How to estimate floating solar capacity from reservoir surface area
You've got a reservoir on the shortlist and someone's asking for a capacity number before you commission a bathymetric survey or call the water authority. This is the back-of-envelope math that gets you there, and the caveats that keep it honest.
The quick formula
Floating-PV density runs roughly 0.4 to 0.6 MW per hectare of panel coverage, depending on module efficiency and mounting pitch. That's the number most developers reach for first, but it applies to the area covered by floats, not the full water surface.
Reservoirs rarely get covered edge to edge. Owners want clearance around intake towers, aeration lines, and spillways, plus a setback from the bank for maintenance boats. A 30 to 70 percent coverage ratio is typical, with the lower end common on drinking-water reservoirs and the higher end on quarry lakes or settling ponds with fewer competing uses.
So the full calculation is:
Surface area (ha) × coverage ratio × MW/ha density = estimated capacity
A 40-hectare reservoir at 50 percent coverage and 0.5 MW/ha works out to 10 MW. Run the same reservoir at 70 percent coverage, which a quarry pond with no potable-water function might allow, and you're closer to 14 MW. That spread is why a single "MW per hectare" figure from a vendor brochure sets a starting point for diligence, well short of a number you'd quote a client.
Where the per-hectare number breaks down
The density range above assumes a reasonably regular shoreline and a water level that doesn't swing far between wet and dry season. Neither assumption holds everywhere.
A reservoir with a steep seasonal drawdown, the kind that drops a meter or more between spring and late summer, forces anchoring and mooring design that eats into usable area and sometimes rules out sections of the footprint entirely. Shade from a tree line on the south bank cuts producible hours even where the float count looks fine on paper. And a reservoir with no graded shoreline access, just rock riprap or a steep bank, adds cost and limits where you can stage construction, which tends to push the actual buildable footprint down from the gross surface area you measured off a map.
None of that shows up in a hectare count. It shows up in water-level records, shoreline grading, and shade studies, the stuff that usually means pulling data one reservoir at a time: a level gauge record from the utility, a site walk for shoreline condition, a shadow study run by hand for whichever trees are tall enough to matter.
That's the gap between a capacity estimate that works for a pitch deck and one you'd take to a developer's investment committee. The formula tells you the ceiling. Level stability, shading, and shoreline access tell you how much of that ceiling you can build to, and which reservoirs on a long list aren't worth the site visit at all.
A faster way to rank the list
If you're screening a dozen reservoirs, quarry lakes, or settling ponds and need to know which ones clear the basic bar before spending survey budget on any of them, Floating Solar Screening turns satellite imagery into a per-water-body candidate map covering exactly those four factors (surface area, level stability, shading, and shoreline access) on an annual cadence, so you're not requesting gauge data and walking shorelines for every candidate on the list just to find out half of them don't clear the bar.
Once you've got the two or three reservoirs that hold up, the MW/ha math above gets you a defensible number to put in front of whoever's approving the next phase of diligence.