Understanding sizing by square footage
Getting from square footage to a system size
Multiply your floor area by a typical energy use intensity for your business type to estimate annual usage, then divide by your location's expected production per kW. A 25,000 sq ft, 2-story office building using about 15 kWh per sq ft a year would need roughly 261 kW to fully offset usage, though a typical roof footprint often can't fit a system that large. That gap between usage-based sizing and roof-based sizing is the most common surprise in early planning. Once you have a target size, the solar panel calculator in our system sizing section can translate it into an actual panel count.
Why the number of stories changes everything
Your roof footprint is your total floor area divided by the number of stories, since only the roof, not every floor, has room for panels. A 3-story building with 30,000 square feet of total floor area only has about 10,000 square feet of roof to work with, a major constraint a single square footage number misses. Run the warehouse sizing calculator instead if your building is a single large single-story footprint, since the math simplifies considerably. The Department of Energy's guide to going solar covers this same roof-versus-usage sizing logic at a more general level.
Picking the right energy use intensity
Energy use intensity varies widely by business type: offices commonly run 10 to 20 kWh per square foot a year, retail stores 12 to 18, restaurants 40 to 80 due to kitchen equipment, and warehouses as low as 5 to 10. ENERGY STAR's own national median energy use intensity reference table confirms figures broadly in this range, built from the same underlying CBECS survey data, with offices at roughly 15.5 kWh per square foot and restaurants running well over 90. Use your actual utility bills divided by square footage for the most accurate number instead of an industry average, since the built-in figures here are meant as a starting point, not a substitute for your real data. Check the retail storefront sizing tool or restaurant sizing tool for a version tuned to those specific business types.
When roof space falls short of full offset
A roof-constrained system still meaningfully offsets your usage, just not all of it. Many businesses add a carport or ground-mount array to close the gap if they have available parking or land, or simply accept a partial offset and keep the rest of their usage on the grid. Check the carport cost estimator if extra parking space could help close that gap, and the roof space vs. output estimator in our system sizing section for the same roof-to-output tradeoff from a residential angle. If demand charges are a big part of your bill, the backup runtime estimator in our Battery & Storage section can show whether a battery would shave your peak enough to pay for itself.