Understanding portfolio solar sizing
Why aggregate sizing can outperform individual buildings
Aggregating buildings pools their combined roof capacity against their combined usage, which often produces a better offset than any single building would show alone. A 3-building portfolio totaling 46,000 sq ft and about 801,000 kWh a year can support roughly 446.9 kW of roof-constrained capacity, covering about 80.1% of total usage, even though individual buildings in the mix might land well below that on their own. A low-usage, large-roof building like the ones in the warehouse sizing calculator can offset the shortfall from a high-usage, roof-constrained building elsewhere in the portfolio. The roof space vs. output estimator covers the system sizing angle of the same decision.
What this tool does and doesn't assume
This tool assumes each building gets its own rooftop system sized to its own roof, with the results simply added together for a portfolio total. It doesn't account for shared infrastructure like a single interconnection point, or behind-the-meter aggregation across sites, a real option in about 17 states through what's usually called aggregate or virtual net metering, which lets one system's generation offset usage at multiple meters owned by the same entity. The DSIRE database tracks which states currently allow this and under what conditions. For a single site's exact numbers, the system size by square footage tool gives the same underlying math for one building at a time.
Handling a portfolio larger than three buildings
Run the buildings in groups of three and add the resulting system sizes and usage totals together, or size your highest-usage buildings first since they typically drive most of the portfolio's total offset potential. The underlying math per building is the same one used in the single-building sizing calculators, including the office building sizing calculator for any multi-story properties in the mix, and the solar panel calculator in our system sizing section for translating a final total into an actual panel count.
Deciding which buildings to prioritize
Buildings with the best roof-to-usage ratio, like single-story warehouses or ag buildings, typically deliver the best return and are worth prioritizing first. Buildings with heavy usage and limited roof, like multi-story offices or restaurants, may make more sense to leave out of an initial phase and revisit with a parking canopy or off-site option later. Run each candidate building through the commercial solar payback calculator individually to rank them before committing to a phased rollout. The U.S. Energy Information Administration's commercial buildings energy survey is a useful benchmark for comparing each building's usage intensity against national averages by type, and the roof space vs. output estimator in our system sizing section covers the same roof-to-output tradeoff from a residential angle.