Flat Roof Solar Panels: South vs East-West, and How Many You Can Fit

Flat roof solar panels come with a freedom that pitched roofs do not give you: because the roof is level, you get to choose which way the panels face and how steeply they tilt. On a normal sloped roof you largely take what the building gives you. On a flat roof, orientation and angle are design decisions — and the most important one is whether to face the panels south or split them east–west.

When I added solar to my own flat roof I went east–west, with a home battery, and the deciding factor was not some clever tariff trick. It was simply how many panels I could fit. This guide explains why that is often the right call, and walks through the tilt, mounting, planning rules and money so you can make the decision for your own roof.

Do solar panels need to face south?

No — south is the best direction, but it is not the only viable one. In the UK the highest annual output comes from panels facing due south at a tilt of roughly 35–40 degrees, and that is the benchmark every other layout is measured against. Panels facing east or west typically generate around 15–20% less energy per panel over a year than the same panels facing south. So per panel, south wins.

The catch is that “per panel” is the wrong way to think about a flat roof. What actually matters is how much electricity the whole roof produces — and that depends on how many panels you can fit as much as which way each one points.

Why east–west can beat south on a flat roof: panel count

This is the heart of the decision. To make panels face south on a flat roof, you have to prop them up on tilt frames. Tilted rows then cast shadows on the row behind them, so you must leave a gap between rows to stop them shading each other in the low winter sun. Those gaps eat up roof space, which means fewer panels fit.

An east–west layout sidesteps this. The panels sit back-to-back in a shallow “A-frame” at a low tilt, one bank facing the morning sun and the other the afternoon sun. There is very little self-shading, so the rows pack tightly together. On the same roof you can often fit a good deal more panels east–west than south — frequently enough extra capacity to more than cancel out the lower per-panel yield.

That is the trade-off in a sentence: south gives you more output per panel; east–west gives you more panels. On a space-limited flat roof, more panels usually wins on total kilowatt-hours. If you are working out how many panels your home actually needs in the first place, our guide to how many solar panels you need is the place to start.

How the battery changes the maths

There is a second, well-known argument for east–west: it spreads generation across the day. A south array produces a single sharp peak around midday, when many homes are empty; an east–west array gives a flatter curve with a morning shoulder and an evening shoulder, which lines up better with when people are actually at home using electricity. More of what you generate gets used in the house rather than exported cheaply.

Here is the honest nuance, though: once you add a battery, that self-consumption advantage shrinks. A home battery stores a south array's midday surplus and releases it in the evening, doing much the same job that the east–west curve does naturally. So with storage in the mix, the “east–west matches my usage better” argument matters less — which is exactly why, for me, it came back to raw panel count and total generation. The battery handles when the energy is used; the panel count decides how much there is to use.

What is the best angle for flat-roof panels?

On a flat roof the tilt is usually much shallower than the textbook optimum. Ballasted flat-roof systems commonly sit at around 10–15 degrees rather than 35. A low tilt packs more panels in, and just as importantly it presents less area to the wind, so the mounting needs less weight to hold it down (more on that below).

The price of a very shallow angle is that rain does less to keep the panels clean — below roughly 10–15 degrees, dirt and pollen rinse off less readily, so panels can need an occasional clean. For most homes that is a minor maintenance point, not a reason to chase a steeper, lower-capacity layout. In short, the best direction and angle for solar panels on a flat roof is the combination that fills the roof with the most panels your structure and budget allow.

Mounting: ballasted, non-penetrating and the weight question

Most flat-roof solar uses a ballasted mounting system: the frame sits on the roof and is weighed down with blocks rather than being bolted through the surface. The big advantage is that nothing penetrates the waterproof membrane, so you do not create new leak points and you keep the roof covering intact. Extra ballast is concentrated at the edges and corners, where wind uplift is strongest.

The thing to check before you commit is weight. A ballasted array adds a meaningful load across the whole roof, so a competent installer will confirm the structure can carry it — ideally with a structural assessment — before anything goes up. This is routine, but it is not optional.

Planning rules for flat-roof solar

In England, solar panels on a house are normally permitted development, meaning no planning application is needed, provided they meet a few conditions. For a flat roof the key one is height: the panels and their frame must not stand more than 0.6 metres above the highest part of the roof (excluding the chimney). That allowance was raised from the old 0.2-metre limit in a late-2023 update, which is what made tilted flat-roof arrays workable under permitted development.

The usual exceptions apply: permitted development does not cover listed buildings or scheduled monuments, and on protected land such as a conservation area, National Park or Area of Outstanding Natural Beauty you may need prior approval from the council. Always confirm your own situation with the local planning authority before ordering. (You may read elsewhere that panels must sit a metre back from the roof edge — that is sensible practice for wind and ballast, but it is not actually a permitted-development rule.)

The money: VAT, SEG and exporting your surplus

Two financial points are worth knowing. First, installing solar panels — and a battery alongside them — currently attracts 0% VAT on homes in Great Britain, a relief that runs until 31 March 2027 before reverting to 5%. Battery storage was added to that zero rate in February 2024, so a solar-plus-battery install is fully covered today.

Second, you can be paid for the electricity you send back to the grid through the Smart Export Guarantee (SEG). Larger suppliers must offer an SEG tariff, they set their own rate (it only has to be above zero), and the best fixed rates have recently sat in the low-to-mid teens of pence per unit — though rates move around, so it pays to shop the tariffs. Claiming SEG requires an MCS-certified installation, which is another reason to use an accredited installer. Our guide to the best SEG export rates goes into who pays what, and the wider picture sits in our solar panel cost and savings guide.

So, south or east–west?

Use this rough rule of thumb. If your flat roof is tight on space — which most are — an east–west, low-tilt layout that packs in the maximum number of panels will usually give you the most total generation, and a battery smooths out the rest. If you have abundant roof space and no battery, a south-facing array can edge ahead on pure yield. For my own flat roof the space was the constraint, the battery was going in anyway, and east–west simply let me fit more panels — so that is where the numbers landed.