Can an 18-Unit Cassette Aircon Office Run Entirely on Rooftop Solar?

Start with the cooling load, not the floor area. Eighteen 5-ton cassette units add up to 90 tons of cooling capacity. As a planning rule of thumb, commercial cassette units draw somewhere around 0.8 to 1.2 kW of electricity for every ton of cooling, depending on whether the units are inverter-type or fixed-speed, so the full bank of 18 units could pull anywhere from roughly 70 kW to over 100 kW if every unit ran at full output at the same moment. In practice, aircon units cycle on and off to hold a setpoint, so not all 18 run at 100 percent simultaneously; applying a typical diversity (coincidence) factor of around 0.7 brings the realistic peak draw down to an estimated 65 to 70 kW. On top of that, a 3-storey, 1,200 sqm office building (400 sqm per floor) carries lighting, plug loads, and elevators, which for a general office in the Philippines typically adds another 15 to 25 watts per square meter, or roughly 18 to 30 kW. Added together, a realistic business-hours peak demand for this building lands in the neighborhood of 85 to 100 kW. That number, not the building's floor area, is the real target a solar-plus-battery system has to be sized against, and it is the first thing any solar proposal for this building should state clearly before quoting panel counts.

A 3-storey building with 400 sqm per floor does not have 1,200 sqm of usable roof. Unless the design steps back at each level, the rooftop is limited to roughly the footprint of a single floor, so the solar array has about 400 sqm to work with, not the building's total floor area. Usable roof space is smaller still once access ways, water tanks, exhaust vents, and fire-code clearances are subtracted; a commonly used planning assumption is that 70 to 80 percent of a flat commercial roof is actually available for panels, which brings this building's usable area down to roughly 280 to 320 sqm. Commercial rooftop solar installations typically need about 6 to 7 sqm of roof per kWp of installed capacity once panel spacing, tilt, and walkway clearances are accounted for. Dividing 280 to 320 usable sqm by that figure gives a realistic ceiling of approximately 40 to 50 kWp of solar that can physically be mounted on this roof. That ceiling matters because it is well below the 85 to 100 kW peak demand calculated from the aircon and office load, which is the core reason a building with this footprint and this aircon load cannot be powered entirely by its own rooftop, regardless of how efficient the panels are.

The Philippines averages roughly 4.5 to 5 peak sun hours per day, meaning a solar array produces at its rated output for the equivalent of about 4.5 to 5 hours daily, even though the sun is up for 10 to 12 hours. After accounting for system losses such as inverter efficiency, wiring, soiling, and temperature derating (commonly estimated at 15 to 20 percent for a well-designed system), a 40 to 50 kWp array in the Philippines can be expected to generate roughly 150 to 190 kWh on an average clear day. Compare that to the building's own daytime consumption: at an estimated average running load of 50 to 60 kW across a typical 10-hour operating day (lower than the 85 to 100 kW peak because not everything runs at maximum continuously), the building could consume somewhere around 500 to 600 kWh on a typical weekday. Set side by side, the rooftop array's output covers roughly 25 to 35 percent of that daily consumption. This is a meaningful and financially worthwhile offset under net metering, but it is not close to covering the building's full daytime electricity needs, let alone powering it through the evening hours when aircon usage can still be high but the sun has set.

Going fully off-grid would mean sizing a battery bank large enough to cover the gap between what the 40 to 50 kWp roof can generate and what an 85 to 100 kW peak, 500 to 600 kWh per day building actually uses, including on cloudy and rainy days when Philippine solar output can drop by 40 percent or more. At this building's scale, that battery capacity requirement runs into the hundreds of kWh, which is a very different capital outlay than the solar array itself and is rarely the first step recommended for a commercial building of this size. The more common and more cost-effective path in the Philippines is a grid-tied system under the net metering framework established by the Renewable Energy Act (RA 9513) and implemented through the local distribution utility: the rooftop array offsets daytime consumption directly, any excess generation is exported to the grid for bill credit, and the grid continues to supply the building during early morning ramp-up, cloudy stretches, and all nighttime load. For a building with this aircon-heavy load profile and this roof footprint, a grid-tied net-metered system sized to the 40 to 50 kWp roof ceiling is the realistic recommendation, with battery storage treated as an optional add-on for specific priority circuits rather than a requirement for full building coverage.

Three factors move the numbers in the previous sections. First, switching from fixed-speed to high-efficiency inverter-type cassette units can lower the kW-per-ton draw toward the lower end of the 0.8 to 1.2 kW/ton range used above, which directly reduces both the peak demand figure and the daily consumption estimate. Second, shifting non-essential heavy loads (pre-cooling, equipment testing, non-urgent charging) into midday hours when solar output peaks improves how much of the array's generation is actually used on-site rather than exported, which is usually the more valuable outcome under net metering. Third, if the building's design allows mounting panels on a canopy, carport, or adjacent structure rather than relying solely on the roof footprint, the 40 to 50 kWp ceiling calculated above can be raised, which is often the single biggest lever available once the roof itself is maxed out. None of these changes make a 90-ton, 1,200 sqm building fully self-sufficient on solar alone, but each one narrows the gap between what the roof can generate and what the building actually consumes, which is the number that ultimately determines the size of the electricity bill, not whether the system is labeled 'entirely solar.'