Quick answer
Start with the required road lighting and nightly load. Size usable battery energy for the agreed reserve, then size PV against the worst relevant month and the required recharge time. Verify battery charging limits, controller compatibility, wind loads and maintenance access. An off-grid design needs a complete energy balance, not a fixed LED-to-panel wattage ratio.
1. Define the site and lighting duty
- Road class, width, surface, mounting height, spacing and an IES/LDT file for the actual luminaire.
- Operating hours and measured power at every dimming step.
- Location, shading, PV orientation and seasonal plane-of-array solar resource.
- Required continuity, reserve days and an acceptable low-energy operating strategy.
- Temperature, wind, corrosion, access and the cost of replacing components.
Choose the applicable road criteria with the project designer. A rural or municipal label alone does not define a universal lux target. DIALux can test the actual geometry, distribution and arrangement.
2. Calculate nightly energy
This teaching example assumes a measured LED load of 50 W for five hours and 15 W for seven hours. Brightness percentages should not be treated as electrical power percentages unless verified.
| Load step | Power | Hours | Energy |
|---|---|---|---|
| Full output | 50 W | 5 h | 250 Wh |
| Reduced output | 15 W | 7 h | 105 Wh |
| LED total | — | 12 h | 355 Wh |
Add controller standby, communication, heating and driver losses if outside the stated measurement boundary. A constant 50 W for twelve hours would correctly total 600 Wh; use the operating profile that is actually required.
3. Size nominal battery energy
For two nights, 80% usable depth of discharge and 90% battery-to-load efficiency, nominal energy = 355 × 2 ÷ (0.80 × 0.90) ≈ 986 Wh. At 12.8 V this is about 77 Ah before temperature, ageing and additional-load allowances. Nominal Wh, usable Wh and energy reaching the LED are different quantities.
These assumptions are not a MCL product performance promise. Use the actual battery datasheet and validated operating conditions. Reserve days depend on weather sequence, failure consequences and service access, not a universal two-, five- or seven-day number.
4. Size PV and recovery separately
With 3.5 equivalent peak sun hours in the chosen month and an illustrative PV-to-load factor of 0.75, daily balance requires 355 ÷ (3.5 × 0.75) ≈ 135 Wp. The factor must identify the included losses; do not count the same loss twice.
Recovering two consumed daily reserves over the next three suitable solar days adds 355 × 2 ÷ 3 ≈ 237 Wh/day. Daily demand plus recovery is about 592 Wh/day, giving about 226 Wp under the same simplified assumptions. This assumes sufficient solar resource on each recovery day; weather, charge-current limits and BMS permission still need checking.
5. Set charging for the actual battery chemistry
Confirm charge and discharge temperature limits separately and measure cell temperature where required. A lithium profile must follow the battery manufacturer. Lead-acid voltage temperature compensation should not be applied to LiFePO4 unless that manufacturer explicitly permits it. Lithium temperature protection or heating is a different function.
For example, Victron Lithium Smart specifies its own charger settings and temperature conditions. It illustrates why settings are model-specific; it is not the datasheet for MCL batteries. Check low-temperature charge blocking, restart conditions and every charging source.
6. Complete the electrical and structural design
- Check cold PV open-circuit voltage, input current and operating voltage against the controller.
- Specify cable lengths, voltage drop, protection, connectors and enclosure access.
- Calculate pole, panel, brackets, anchors and foundation using site loads and soil information.
- Choose corrosion protection and drainage for the exposure; component IP ratings do not describe every assembly.
- Arrange spares, safe access and a local operating mode if remote communication fails.
7. FAQ
Should a sunny desert site always use a small panel?
No. Solar resource alone does not establish adequacy; load, module temperature, dirt, battery recovery and seasonal conditions also matter.
Does off-grid mean no electrical safety work?
No. PV, battery and DC wiring still require appropriate protection, installation and maintenance procedures.
What should be submitted before ordering?
The load table, energy and recovery calculation, component datasheets, lighting study, structural design and agreed acceptance and warranty documents.
Request a project-specific proposal
Send MCL Solar the location, road layout, mounting height and spacing, quantity, lighting requirement, nightly profile and reserve target. Include environmental exposure, delivery scope and the BOQ or tender specification. Request the ordered configuration’s datasheets, calculations and written warranty terms.
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