Off grid solar street light: autonomy is the specification
On a road with no mains, the fixture is the easy part. The system has to carry the load through the worst week of the year, on the sun the site actually gets, with a battery that survives the cycle.
Specification
What the sheet states for this build, and what is confirmed per project.
| Topic | What the sheet states |
|---|---|
| Load | Lamp and controls, with the nightly hours agreed |
| Autonomy | Days of poor weather the system must carry |
| Design month | The worst month at the site latitude |
| Panel | Rated output, angle and azimuth for the site |
| Battery | Chemistry and usable capacity at the depth of discharge |
| Controller | MPPT, with a programmed profile |
| Mount | All in one on the pole, or split with the panel separate |
| Certification | File per model and market |
| Data sheet | Current revision, naming the delivered model |
Autonomy is a client decision, not a product feature. Ask how many poor days the road can go dark, and size to that answer.
The arithmetic, stated plainly
Nightly energy is the load multiplied by the hours. Daily harvest is the panel output multiplied by the site's sun hours in the design month, less losses. The battery carries the shortfall across the autonomy days. If the harvest in the design month is below the nightly load, no battery size fixes the system for long.
Writing those four numbers into the order is the specification: load, hours, autonomy and design month. A supplier who will not state them is quoting a panel, not a system.
- The design month at the site latitude, not the annual average.
- The nightly hours, and whether dimming or sensing is acceptable.
- The autonomy the client accepts, in days.
All in one or split
An all in one unit puts panel, battery and head on one pole. It installs quickly and it looks tidy, and it puts the panel wherever the pole goes, which on a shaded verge is the wrong place. A split system costs more cable and installs where the sun is.
On a tree lined road the split is usually the honest choice, even though it is the more expensive quotation. On an open site the all in one is fine and cheaper.
- The shading at the proposed pole positions, at midday and in winter.
- Whether the panel can go on a separate mast or a roof.
- The cable route, if the system is split.
Year two, and what fails
These systems rarely fail at the lamp. They fail when the panel is dirty, the shading grew, the battery was cycled deeper than its rating, or the controller profile kept the lamp on longer than the budget allowed.
Ask for the controller settings in writing, and for the cleaning interval. Then put both in the handover document, because the person who maintains the road in year three was not in the meeting.
- Controller settings, documented and locked or recorded.
- A cleaning interval, with who does it.
- Battery cycle life at the depth of discharge actually used.
Before the order
- Design month and autonomy in the order, as numbers, not adjectives.
- Shading survey with a winter midday photograph.
- Controller profile documented for the maintenance team.
- Build
- Off grid solar street light
- Line
- Solar lights
- Ask for
- Specification sheet, photometric file, aiming drawing
Send the space, get the sheet.
Build, specification sheet and the option list for each line item, in one reply.