 90° and 270°), sum the per-roof energies, and compare to the equivalent south-only POA. East-west saves on annual kWh but improves daytime self-consumption — the solar self-consumption calculator quantifies the trade-off.
• Cross-check against PVGIS before finalising any MCS design submission. PVGIS uses a Perez transposition that is 1–3 percentage points more accurate than the isotropic model used here at UK latitudes; expect agreement within ±3 %.

How POA feeds the rest of your UK design

POA is the upstream variable for almost every other calculator on this site:

• The solar panel output calculator takes POA × PR as its core energy estimate.
• The solar system efficiency calculator inverts the relationship — given measured AC kWh and POA, it returns a real-world PR you can benchmark against the Solar Energy UK 2024 PV Performance Report median (0.78).
• The solar panel tilt calculator and solar panel azimuth calculator feed directly into the AOI term of the beam component.
• The solar bill savings calculator divides POA-derived annual kWh between self-consumed (offset at the retail Ofgem cap, 24.5 p in 2026) and exported (paid at the Smart Export Guarantee tariff your supplier offers).

Authority sources

• PVGIS 5.2 — re.jrc.ec.europa.eu/pvg_tools — JRC Joint Research Centre, the EU reference irradiance database, accepted by all UK DNOs and MCS bodies.
• MCS MIS 3002 v4.0 — Microgeneration Certification Scheme PV design and installation standard. Defines the irradiance and PR conventions for residential commissioning paperwork.
• Solar Energy UK 2024 PV Performance Report — fleet study of 8,400 UK domestic systems giving the 0.78 median PR figure used as the calculator default.
• Met Office MIDAS Open — ground-station irradiance data for ~280 UK sites, useful for cross-validating PVGIS satellite estimates if your site sits in a microclimate.
• Loughborough CREST and BRE National Solar Centre — academic references for UK-specific Perez vs isotropic transposition error bounds (both within ±3 % at typical UK tilts).