Sizing a solar street light is a short calculation with five inputs. Getting it wrong in either direction is expensive: undersize and the lamp dies during the rainy season; oversize and you have paid for storage that never discharges. Here is the method, in the order the numbers are needed.
Input 1 — Nightly energy load
Multiply the LED wattage by the hours it runs at that output. A 100W luminaire running six hours needs 600 watt-hours per night. If the fixture dims after midnight — say 100W for five hours then 50W for five more — sum the segments: 500Wh + 250Wh = 750Wh. Dimming profiles and PIR motion boost change this figure substantially, so define the runtime profile before sizing anything else.
Input 2 — Autonomy days
Autonomy is how many consecutive days the system must run with no meaningful charging. Standard Mast Lighting configurations cover 3 to 7 rainy days. Choose from the local climate: an equatorial site with brief afternoon storms sits at the low end; a monsoon or long-overcast-winter site needs the high end or beyond, which is customizable.
Input 3 — Depth of discharge
You cannot use a battery's full nameplate capacity without shortening its life. LiFePO4 tolerates deep cycling far better than lead-acid, and a design depth of discharge around 80% is a reasonable working assumption for lighting service.
Input 4 — System losses
Controller efficiency, wiring losses and temperature effects consume part of every stored watt-hour. A combined system efficiency of roughly 0.85 is a conservative planning figure.
Input 5 — Peak sun hours
Peak sun hours (PSH) is the number of hours per day equivalent to full-strength sunlight at the site. It varies by latitude and season, and the value that matters is the worst month, not the annual average. Sizing to an annual average guarantees a system that underperforms every winter.
The two formulas
Battery capacity (Wh) = nightly load × autonomy days ÷ (depth of discharge × system efficiency)
Battery capacity (Ah) = battery capacity in Wh ÷ system voltage (12V or 24V)
Panel power (W) = (nightly load ÷ system efficiency) ÷ peak sun hours, then apply a safety factor so the battery can also recover after an overcast stretch rather than merely break even each day.
Worked example
A 100W luminaire, six hours per night, four days of autonomy, 80% depth of discharge, 0.85 system efficiency, 4.5 peak sun hours, 24V system:
- Nightly load = 100W × 6h = 600Wh
- Battery = 600 × 4 ÷ (0.8 × 0.85) = ≈3,530Wh
- At 24V = 3,530 ÷ 24 = ≈147Ah
- Panel = (600 ÷ 0.85) ÷ 4.5 = ≈157W, with recovery headroom applied → ≈200W+
Our solar street light calculator runs exactly this calculation interactively, so you can test different runtime profiles and autonomy targets before requesting a quotation.
What the calculation does not tell you
Sizing tells you the system will stay charged. It does not tell you whether the road is adequately lit — that depends on wattage, optics, pole height and spacing, and is answered by a photometric calculation. Mast Lighting provides a free Dialux simulation alongside the sizing so both questions are settled before manufacture.
