Modelling for Solar Sizing

Solar sizing starts with the roof, not with the electricity bill. A model estimates how much sunlight reaches each roof plane over a year, given its orientation, pitch, location and shading from trees, chimneys, neighbouring buildings and roof features. That solar resource, combined with panel efficiency and system losses from inverters, wiring and soiling, gives an estimated annual generation for a given array size. Only then does household electricity use enter the picture, to estimate how much generation will be used at home versus exported.

That order matters because generation and use follow different daily and seasonal patterns. Solar generation peaks at midday in summer, while household use often peaks in mornings and evenings and, for heating, in winter. A model that matches annual generation to annual consumption without considering timing will overstate self-consumption and understate export, and it will flatter the economics. Good sizing models therefore consider timing, whether through hourly profiles or through a self-consumption estimate that reflects when the household actually uses electricity.

Roof constraints shape sizing more than any other factor. Usable roof area, after allowing for edges, access, vents and shading, sets a maximum array size, and orientation and pitch set how productive each panel will be. A south-facing roof at a moderate pitch in a sunny location generates more per panel than a north-facing or heavily shaded roof, and a model should show generation per panel by roof plane, not merely a total for the whole array. Where a roof is split across orientations, each plane should be modelled separately.

Shading deserves particular attention, because even partial shading can disproportionately reduce output, depending on panel and inverter design. A model that ignores a chimney shadow in winter, or a tree that leafs in summer, will overestimate generation. A shading analysis, ideally based on a site survey rather than satellite imagery alone, should be part of any sizing proposal, and its assumptions should be stated.

Before accepting a proposed size, ask for the roof survey, the shading analysis, the generation estimate by roof plane, and the usage profile used for self-consumption. Check whether the usage profile reflects the household's actual pattern, including working hours, electric heating or cooking, and any electric vehicle charging, and whether future changes, such as a heat pump or an electric vehicle, are included or excluded. A sizing that assumes daytime use that does not exist will overstate self-consumption.

Also ask about system losses and degradation. Inverters, wiring, temperature effects and soiling reduce output below panel nameplate figures, and panels degrade slowly over time. A generation estimate that quotes nameplate capacity without losses, or that assumes no degradation, is optimistic. A realistic estimate states its loss assumptions and its first-year and later-year generation.

By place, solar sizing conventions and economics differ. In the UK, MCS-based design and installer certification set expectations for domestic solar, while export tariffs and grid connection rules shape the economics. In EU member states, national schemes, net-metering or feed-in arrangements, and grid rules vary widely, and a sizing that is economic under one country's export terms may not be economic under another's. Local rules, not general solar principles, determine the financial case.

Roof and planning constraints also vary. Listed buildings, conservation areas and multi-unit blocks may restrict panel placement, and those constraints should be checked before sizing effort is spent on an array that cannot be installed as modelled.

A common mistake is to size an array to match annual consumption exactly, without considering timing, and then to be disappointed by export volumes and self-consumption. Another mistake is to maximise array size regardless of roof suitability, placing panels on shaded or poorly oriented planes that contribute little generation for their cost.

A further mistake is to ignore the interaction with other systems. A heat pump, a battery or an electric vehicle changes the usage profile and the value of solar generation, and sizing solar in isolation from those plans can lead to an array that is too large or too small for the home as it will be, rather than as it is.

Keep the roof survey, shading analysis, generation estimate, usage profile and installer proposal together, with dates. That file supports future battery decisions, tariff comparisons and any query about whether generation matches the estimate.

Questions to ask include: what is generation by roof plane, what shading was assumed, and what usage pattern drives the self-consumption estimate. Those questions reveal whether a solar sizing is grounded in the roof and the household, or merely in annual totals.