Very common in the UK, because our houses rarely offer one large south-facing roof. Terraces, extensions, dormers, outbuildings and hipped roofs all mean panels end up split.
It is entirely normal. It has cost implications and it changes some of the equipment decisions.
Why it is often the right answer
More usable area If no single face is big enough, splitting is the only way to get the capacity.
Generation spread across the day An east face produces in the morning and a west face in the afternoon. The daily total is lower than the same panels all facing south, but the output is spread across more hours, which often matches household use better.
That second point is underrated. A household that uses electricity at breakfast and in the evening may get more value from an east-west split than from a south array that peaks at midday when nobody is home. Value is about self-consumption, not just total generation. See how much you will export.
What it costs
More than one array on one roof, for straightforward reasons:
- More mounting work, more separate runs of rail and flashing
- Longer cable runs, sometimes through more of the house
- Possibly more scaffolding, if the faces are on different elevations. This can be the largest single addition
- Panel-level electronics, often needed, see below
Ask each installer to break out what the second roof face adds. If one quote is much cheaper, check whether they have simply priced less scaffolding.
Why the equipment usually changes
Panels facing different directions produce different amounts at any given moment. On a single string that is the same problem as shading: the string gets limited by whichever panels are producing least.
The solutions:
- Separate strings, one per roof face, into an inverter with multiple inputs. Often the cleanest answer and usually the cheapest
- Optimisers, per panel
- Microinverters, per panel
Separate strings work well where each face has enough panels to make a string. Where a face has only two or three panels, panel-level electronics usually make more sense. See microinverters and optimisers.
Roof faces worth using and not
Worth using south, south-east, south-west, east, west. A west face in particular is more useful than its annual total suggests, because it generates into the early evening when demand rises.
Rarely worth it north-facing. The output is low enough that the mounting and cabling cost usually is not justified.
Check separately a garage or outbuilding roof. Often unshaded and unobstructed, but the cable run back to the house can be long and the cost of that run sometimes exceeds the value of the panels.
Practical points
- One system or two? Usually one system with multiple arrays, which keeps it to a single inverter, a single connection and a single certificate. Two separate systems is rarely a good idea.
- Planning. Permitted development conditions apply to each elevation. Panels on a roof slope facing a highway may be treated differently in a conservation area. See can I install panels.
- Monitoring. With panel-level electronics you can see each face separately, which is genuinely useful for spotting a problem.
- Make sure the generation estimate is per face and adds up, rather than one figure for the whole thing.