How the Solar Street Light Energy Calculator Works
How the Solar Street Light Energy Calculator Works
Every quotation conversation eventually reaches the same two questions: "Can this configuration last all night?" and "How big does the battery need to be?" The honest answer used to take hours. An engineer would build an energy balance in a spreadsheet, or wait for a PVsyst run, and the customer would wait with them. We built the solar lighting energy consumption calculator to answer both questions in about five minutes, with a 21-day simulation behind the answer instead of a single sunny-day estimate.
What the tool actually does
The calculator runs a continuous 21-day charge-discharge cycle simulation. It steps through day by day: generation from the panel, storage into the battery within its depth-of-discharge (DOD) limits, discharge to the luminaire at night, and the leftover state of charge carried into the next morning. A dynamic remnant state-of-charge algorithm tracks what is actually left after each cycle, so a cloudy Tuesday spends energy banked on a sunny Monday.
That is the difference between this and a single-day estimate. A one-day calculation tells you the system works on a good day. The 21-day run tells you whether it survives the bad sequence, which is the only question that matters in the field.
The inputs, and where to find them
Everything the calculator needs comes off a datasheet or a drawing:
| Parameter | Typical range | Source |
|---|---|---|
| Peak sun hours | 3–5 h in tropical/subtropical regions | Built-in map — click the project location |
| Panel power | 100–400 W | Product datasheet |
| Panel quantity | 1–4 pcs | Design drawings |
| Battery voltage | 12 / 24 / 48 V | Product datasheet |
| Battery capacity | 50–300 Ah | Product datasheet |
| Battery quantity | 1–4 pcs | Design drawings |
| DOD | 0.8–0.9 lithium; 0.5–0.7 lead-acid | Battery type |
| Luminaire power | 30–200 W | Product datasheet |
| Luminaire quantity | 1–2 pcs | Design drawings |
| Lighting hours | 10–12 h | Customer requirement |
| Dimming mode | Full brightness early night, dimmed at midnight, full before dawn | Customer requirement |
Peak sun hours is the parameter people get wrong most often, so the tool fetches it from a built-in map: click your project location and the local meteorological value loads automatically. No manual searching, no borrowing a number from a neighboring city.
The math behind the result
The model is deliberately simple, because a simple model you can explain to a client beats a complex one you cannot:
- Daily generation = panel power × quantity × peak sun hours × system derate. The derate covers controller, battery round-trip, and wiring losses; we use roughly 0.75–0.85 as a typical band, and your site's temperature and wiring run govern the exact figure.
- Nightly consumption = luminaire power × quantity × lighting hours, adjusted for the dimming profile. A profile that runs full output for four hours, 50% for six, and full again for one draws far less than flat full output for eleven.
- Usable battery energy = voltage × capacity × quantity × DOD. This is the number the simulation spends against, and it is why a lead-acid bank at DOD 0.6 needs roughly half again the nameplate capacity of a lithium bank at DOD 0.85 for the same nights of autonomy.
Why 21 days
Solar systems fail on sequences, not on averages. Three cloudy days in a row in the rainy season is the event that empties a marginal battery, and no single-day calculation will ever show it. A 21-day window is long enough to contain a realistic worst-case cloudy stretch for most climates, and short enough to run instantly. When the simulation ends green, the system has survived the sequence; when it ends red, you have found the failure before the customer did.
Reading the results
The 21-day table is color-coded:
- Green: the battery stays above its reserve through the whole window. The configuration carries the lighting requirement.
- Red or warning: the battery depletes within the window. The fix is usually one of three levers: more panel, more battery, or a more aggressive dimming profile. The calculator shows which one moves the result.
What the calculator does not model
Honesty about the boundaries is what makes the tool trustworthy:
- Temperature derating. Battery capacity and panel output both move with temperature. The simulation uses nominal values; cold winters and hot summers shift the real numbers.
- Soiling and shading. A dirty panel or a tree canopy is site-specific and not in the model. Shading studies are a separate exercise.
- Battery aging. Capacity fade over years is not simulated. Size with margin, or plan a mid-life pack replacement.
- Final engineering design. For formal submissions to governments or large owners, the calculator output is a screening result, not a stamped design.
When to use it, and when to call us
The calculator is the right tool for on-site preliminary assessment, quick verification of a proposed configuration, customer quotation support, and teaching demonstrations. It is the wrong tool for final engineering design; if the project requires formal submission, send it to our engineering team for full professional verification.
Start with the calculator, and if the configuration needs work, the battery sizing calculator and solar autonomy calculator narrow down which component to change. For complete systems sized with these same models, see our all-in-one solar street lights line.
What this page does not cover
This page explains how the energy consumption calculator works and where its boundaries lie. It does not cover photometric lighting design (lux levels and layout), structural or foundation design, or PVsyst-grade meteorological modeling — those are separate tools with separate disciplines. For lighting layout, start with our lighting design tool.