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Set the place, pick the day, drag the time. The sun is placed from the real astronomy, using the same sun-position maths as Installa’s 3D designer, and the shadows fall from it onto a stand-in house. The shade figure is this page’s own estimate from the shapes in the scene, not the designer’s.
The shade figure is estimated here from the tree, chimney and neighbour you can see. Installa’s designer does not work shade out this way: for a customer’s roof it takes shading from modelled irradiance data for the actual roof (Google Solar), where that data is available, and uses it in the MCS yield.
Sunrise – · solar noon – · sunset –
Climate band for the year’s total: –
Drag the dial or type a bearing.
Array: 5.28 kWp.
Drag the 3D view to orbit and scroll to zoom. Times are the longitude’s standard time unless you switch summer time on. The year’s figure uses the climate band’s clearness; the shade figure comes from casting a ray from every panel to the sun every half hour on the fifteenth of each month. A panel facing away from the sun is not counted as in shade.
Its height and bearing come from the standard astronomical formulas (the ones in the SunCalc library) for the latitude, longitude, day and time you set. Sunrise, sunset and solar noon are from the same source. Installa’s designer places the sun over a customer’s roof with the same formulas.
A light at the sun’s position casts the shadows of the house, the chimney, the tree and the neighbour onto the roof and the ground. For the power figure, a ray is cast from the centre of every panel towards the sun; a panel whose ray hits something loses its direct light and keeps only the diffuse light from the sky. Installa’s designer does not deduct shade this way; for a real roof it takes shading from modelled irradiance data (Google Solar).
For each moment: the angle between the sun and the panels, a 22% diffuse share, an air-mass factor for low sun, and 15% off for the inverter and wiring. The day’s kWh is the area under the curve. The year is 365 of those days multiplied by a clearness factor for the climate (0.47 for Britain), and the shade loss is estimated every half hour on the fifteenth of each month.
The diagram is the sky seen from above: the edge of the circle is the horizon, the centre is straight overhead, and the rings are 30° and 60° up. The sun rises on the east side and travels along the arc to the west through the day; the hour marks show where it is at six, nine, noon, three and six. The dark diamond is the direction the panels face: the closer the arc passes to it, the harder the sun hits them.
Try this: London, 21 December, and drag through the afternoon with the tree on. Then turn the panels east and compare the morning with the afternoon. Then move the house to Madrid and do it all again.
The designer builds the house and its surroundings from imagery and a height model, lays the panels on the real roof planes, and shows a year of modelled sunlight on every surface. See the 3D designer.
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