Does your roof need to face south?

How much does direction change the output?

Compared with a roof facing due south at a typical pitch, roughly:

These are averages, so a clear south-west roof can beat a south roof that has a large tree beside it. It is why I look at the real roof rather than working from a rule of thumb.

What is different on the south coast

• Plenty of sun. The south coast is one of the sunniest parts of the UK. As a rough guide, a well-placed south-facing roof here can generate somewhere around 1,000 to 1,100 kWh per kW of panels each year. Published figures vary, so treat this as a ballpark. Your own roof can be modelled properly once I know the address.
• Terraced and semi-detached homes. Many have a roof running along the street, which often means one slope faces east and the other west. Panels on both slopes can add up to more capacity than a single south slope.
• Shade matters more than most people expect. Trees, chimneys, dormers and neighbouring buildings in built-up streets can cost more output than turning from south to west.
• Wind. Roofs near the coast and on higher ground are more exposed. The mounting system and fixings need to suit your roof and how windy the location is.
• Salt air. Close to the sea, salt is harder on metal. Corrosion-resistant mounting parts are worth having, and it is sensible to have the array checked from time to time.
• Gulls and pigeons. Birds like to nest under panels, and the droppings and mess can affect output and add maintenance. Bird-proofing mesh is common on the south coast, so it is worth budgeting for.
• Older streets. If your home is listed or in a conservation area, there can be extra rules about panels, so check before you commit.
• The grid connection. Your local network operator, which in much of Hampshire is SSEN, needs to be told about your system, and larger systems can need approval first.

Why output is not the whole story

What matters most is how much of your solar power you use yourself. Each unit you use saves you the price you would pay to buy it, which has been around 26p per unit under the recent price cap. A unit you export earns far less, often somewhere between about 3p and 15p depending on your tariff (see my guide to the Smart Export Guarantee).

A south-facing roof produces a big burst around midday, when many households are out. An east-west split produces a flatter curve, with power in the morning and again in the afternoon. If you are out all day, that can mean more of your solar is used at home. If you are in during the day, a south-facing roof can do just as well or better. A battery changes the picture again, as it stores the midday surplus for the evening.

When south is best, and when it is not

South tends to win when: you are home in the middle of the day, you have a battery, or you have one large south-facing slope and little else.

East-west tends to win when: you are out most of the day and have no battery, your roof only has east and west slopes, or you can fit more panels across two slopes than one.

Often the difference is small. That is a good reason to design around your roof and your habits, not the compass.

What I check on your roof

• Direction and pitch, slope by slope.
• Shade across the year, including trees and neighbouring buildings.
• The condition and structure of the roof.
• How windy and exposed the location is, and how close to the sea.
• Bird-proofing needs.
• How and when you use electricity.