From Storage Tank to Solar Platform: The Maui Project

A County of Maui water utility transformed its elevated concrete storage reservoirs into platforms for solar generation, tackling one of the most stubborn energy problems for drinking-water systems: pumping represents roughly 80% of the electricity cost for municipal water processing and distribution, and energy often accounts for 25–30% of operation and maintenance expenses. With flat, sunlit land scarce on the islands, the utility looked upward—to the roofs of its own circular tanks.

Working with Johnson Controls as the program contractor, the utility mounted non-penetrating, ballasted photovoltaic arrays rated between 105 and 141 kW-DC on multiple reservoir roofs. The installations directly offset the electrical load of wells, booster stations, and treatment facilities at the same sites where the generation is located, putting unused infrastructure to work and lowering a major controllable operating cost.

The approach was not a one-off experiment but a repeatable method. By solving for structural weight limits, seismic loading from the water inside the tank, wind uplift, and the awkward fit of rectangular panels on a circular roof, the team proved that a drinking-water tank need not be sacrificed as a solar site. The arrays were ballasted rather than penetrated, and all heavy electrical equipment—inverters, disconnects, panelboards—was placed at grade, leaving the tank structure essentially untouched.

Engineering the Balance: How Tank-Mounted PV Overcomes Critical Constraints

Why the Circular Roof Is the Hardest Shape for Solar

Photovoltaic modules and racking are rectangular, but reservoir roofs are not. Fitting the array into the inner disc while preserving clear zones around hatches, vents, and the tank perimeter was the defining layout puzzle. The design satisfied International Fire Code access-pathway requirements, using a four-foot perimeter exception (IFC Section 1204.3.1) that applies when an axis of the structure is 250 feet or less—recovering roof space without compromising firefighter access. The resulting field reads as a dense rectangular block stepped inward at the edges, a shape dictated by the tank’s service needs, not aesthetic preference.

Advertisement

Where Wind and Earthquake Collide

Unlike a typical rooftop, a tank-mounted array faces a more severe dual demand. Tanks sit on high, open ground, subject to full wind loads, so ballast must resist uplift. But the same mass that holds panels down in a storm pushes laterally during an earthquake—and the tank already carries a heavy seismic demand from the water sloshing inside. The binding constraint is not the roof’s gravity capacity but how much added seismic load the ballast introduces. Engineers sized the ballast to meet wind uplift (per ASCE 7), then checked the seismic case, iterating until both were satisfied. Keeping the modules low and flat reduced wind drag, minimizing the ballast needed in the first place.

Protecting the Tank’s Primary Job: Water Quality and Access

A PV array lasts decades; the reservoir beneath it is expected to serve far longer. The team ensured that all access hatches, vents, and inspection points remained clear—a sanitary imperative for finished-water storage where open or damaged screens risk contamination. Corridors were preserved for future interior inspection and recoating, and conduit routing avoided low spots that could trap water against the structure. By placing inverters and panelboards on a freestanding rack at grade, the design kept the heavy electrical balance-of-system completely off the tank wall, with only conductors run down the side in appropriately anchored conduit.

A Template for Land-Constrained Utilities

Treating the tank not as an obstacle but as a generation asset is the core insight. The discipline is consistent: structural assessment first, a ballasted array that never penetrates the roof, wind and seismic solved together, layout optimized inside a circle, and the tank’s maintenance functions treated as non-negotiable. For water agencies short on flat land and long on pumping load—especially on islands or in dense urban corridors—the reservoir roof often represents the best solar real estate they already own. The Maui case demonstrates that the hurdles are engineering problems with proven solutions, not feasibility blockers.

What Other Water Utilities Can Take From Maui’s Approach

  • Start with a structural assessment that establishes the tank’s ballast capacity, factoring in the added seismic load from water sloshing. The roof’s gravity capacity is rarely the limit; the binding constraint is the earthquake case, so original drawings or field evaluation by a structural engineer is step one.
  • Work with a solar engineering team experienced in non-penetrating, ballasted systems. Wind uplift (ASCE 7) and seismic demand must be modeled together, not independently. Iterative reconciliation is required to keep the total ballast mass within the tank’s structural envelope.
  • Design the array layout for the usable inner disc, not the full circle. Maintain a minimum four-foot clear perimeter and four-foot clearance around each roof hatch, using the IFC Section 1204.3.1 exception when the tank axis is 250 feet or less to maximize generation while keeping firefighters’ access.
  • Keep inverters, disconnects, and panelboards on a freestanding rack at grade. This eliminates the weight and anchoring complexity of heavy equipment on the tank structure and allows straightforward conduit runs down the wall with anchors suited to the tank’s material.
  • Protect all existing access hatches, vents, and inspection corridors. Layout must treat these as first-order constraints, not afterthoughts, to avoid compromising sanitary protection and future tank maintenance.