Solar Street Light Autonomy Days Calculation (Backup Nights) — Tender-Ready
This guide shows how to calculate solar street light autonomy (backup nights / autonomy days) using auditable inputs: Wh, dimming profile, nightly runtime, and usable battery energy. It also includes RFQ clauses to make supplier quotes comparable.
TL;DR (Use this formula)
- Nightly load energy (Wh/night) = Average power under dimming (Wavg) × Night runtime (hours)
- Usable battery energy (Whusable) = Battery nominal energy (Whnominal) × Usable factor
- Autonomy nights (days) = Whusable ÷ Wh/night
If a supplier cannot provide the energy budget sheet (Wavg, hours, Whusable assumptions), the autonomy claim is Not Comparable / High Risk.
Where This Guide Fits
- Start here: Buyer’s Guide Hub
- Related system pages: Split Type Solar Street Lights, All-in-One Solar Street Lights
- Evidence centers: Spec Definitions · IES/DIALux · Quality Evidence
Quick Jump
What Inputs You Need
Collect these from the supplier (or define them in your RFQ):
| Input | Example format | Why it matters |
|---|---|---|
| Dimming profile | Time steps or PIR behavior | Autonomy depends on average power, not max watt |
| Average power (Wavg) | “Base 30% + boosts” → computed Wavg | Converts schedule into energy |
| Night runtime (h) | e.g., 11.5 h | Changes Wh/night directly |
| Battery nominal Wh | Wh (not Ah only) | Correct energy unit for autonomy |
| Usable factor | e.g., 0.70–0.85 (declared) | Converts nominal to usable Wh |
| Environment note | cold / hot / rainy season | Impacts usable factor & charging |
Step-by-Step Autonomy Calculation
Step 1) Define the dimming profile
A dimming profile must be described in one of these ways:
- Time-based steps (e.g., 100% for 2 hours, 60% for 4 hours, 30% for 6 hours)
- PIR-based logic (base brightness + boost duration + expected triggers)
If the profile is not declared, autonomy claims are not auditable.
Step 2) Calculate average power
For time steps, use:
Wavg = (W1×h1 + W2×h2 + … ) ÷ total hours
For PIR logic, require the supplier to disclose:
- Base power level and duration
- Boost power level and typical boost minutes per night (assumption must be stated)
Step 3) Calculate nightly load energy
Wh/night = Wavg × runtime hours
Step 4) Calculate usable battery energy
Whusable = Whnominal × usable factor
The usable factor must be declared. It accounts for protection margins, temperature effects, and reliability strategy (conservative vs aggressive).
Step 5) Calculate autonomy nights
Autonomy = Whusable ÷ Wh/night
How to Choose a Usable Factor
Use this rule: the supplier must declare the usable factor and explain what it includes.
A procurement-grade battery statement should include:
- Battery chemistry (e.g., LiFePO4)
- Nominal Wh
- The usable factor assumption (single number used in calculation)
- BMS protections list (at minimum)
If the usable factor is not declared, treat autonomy as “Marketing claim”.
Recharging Check
Autonomy alone is not enough. A roadway system must also recharge reliably.
Ask for a sizing statement that discloses:
- PV module Wp
- The “worst-month” (or conservative equivalent) solar resource assumption
- Overall charging efficiency assumption (declared)
Simple sanity check structure (supplier must fill values):
- PV daily energy (Wh/day) = PV_Wp × Peak Sun Hours × charging efficiency
- Net daily balance = PV daily energy − daily load energy
If net daily balance is negative during worst-month assumptions, the system will drift into low SOC and fail earlier in extended bad weather.
For rainy/monsoon sizing, see:
/solar-street-light/guides/rainy-season-sizing/
PASS/FAIL Rules
Mark a bid FAIL (Not Comparable) if it lacks:
- An energy budget sheet (Wavg, runtime hours, Wh/night)
- Battery nominal Wh + declared usable factor
- Dimming profile description used in the calculation
For highway/main-road projects, also require photometrics:
- IES files + DIALux-ready inputs (if spacing is claimed)
Internal resources:
- Spec Definitions Hub: /solar-street-light/resources/spec-definitions/
- RFQ & BOQ Kit: /solar-street-light/resources/rfq-boq-kit/
- Quality & Testing Evidence: /solar-street-light/resources/quality-testing/
- IES & DIALux Center: /solar-street-light/resources/ies-dialux/
Copy/Paste RFQ Clauses
Clause 1 — Energy budget sheet
Supplier shall provide an energy budget sheet showing:
- dimming profile used
- average power under dimming (Wavg)
- nightly runtime (hours)
- battery nominal Wh and usable factor assumption
- resulting autonomy nights (days)
Clause 2 — Battery statement
Supplier shall declare battery chemistry, nominal Wh, usable factor assumption, and BMS protections list.
Clause 3 — Bid comparability rule
Bids missing the energy budget sheet and usable Wh assumptions shall be labeled “Not Comparable / High Risk” before price comparison.
Autonomy Worksheet
Get the editable worksheet used to standardize autonomy claims:
- Dimming profile to Wavg calculator
- Wh/night energy budget sheet
- Usable Wh assumptions block (supplier-fill)
- Worst-month recharge sanity check block
- PASS/FAIL screening checklist
Want a quick engineering sanity check?
Share your road class, pole height, target lux/uniformity, and worst-month PSH assumptions.
Next Steps (Keep Your Project Tender-Ready)
Ensure your solar street light project meets all procurement and engineering standards.
Templates & Evidence
- 📑 Buyer’s Guide Hub — Pick a scenario & follow evidence-first path.
- 📊 Resources Library — Specs, QC evidence, & IES workflows.
- 🔍 Spec Definitions Hub — Normalize Wavg, Wh, & IP/IK evidence.
- 💡 IES & DIALux Center — Spacing & uniformity validation.
- 🛡️ Quality Testing — Verified proof vs marketing claims.
- 📋 RFQ & BOQ Kit — Templates, clauses & scoring rubrics.
Shortlist a System Type
- 🚀 All-in-One Solar Street Lights
- 🌓 All-in-Two Solar Street Lights
- 🛠️ Split Type Solar Street Lights
- 📏 Solar Pole Lights
- ✨ Decorative Solar Street Lights
- ⚠️ Specific Solutions (Explosion-proof/Hybrid)
FAQ
What’s the difference between autonomy “days” and “nights”?
They are commonly used interchangeably in solar street lighting. Always define the runtime hours per night to avoid confusion.
Why must autonomy be calculated with average power (Wavg)?
Because dimming profiles change power throughout the night. Autonomy depends on total Wh per night, not peak wattage.
Why is Wh required instead of only Ah?
Ah alone is not energy unless voltage is included. Wh is the correct energy unit for comparing autonomy.
What makes an autonomy claim not comparable?
Missing a dimming profile, missing Wavg/runtime assumptions, or missing usable battery Wh assumptions.