| Scenario | Lamp (W) | Hours | PSH | Autonomy | Voltage | DoD | Panel (W) | Battery (Ah) |
|---|---|---|---|---|---|---|---|---|
| Urban street, warm climate | 40 | 12 | 5.0 | 2 days | 12 V | 60% | 200–300 | 160–220 |
| Rural road, cloudy season | 60 | 12 | 3.5 | 3 days | 24 V | 70% | 450–650 | 150–220 |
| Pathway light, short runtime | 20 | 8 | 4.5 | 2 days | 12 V | 60% | 120–180 | 90–140 |
Panel_W_with_margin = Panel_W × (1 + SafetyMargin)
Battery_Ah = Battery_Wh ÷ (Voltage × DoD)
- Enter the luminaire power and expected nightly runtime.
- Set peak sun hours for your site and worst season.
- Choose autonomy days based on reliability targets.
- Select battery voltage and a safe depth of discharge.
- Adjust efficiency and loss values to match your design.
- Pick panel and battery unit sizes to get quantities.
- Press Calculate to view results above the form.
Define the lighting load and runtime
Start with the luminaire input power, not just watt-equivalent claims. Multiply lamp watts by planned nightly hours to get daily energy in Wh/day. For example, a 40 W fixture running 12 hours needs 480 Wh each night, before conversion losses. Add any auxiliary loads such as motion sensors, controllers, or communications modules.
Use conservative peak sun hours
Peak sun hours represent equivalent full-sun energy and vary by season, dust, tilt, and shading. Size using the lowest monthly PSH for the site, not the annual average. A design based on 3.5 PSH will often outperform one sized at 5.5 PSH during cloudy periods. If the pole is near trees or buildings, reduce PSH further because even short morning shade can cut daily harvest significantly.
Account for derating and efficiencies
Real systems deliver less than nameplate. Apply panel derate for temperature, soiling, and aging, then reduce further for wiring loss. Controller efficiency and battery round‑trip efficiency also matter. The calculator combines these as an effective derate so array power reflects usable energy. In hot regions, a 5–10% temperature loss is common, and poor cabling can add another 2–5% loss unless wire gauges are selected for low voltage drop.
Select battery autonomy and depth of discharge
Autonomy days define resilience during storms or short winter days. Battery capacity is sized in Ah from required backup energy, system voltage, and allowable DoD. Lower DoD (for example 60%) increases battery size but improves cycle life and reduces unexpected dimming. For two days of autonomy, a 480 Wh/day load needs about 960 Wh stored, then more after efficiency allowances.
Verify controller current and expandability
Controller current is estimated from total array watts divided by battery voltage, with a safety margin. Confirm the controller’s PV input voltage and temperature limits, then choose the next standard rating. If future upgrades are likely, keep spare controller capacity and pole-space for another panel. Overheating and water ingress are common causes of early system failures.
1) What peak sun hours value should I enter?
Use the lowest typical monthly value for your site and season. If you only have an annual average, subtract a safety buffer. Shading, dust, and tilt errors can reduce harvest, so conservative PSH improves reliability.
2) How do I choose autonomy days?
Two days suits many urban streets. Use three or more days for remote roads, monsoon seasons, or critical routes. Higher autonomy increases battery size and cost, but reduces outage risk during consecutive cloudy days.
3) What depth of discharge is reasonable?
For longer battery life, keep DoD around 50–70% for lead‑acid and 70–90% for many lithium packs, depending on the manufacturer. Lower DoD increases required capacity but reduces cycling stress.
4) Why does the calculator include derate and losses?
Nameplate ratings assume ideal test conditions. Real output drops from temperature, dirt, wiring voltage drop, and conversion inefficiency. Including derates prevents undersizing, which otherwise causes dimming, reduced night hours, or premature battery wear.
5) How should I size the charge controller?
Select a controller current rating at or above the calculated value, then move to the next standard size. Also confirm PV input voltage limits, temperature ratings, and compatibility with your battery chemistry and system voltage.
6) Do I need to change panel and battery unit sizes?
Yes, set them to the products you plan to purchase. The calculator converts required watts and amp‑hours into practical quantities. Adjust unit sizes to explore different combinations and minimize unused capacity while keeping a safe margin.