What is cover ratio in floating solar design?

Cover ratio shows up in almost every early floating PV conversation, usually right after someone asks "how many megawatts could we fit here." It's the percentage of a water body's surface area that gets occupied by panels, floats, and the walkways between array blocks. A 10-hectare reservoir with a 30 percent cover ratio has roughly 3 hectares of water under PV, with the rest left open.

That's the easy part. Setting the actual number for a given reservoir is where most of the work happens, and it's rarely a single input you plug into a formula.

Why cover ratio isn't a fixed number

Three factors routinely cut the cover ratio below the array layout's theoretical maximum.

Evaporation and ecology rules come first. Utilities that use a reservoir for irrigation storage or municipal supply often want enough open water left for mixing, aeration, and fish passage. Some state and local permitting bodies cap floating arrays at a set percentage of surface area for exactly this reason, and that cap can be well below what the panel layout alone would support.

Water-level swing matters just as much. A reservoir that draws down several meters between wet and dry season needs anchoring and mooring slack built into the design, and that affects how tight the array blocks can be packed near the shoreline. A reservoir that holds a stable pool year-round can run array blocks closer to the bank.

Shading and wind loading set the third constraint. Treelines, dam faces, and inlet structures throw shadow across part of the water at different times of day, and that part of the surface usually gets excluded from the usable footprint even if nothing else about the site changes.

How developers use the number

Once a cover ratio is set for a site, it becomes the multiplier that turns surface area into a rough capacity estimate. Multiply usable water-body area by cover ratio, then by the output you'd expect per hectare from the panel and float configuration being used, and you get a first-pass megawatt figure. That number drives whether a reservoir is worth a site visit at all.

The catch is that developers often don't know the usable surface area until someone has already checked level stability, shoreline access, and shading for that specific water body. That's a few hours of digging through water-level records, aerial imagery, and permitting notes per reservoir, and most portfolios have dozens of candidates to get through before a single one earns a site visit.

Our take, at floatingsolarscreening.com, is that this ranking step belongs earlier in the process than it usually happens. Screening every reservoir, quarry lake, and settling pond in a portfolio against surface area, level stability, shading, and shoreline access from satellite imagery gets you a ranked candidate map before anyone pulls a single water-level record by hand.

What drives cover ratio down in practice

A few site conditions tend to show up again and again once a candidate list gets past the first screening pass:

• Reservoirs with seasonal drawdown greater than a couple of meters, which push mooring and anchoring design toward more conservative spacing
• Water bodies used for drinking water supply, where regulators frequently set a hard cap on surface coverage regardless of what the layout could support
• Narrow or irregularly shaped reservoirs, where shading from the shoreline eats into usable area disproportionately compared to a round or square water body of the same size
• Sites with existing aeration or irrigation infrastructure, where equipment clearances carve out exclusion zones inside the floatable area

None of these rule out a reservoir outright. They just mean the real cover ratio for that site will land lower than a back-of-envelope number based on surface area alone, which is exactly the kind of detail that's cheap to check against satellite imagery and expensive to find out after a developer has already committed a site visit to the wrong reservoir.

If you're ranking a portfolio of reservoirs and want the surface area, stability, and shading picture before you spend a week on records requests, that's the problem this site exists to solve.