Effective Projected Area (EPA) for Solar Street Lights
-- How to Calculate
Effective Projected Area (EPA) for Solar Street Lights: How to Calculate
Effective Projected Area (EPA) is the surface area a wind "sees" when it hits your lighting assembly. It is the first number on every pole order we quote, because EPA plus the local design wind speed decides whether the pole and foundation survive the first serious storm. Guessing it is how cheap projects fail: a pole that meets code at 90 mph can be under-specced at 150 mph with no visible difference on paper. This page gives the formulas, the components that feed the number, and the wind-speed ranges we design against.
The two formulas you need
For rectangular components — a solar panel, a fixture housing:
EPA = Length × Width × shape coefficient
For cylindrical components — the pole itself:
EPA = Diameter × Height × shape coefficient
The shape coefficient accounts for aerodynamics: a flat panel catches more wind per unit area than a round pole does. The values come from the wind-load provisions of your building code; in the U.S. that is normally ASCE 7 (the American Society of Civil Engineers' minimum design loads standard). Use the code's force coefficients, not a rule of thumb.
What adds to EPA on a solar street light
Three components drive the number, and the panel is the one people forget:
- Solar panel orientation. The tilt angle and azimuth of the panel change its exposed area. A panel tilted for optimal energy production presents a larger face to the wind than one lying flat — energy optimization and structural economy pull in opposite directions, and EPA is where you settle the trade.
- Fixture design. Larger housings and decorative elements add projected area. Ornate fixtures that photograph well are EPA you pay for twice: once in the fixture and once in the pole.
- Pole shape. A tapered pole presents less area to the wind than a straight pole of similar height, which is why tapered sections dominate commercial installations.
For an all-in-one unit, add the battery box to the list — on integrated designs it sits at the top of the pole under the panel and contributes its own projected area at the worst wind angle.
The wind-speed side of the equation
Commercial solar street lights must be designed to the wind speeds in local building codes, and the spread is wide. Common design criteria run from about 90 mph (40 m/s) for most inland areas up to 150 mph (67 m/s) for hurricane-prone coastal regions. Higher EPA values require stronger pole foundations and structural supports — the two numbers multiply into each other. A coastal project with an oversized panel mount is the classic over-limit combination.
Reducing EPA without reducing function
Where the structure needs help, four strategies work, in rough order of value:
- Aerodynamic panel mounting systems that reduce the panel's effective wind resistance.
- Compact LED fixtures with lower projected area — smaller housings, fewer decorative surfaces.
- Wind deflectors on pole tops to break up flow around the head assembly.
- Smaller panels paired with MPPT (Maximum Power Point Tracking) charge controllers , so the array occupies less area while still harvesting enough energy. This is the strategy we prefer on constrained sites: it attacks EPA at the source instead of thickening the steel downstream.
Wildlife-friendly and dark-sky-sensitive installations often push the same direction — reduced fixture sizes with directional lighting, low-profile panel mounting, and dark-sky-compliant optics — so the structural and environmental requirements usually reinforce each other rather than compete.
What this page does not cover
This page explains EPA and its inputs. It is not a substitute for a structural engineer's verification: for permitted projects, have the EPA values and wind-load assumptions checked against the local code before the foundation is poured. Our wind load calculator runs the arithmetic for the pole and head assembly, and our foundation design page covers what the resulting loads mean for the base.