East and West Roofs on One String: The Hidden Solar Loss

One string means one current — and the weaker face sets the pace

A series string is a chain. Voltages add up along the chain, but the current has nowhere else to go: whatever flows through one panel flows through every panel in that string. Panels are current devices in the way that matters here — the more light landing on the cells, the more current they can produce. So when you put a brightly lit east slope and a shaded, low-angle west slope into the same chain, the string cannot run two currents at once. It settles somewhere in between, and both faces are pulled away from where they would each individually prefer to sit.

This is not a fault, a defect, or a sign of bad panels. It is simply how series wiring behaves. The industry word for it is mismatch: any time panels in one string are exposed to meaningfully different conditions — different orientation, different pitch, a chimney shadow, a dormer, a different panel model — the string operates at a compromise rather than the sum of its parts. Orientation mismatch is the most predictable kind, because it happens every single day, twice a day, by design.

What an 'operating point' actually is, and why trackers exist

Every solar array has a curve of voltage against current, and somewhere on that curve there is a single combination that yields the most power at that moment. Move the voltage up or down from there and power falls away. That sweet spot shifts constantly — with irradiance, with cell temperature, with cloud, with the sun's position through the day.

The inverter's job is to hunt for that point continuously. The function is called Maximum Power Point Tracking, or MPPT. A tracker sweeps the voltage, watches the power, and parks the array where output is highest, revising the decision every few seconds. It is quietly one of the cleverest things in your loft or on your side passage wall.

The catch is arithmetic. One tracker can find one best point. If two roofs with genuinely different light are wired into one tracker, the tracker finds the best available compromise — which is the best single point for the combined array, not the best point for either roof on its own. It is doing its job perfectly. It has simply been given an impossible brief.

Two inputs, two peaks: how the fix is actually wired

Most modern residential string inverters have more than one MPPT input. The fix is unglamorous and cheap in engineering terms: run the east panels as one string into MPPT 1, run the west panels as a separate string into MPPT 2. Same inverter, same meter, same consumer unit, same roof — two independent circuits that never electrically touch until the DC has been converted.

Now each tracker has one job and one roof. The east string runs at its own best point through the morning while the west string idles at its own lower, but honest, output. As the sun swings west through the afternoon, the roles reverse without either side dragging the other. The combined figure rises above where the single shared string would have sat, and the generation profile broadens the way an east–west layout is supposed to.

Where an inverter genuinely has only one usable tracker, or where the roof geometry is more complicated than two clean faces, module-level electronics are the usual alternative: per-panel optimisers or microinverters give each module its own operating point, at the cost of more components on the roof. Neither is automatically better. The right answer comes from the survey, the shade study and the actual panel count on each slope, not from a product preference.

When a shared string is genuinely fine

Not every multi-slope array needs splitting, and we are not in the business of manufacturing problems. A single string is perfectly reasonable when the panels all sit on one face with the same pitch and no meaningful shading, or when a second face differs only slightly — a small azimuth offset on the same broad orientation, for example.

There are also practical constraints that pull the other way. Each MPPT input has a voltage window it needs the string to sit inside, and a minimum sensible string length to get there. If you only have three panels for the west slope, a two-panel string may not reach the inverter's start-up voltage in winter, and forcing the split could cost you more than the mismatch would have. Sometimes the correct engineering answer is a different inverter, sometimes it is optimisers on the smaller slope, and sometimes it is deciding those three panels do not belong in the design at all.

This is also a reminder worth keeping in mind for the whole conversation: UK homes still generate useful solar power on overcast days. A roof that is 'only' facing east is not a dead roof, and diffuse light on a grey Tuesday is still real generation. The goal is not to chase perfect orientation. It is to stop good panels being held back by the wiring around them.

Follow-up questions people ask about east–west solar

Is an east–west array worth doing at all? Often, yes. It flattens the generation curve, producing earlier in the morning and later in the evening, which can line up better with how a household actually uses electricity than a narrow midday peak. The condition is that it is wired to behave like two arrays, because that is what it is.

Can I just add the west panels to my existing string later? That is exactly the situation this episode is about. Adding a different-orientation face to an existing string is the cheapest-looking option and the one most likely to disappoint. Check whether your current inverter has a spare MPPT input first.

Do optimisers make MPPT inputs irrelevant? No. Module-level electronics solve mismatch at the panel level and are genuinely useful on complex or shaded roofs, but they add components, cost and failure points. They are a tool, not a default. Will splitting the strings change my export or metering? No. The split happens on the DC side, before the inverter. Everything downstream — the AC connection, metering and any export arrangement — is unaffected by how the DC is grouped. Does hot weather help? Counter-intuitively, no. Solar panels generate more efficiently in cold, bright conditions than in hot ones, because cell voltage falls as temperature rises. A crisp, clear February morning can be a surprisingly good day for a solar roof. Can I tell from my app whether this is affecting me? Sometimes. If your inverter reports per-MPPT or per-string figures, compare the morning and afternoon shapes. If everything is reported as one lumped number, you are looking at a shared string and you will not see the mismatch directly — only the total that it has already reduced.

Video transcript

Mo: East face, west face, one string. Fine? RoboMo: No. One string means one current. The weaker face limits both.

Mo: So how much are we actually losing? RoboMo: Each face is held off its own best point. The shortfall is the gap between the shared point and both peaks.

Mo: And two inputs fixes that? RoboMo: Yes. Separate trackers let each roof find its own peak, independently, all day. Mo: East face, west face, one string. Fine? RoboMo: No. One string means one current. The weaker face limits both. / Mo: So how much are we actually losing? RoboMo: Each face is held off its own best point. The shortfall is the gap between the shared point and both peaks. / Mo: And two inputs fixes that? RoboMo: Yes. Separate trackers let each roof find its own peak, independently, all day.

Wrapping up

Two roofs are not one roof, and wiring them as though they were is a design decision with a daily cost. The physics is simple enough to hold in your head: one string, one current, one operating point — and a compromise that suits neither face. The fix is equally simple, and usually already sitting unused inside the inverter you were going to buy anyway. What matters is that someone looks at your actual slopes, your actual shading and your actual panel counts, and decides deliberately rather than by habit. Good solar is not about heroic components. It is about not leaving free output on the roof because the wiring took the shortest route.