East–west versus south-facing: the real numbers

The annual number, first

At the same 35° pitch, the same total capacity, across our 25 UK reference locations, an east or west-facing plane yields 79% of due south (range 77–81%).

That figure is remarkably stable — a four-point spread across ten degrees of latitude. So the annual penalty for going east–west is effectively a national constant, whatever else varies.

But that is not the comparison most people face

Nobody chooses between 4 kWp facing south and 4 kWp facing east–west on the same house. The real choice is usually: use one roof plane, or use two.

A typical semi-detached house with its ridge running north–south has an east plane and a west plane and no south plane at all. A house with the ridge running east–west has a south plane and a north plane. Very few have a large south plane and nothing else.

So the honest comparison is per house, not per kilowatt:

Six kWp split east–west beats four kWp facing south in every location. That is the argument for the split layout, and it is a good one — but notice what it actually rests on: more panels, not better geometry. Whether the extra capacity is worth its cost depends on price and on what you do with the output, not on orientation.

Hour by hour, in June

This is the claim worth testing properly. In-plane irradiance through an average June day in London, at 35°.

The south plane peaks at midday UTC; east peaks around 09:00 and west around 14:00. Averaging the two gives the curve a half-and-half array actually sees.

Read the bottom four rows carefully, because they are less flattering to the split array than the usual telling.

The broadening is real but modest. One extra hour above 40% of peak. At 50% it is a tie. Above 60% the south array wins, and above 75% it wins comfortably.

What the split array genuinely does is move energy from the middle of the day to the shoulders: earlier in the morning and later in the evening. What it does not do is hold a high output across a much wider window. The often-repeated image of a broad flat plateau against a narrow south-facing spike is not what the model shows.

Hour by hour, in December

Now the part almost nobody publishes.

The split array collects less than half as much. In December the sun rises late in the south-east, tracks low across the southern sky and sets early in the south-west. It spends the whole short day in the sector a south roof faces and an east or west roof largely does not.

So the annual 79% is not a uniform 79% month by month. It is roughly 94% in June falling to 49% in December — and the shortfall lands in exactly the months when UK household demand peaks and when every unit generated is much more likely to be used rather than exported.

Where east–west is straightforwardly better

Two places, and both are real.

Inverter sizing. The split array’s peak is around 17% lower in June, so a given Inverter can carry more panel capacity before it starts clipping. A higher DC:AC ratio is easier to justify on a split roof, and the inverter is often the second most expensive component.

Export limits. Where a DNO imposes Export limitation , the constraint is on instantaneous export in kW. A lower peak means less of the year spent bumping against the limit, so more of the installed capacity is usable.

Both are consequences of the lower peak, which is usually presented as a downside. Here it pays.

Bringing it back to money

Whether 79% of the yield is acceptable depends on what the units are worth, and that is set by Self-consumption rather than by orientation.

A unit you use is worth your import rate. A unit you export earns your SEG rate, and in Ofgem’s latest annual report the best untied rate was 12p while many were 4.1p or less. So the same kilowatt hour is worth roughly 26p or roughly 4p depending only on whether somebody was in to use it.

That ratio is why the shape of the day is worth arguing about at all — and why the December finding above matters more than the June one. Put your own figures into the rooftop calculator; it takes orientation and a self-consumption range and shows you the spread rather than picking a flattering point inside it.

A short summary you can act on

• No south roof? Use both east and west planes. The per-panel penalty is real and the alternative is generating nothing on half your roof.
• South roof large enough for the array you want? Use it. Adding east or west panels beyond that point buys output at 79% efficiency and full cost.
• South roof too small? Fill it, then extend onto east or west. That ordering is almost always right.
• Being sold east–west as superior? It is not, per panel, and the winter figures are the ones to ask about.
• Choosing an inverter for a split array? The lower peak is a genuine argument for a higher DC:AC ratio, and worth raising with your installer.