How many solar panels does an office building need?

The wrong way to size office solar

The most common bad answer to “how many panels does my office need?” goes like this: divide your roof area in square metres by 5.5, and that’s roughly how many panels you can fit.

That answer tells you what your roof can physically hold. It tells you nothing useful about what your building actually needs to capture the economic value of solar PV. We’ve seen office buildings with 4,000 sqm of clear roof where the right system size is 280 kWp (about 520 panels), not the 700 kWp the roof would physically support — because the building’s electricity demand can’t economically absorb more than 280 kWp without exporting most of it to grid at low SEG tariffs.

The right way to size office solar starts with load.

The load-first sizing method

Every office building has a half-hourly load profile — a 48-data-point-per-day record of how much electricity the building draws from the grid. Your DNO and your supplier both hold this data, and either will release it on request (often within 48 hours for the supplier route).

Twelve months of half-hourly data tells you everything that matters for sizing:

Total annual consumption in kWh — sets the maximum economic system size
Daytime vs night-time split — daytime drives self-consumption ratio
Summer vs winter split — summer cooling load matches PV generation; winter dark months export to SEG
Weekend vs weekday split — Monday-Friday offices have weekend export; 7-day offices use generation directly
Peak demand timing — sets battery storage economics

For a typical Monday-to-Friday office with HVAC and IT baseload, the right system size is whatever delivers around 70-80% of annual kWh consumption. Below 70%, you’re under-using available roof. Above 80%, you start exporting more than you self-consume, and SEG tariffs (currently 5-12p/kWh) are usually lower than your grid retail rate (currently 25-40p/kWh).

The maths in practice

A worked example: a 4,500 sqm office in central Manchester, 380 employees, hybrid working pattern with ~70% Tuesday-Thursday occupancy.

Annual electricity consumption: 1.08 GWh (1,080,000 kWh)
Target coverage: 75% of annual demand = 810,000 kWh
UK irradiance yield: ~920 kWh per kWp installed per year
Required system size: 810,000 / 920 = 880 kWp
Panel count at 450W per panel: 1,955 panels
Roof area required at 5.5 panels per sqm: 355 sqm of usable roof

The building has 3,200 sqm of physical flat roof, of which around 60% (1,920 sqm) is usable after plant exclusions, edge zones, and walkways — comfortably enough for the 880 kWp design.

What about the remaining 1,565 sqm of usable but unused roof? It could host an additional 500 kWp pushing total system size to 1,380 kWp — but the load can’t absorb the extra generation. Most of it would export to SEG at 9-10p/kWh while the building’s avoided grid cost is 28p/kWh. The extra capex doesn’t pay back.

Why most sizing pitches get this wrong

Sales-led commercial solar pitches almost always sell to roof area, not to load. The reason is simple — bigger system, bigger commission, bigger headline number. The customer ends up with an oversized installation that takes 11 years to pay back instead of 6.

We model from half-hourly data on every proposal. If the optimal system is 280 kWp and you have roof for 700 kWp, we recommend 280 kWp — and explain why the additional 420 kWp would damage your project NPV.

When over-sizing makes sense

There are exceptions. If your building’s load is genuinely growing — recent EV charger installation, planned heat pump conversion, expansion of cooling load — sizing should anticipate future demand, not just current consumption. We typically allow for 15-25% headroom above 12-month historic demand where future growth is documented.

Solar carport over surplus parking can also justify capacity above current load when the carport revenue plus EV charging margin together return positive NPV — and the canopy itself improves tenant experience.

Battery storage shifts the calculation again. With a 200-400 kWh battery, summer over-generation can be stored and discharged on Monday morning peak, lifting effective self-consumption from 75% to 90%+ and justifying systems 20-30% larger than the unbatteried economic size.

The right next step

If you want a load-led system sizing for your office building, the input we need is twelve months of half-hourly meter data (any standard CSV from your DNO or supplier) plus a roof plan or current Google Earth screenshot. We’ll come back within 7 working days with three sizing scenarios — conservative, optimised, and ambitious — and the economics of each.

Request a free feasibility study