East and west-facing roofs in Frodsham

The compass on a Frodsham roof

Due south is the best direction for panels in Frodsham, but it is far from the only workable one. PVGIS figures for the town, run for this guide across eight directions and seven pitches with 14% system losses, put a south-facing roof at 35° at 941 kWh per kWp a year. The table shows every direction at that pitch, with its yearly figure as a share of the south one.

East and west sit close together at about four-fifths of the south figure: 744 kWh per kWp for east and 753 for west. The two north-facing quarters fall to roughly three-fifths, and due north to around 55%. South-east gives up only about 6% and south-west about 5%, which is why a roof a little off south is rarely a problem.

In the centre, the medieval street plan has already made part of this choice. In the 2006 conservation area appraisal, the line of High Street and Main Street is the old coaching route from south-west to north-east, so a range built along it tends to have one slope looking roughly south-east and the other north-west: one side near the top of the table, the other well down it. Houses on side streets and later estates face every which way, which is why the bearing of a particular roof is worth measuring rather than guessing.

Where east and west lose ground

Almost all of the gap between an east or west roof and a south one falls in winter. In June, an east roof at 35° makes about 94% of what the south-facing one does; in December, only about 40%. West follows the same pattern, at 95% and 42%. The sun’s path explains it. In summer it rises well north of east and sets well north of west, so it sweeps across both sides of a pitched roof during a long day. In winter it rises south of east, sets south of west and stays low throughout, spending nearly all its time in the southern sky that only a south roof faces squarely.

So an east or west system concentrates its output in the long days even more than a south one does, and its case rests mainly on spring and summer. Export and self-use follows that monthly pattern through a household’s year.

The hill treats east and west differently

For homes at the foot of the hill south of the centre, the terrain model holds a surprise. A west-facing roof at that point makes 753 kWh per kWp a year, against 753 at the hilltop point where nothing interrupts the sky: the hill takes about 0.03%, which is nothing worth measuring. An east-facing roof at the same spot loses about 4.1% of its year and about 41% of its December. South-east fares worse again, while south-west gives up only a little.

The figures suggest that, seen from the hill-foot point, the ridge stands higher across the morning sky than across the evening one, so it is the low sun of a winter morning that it catches. For a house tucked under the ridge with a choice of slopes, the west one is the safer pick, and a south-west slope keeps most of what a south one would make. The hill and winter sun explains how the horizon was modelled, and the hill check shows which modelled point is nearest a particular home. Up on the ridge itself, as Overton and the hillside shows, neither side loses anything to the terrain.

As everywhere on this site, the terrain model ignores buildings and trees, and its grid is coarse. A west roof facing a tall neighbour or a row of conifers can lose a great deal that the model will never show; trees and shade deals with that.

Morning and evening

Direction decides when the electricity arrives as well as how much. An east roof peaks in the morning and fades by mid-afternoon; a west roof starts slowly and carries on into the early evening. Many households draw the most power from late afternoon, when cooking, washing and lighting overlap, so a west-facing array often lines up better with the hours when the house is busiest. An east one suits homes where mornings are heavy, or where appliances can be set on timers to run early.

Panels on both slopes spread generation over the whole day, with a lower and broader peak. A split array sends less to the grid at midday and covers more of the house’s own use, and it leaves less surplus to store, which matters when weighing up a home battery.

Splitting an array across two slopes

Dividing panels evenly between an east and a west slope gives roughly 749 kWh per kWp a year at 35°, about 80% of a south roof. Because the two halves never peak at the same moment, the combined output never reaches what the full array could in theory produce, and firms sometimes choose an inverter rated below the total panel capacity on that basis. That choice bears on the grid paperwork too, because the threshold for a simple notification is set by the inverter’s output; grid connection explains the limit.

Splitting does add work. Each slope normally needs its own string, or its own input on the inverter, plus a cable route over the ridge or through the roof space. Ask the firm how it would wire a two-slope system and how its output estimate treats each side.

Pitch matters more on east and west

On a south roof, pitches up to about 40° do best over a year. On east and west roofs the opposite holds: the flatter the panel, the more it makes, because a flatter panel sees more open sky and less of the side the sun is not on.

At 10° an east roof makes 782 kWh per kWp and a west one 785; at 45° those fall to 717 and 727. A shallow roof facing east or west, such as a low extension or a gently pitched older roof, is therefore better placed than a steep one.

Facing north

North-facing panels are rarely the first choice, but at low pitches they are less hopeless than people assume: about 708 kWh per kWp a year at 10°, dropping to 515 at 35°. On a steep north slope the output is low enough that most households would spend on other positions first, including a south-facing frame in the garden.

Reading the figures for one house

These are modelled figures for the town from the JRC’s PVGIS tool. A survey adds the real pitch, the real compass bearing (few roofs face exactly east or west), the shade and the panel layout. Output on a Frodsham roof has the full monthly tables, and the question form takes questions about a particular roof.