Solar Production: What Actually Happens vs What the Quote Simulated

A quoted solar generation figure is a simulation, and real-world output typically lands 5 to 15% below it in the first year, for reasons that are well understood and largely unavoidable, not a sign of a bad system. Knowing what drives that gap in advance sets realistic expectations before you sign, rather than causing concern once the system is running.

What does a generation simulation actually model?

A simulation combines your roof's orientation and tilt, Singapore's average solar irradiance data, and the panel and inverter specifications to project annual output. Sunnify's own estimate methodology, for example, applies a 30% system loss factor on top of raw irradiance and Singapore's 4.33 peak sun hours to arrive at a realistic, not optimistic, generation figure. See the full methodology, published in full for every assumption behind that figure.

What real-world factors pull actual output below the simulation?

Several factors compound together. Panel manufacturing tolerance means individual panels can perform slightly below their nameplate rating even when new. Temperature derating reduces output as panels run hotter than standard test conditions, which matters more in Singapore's climate than in cooler countries.

Minor shading from nearby structures or foliage that a simulation may not fully capture, and gradual soiling from dust and haze between cleanings, both reduce real output further. Wiring and inverter conversion losses account for a further small reduction on top of all of the above.

Why do some installers quote a more optimistic figure anyway?

A simulation built on a P50 confidence level, or one that skips a realistic system loss assumption, can produce a headline number that looks better in a sales conversation. This is exactly why comparing the assumptions behind two quotes, not just the final generation number, matters when evaluating installers.

Is this the same thing as the P50/P90 gap covered in Sunnify's estimate methodology?

A related but genuinely distinct concept, worth keeping separate rather than conflating. The P50/P90 gap, covered in full in the P50/P90 estimates guide, describes year-to-year weather variability around a single central estimate, roughly a 4 to 5% spread driven mainly by how sunny or cloudy a given year turns out to be. The simulation-versus-reality gap covered here instead describes the difference between a simulated baseline figure and what a system actually produces once real-world hardware tolerances, heat, shading, and losses are accounted for, a separate, additive effect on top of whichever weather year actually occurs.

Does Sunnify's own estimate methodology already build in this gap, or is it a separate adjustment?

Already built in, which is precisely the point of a realistic, rather than optimistic, simulation in the first place. Sunnify's estimates apply a 30% system loss factor specifically to account for the combined effect of these real-world factors, meaning a Sunnify-generated figure is already positioned close to what a system should realistically achieve, not a best-case number requiring a mental discount applied afterward. This is also why the savings figures behind Sunnify's own documented installations are consistently labelled as estimates or projections rather than guarantees, an honest reflection of real-world variability rather than an attempt to promise a precise number no system can actually guarantee in advance.

When is a generation gap actually worth investigating?

A first-year gap within roughly 5 to 15% of the simulated figure is normal and expected. A gap noticeably larger than that, or one that develops suddenly after the system has been running fine, points to something specific and fixable: new shading from growth or construction nearby, soiling buildup overdue for cleaning, or a genuine equipment issue worth raising with your installer under warranty.

Further reading: see solar payback period in Singapore for how realistic generation feeds into your return, and the maintenance and warranty guide for what to check if a gap looks abnormal. Run the Sunnify solar estimate to see a realistically modelled figure for your own roof.

Does SERIS's own Singapore-specific research back up this typical gap range?

Broadly, yes, and it is worth grounding the 5 to 15% figure in real research rather than treating it as an arbitrary industry rule of thumb. SERIS, based at NUS, conducts solar performance and degradation research specifically calibrated to Singapore's tropical field conditions, precisely because generic simulation assumptions built for temperate climates do not automatically transfer to Singapore's heat and humidity without adjustment. A simulation genuinely tuned to local conditions, rather than a generic global default, is what actually keeps the realistic gap in that 5 to 15% range rather than drifting meaningfully wider.