Do the Hills Around Chesham Shade Solar Panels?
PVGIS terrain-horizon estimates for Chesham roofs by direction and pitch, what the model misses, and what a site survey should check.
Do the Hills Around Chesham Shade Solar Panels?
Chesham's adopted neighbourhood plan refers to the town's many steep hills and valleys. So do the hills take sunshine away from your roof? We put the question to PVGIS, the solar model run by the European Commission's Joint Research Centre (JRC), which can include shading from surrounding terrain.
The short version: in the model, terrain costs a south-facing roof in central Chesham very little over a year, costs an east-facing roof more, and matters most on winter mornings. If you are speaking to solar panel installers in Chesham, these are the figures to ask them to rework for your own address.
Over a year, the modelled terrain horizon trims a south-facing roof in central Chesham by about 1%.
How we ran the numbers
- Location: latitude 51.705, longitude -0.611, in central Chesham beside the HP5 1DA postcode
- Model: PVGIS 5.3, PVGIS-SARAH3 radiation data for 2005 to 2023
- System: 1kWp of crystalline silicon panels, 14% system losses, fixed free-standing mounting
- Method: each roof direction and pitch run twice, with and without the PVGIS calculated horizon
Results are per kWp, so a 4kWp array makes about four times each figure.
What the skyline looks like from central Chesham
PVGIS works out a horizon height, in degrees above level, for each direction around a point:
| Looking towards | Modelled horizon height |
| North-east | 10.7 degrees |
| East | 10.7 degrees |
| South-east | 10.7 degrees |
| South-south-east | 4.6 degrees |
| South | 1.5 degrees |
| West | 0.8 degrees |
The raised skyline runs from south-east round to north-east, while the view south and west is close to flat. On the winter solstice PVGIS puts the midday sun at 14.8 degrees, and that morning the sun stays below the modelled skyline to the south-east until it is roughly 30 to 40 degrees east of due south. At the summer solstice the midday sun reaches 61.7 degrees, far above any modelled terrain.
Output by roof direction and pitch
| Roof faces | Pitch | Without horizon (kWh per kWp) | With horizon (kWh per kWp) | Yearly change | December change |
| South | 30 degrees | 1,010 | 1,000 | -1.0% | -6.7% |
| South | 35 degrees | 1,018 | 1,007 | -1.1% | -7.0% |
| South | 45 degrees | 1,017 | 1,004 | -1.3% | -7.6% |
| South-east | 35 degrees | 966 | 941 | -2.6% | -10.7% |
| South-west | 35 degrees | 951 | 949 | -0.1% | -1.8% |
| East | 35 degrees | 818 | 789 | -3.5% | -13.2% |
| East | 45 degrees | 790 | 756 | -4.3% | -15.9% |
| West | 35 degrees | 796 | 796 | 0.0% | 0.0% |
| North | 35 degrees | 547 | 542 | -0.9% | 0.0% |
What the table shows
- South-facing roofs barely notice the terrain: a yearly cost of 1.0% to 1.3%.
- East and south-east slopes pay most, 2.6% to 4.3% a year, because the modelled eastern skyline blocks morning sun. The 45-degree east slope loses more than the 35-degree one.
- West and south-west slopes lose almost nothing, because the modelled western horizon is under 2 degrees.
- December shows the effect: a south-facing roof loses 7.0% of its December output to terrain, an east-facing roof 13.2%.
- An even east-west split at 35 degrees loses about 1.8% a year, because the west half is untouched.
A different address, a different skyline
Terrain shading belongs to a location, not a town. We repeated the runs for the postcode point of HP5 3HA in Chesham North ward, under a kilometre away:
| Central point (51.705, -0.611) | HP5 3HA point (51.7125, -0.6066) |
| Horizon due south | 1.5 degrees | 5.3 degrees |
| Horizon due east | 10.7 degrees | 4.6 degrees |
| Highest modelled horizon | 10.7 degrees | 8.0 degrees |
| Yearly loss, south at 35 degrees | 1.1% | 0.6% |
| Yearly loss, east at 35 degrees | 3.5% | 1.0% |
A house on a valley side, on higher ground or below a steep bank can have a different horizon, so a generation estimate should be run for your address, not borrowed from a neighbour.
What the horizon model does not capture
PVGIS terrain data has a resolution of 3 arc-seconds, about 90 metres, and the JRC explains that at that resolution the model cannot account for shadows from nearby objects such as houses or trees. The calculated horizon leaves out:
- Trees in your garden, next door and on nearby slopes
- Neighbouring houses, extensions and outbuildings
- Your own chimneys and dormers
- Ground features smaller than the roughly 90-metre terrain grid
PVGIS does let users upload their own horizon file, so a skyline measured on site can replace the built-in one.
Where the Chilterns National Landscape fits
The Chesham Neighbourhood Plan says the town is surrounded by the Chilterns National Landscape and Green Belt. On the government's planning data map, both Chesham points we modelled fall outside the Chilterns boundary, while the centre of Chartridge village, in the same HP5 postcode district, falls inside.
For shading, the designation changes nothing. For planning, it matters for flat roofs: a flat-roof array on a house on National Landscape land, or in a conservation area, needs an application for a prior approval determination before work starts. Pitched-roof panels on an unlisted house follow the same limits either side of the boundary: no more than 0.2 metres from the roof slope, and no higher than the highest part of the roof, excluding chimneys.
What a site survey should check in Chesham
- The direction and pitch of each roof slope, measured on site
- The skyline from the roof, especially to the east and south-east
- Trees on your land and next door, and how much taller they may grow
- Chimneys, dormers and nearby buildings that could shade part of the array
- Whether the property is listed or inside the Chesham Conservation Area
- Whether the supply is single-phase or three-phase, which sets the G98 limit at 3.68kW or 11.04kW
What the output is worth
A 4.5kWp system, the typical domestic size given by the Energy Saving Trust, on a south-facing 35-degree roof in central Chesham would make about 4,530kWh a year. At the Ofgem price cap average unit rate of 26.32p per kWh for 1 October to 31 December 2026, using every unit at home would avoid about £1,192 of electricity. Exporting every unit at Outgoing Octopus's variable 12p per kWh would earn about £544, and Smart Export Guarantee payments require an MCS-certified installation.
Real results sit between the two. Our SEG tariff comparison covers the export side, and home battery storage is the usual way to use more of your own generation.
What to do next
- Note which way each roof slope faces and roughly how steep it is.
- Look east and south-east from an upstairs window: that is where the modelled skyline around central Chesham is highest.
- Ask every installer for a generation estimate for your roof, and whether it allows for terrain and on-site shading.
SOLA UK is an MCS-certified installer based in St Albans, fitting AIKO 485W all-black panels, SigEnergy battery systems and the Tesla Powerwall 3. Domestic solar panels and batteries carry 0% VAT until 31 March 2027, then 5%. When you are ready, call 0800 470 0922.
