| Scenario | Battery Pack | Load | Factors | Estimated Net Runtime |
|---|---|---|---|---|
| Pathway fixtures | 12V, 7Ah, 2S1P | 8 lamps × 6W | ηinv 0.85, ηdrv 0.90, DoD 0.80 | ~1h 25m |
| Greenhouse exit route | 12V, 9Ah, 2S2P | Direct 75W | Aging 0.90, Temp 0.95, Reserve 5m | ~3h 10m |
| Storage shed lights | 12V, 18Ah, 1S2P | 6 lamps × 5W | ηinv 0.90, ηdrv 0.92, DoD 0.70 | ~4h 05m |
- Pack voltage: Vpack = Vbattery × Series
- Pack capacity: Ahpack = Ahbattery × Parallel
- Nominal energy: Whnom = Vpack × Ahpack
- Usable energy: Whusable = Whnom × DoD × Aging × Temp
- Load watts:
- Lamp-based: Wload = (LampCount × LampW) / DriverEfficiency
- Direct: Wload = DirectWatts
- Battery-side power: Wbatt = Wload / InverterEfficiency
- Runtime: Hours = Whusable / Wbatt, then minutes = hours × 60
- Net runtime: MinutesNet = max(0, Minutes − ReserveMinutes)
- Enter battery voltage and amp-hours for the battery type you use.
- Set series and parallel counts to match your pack wiring.
- Select lamp-based mode for fixtures, or direct watts for measured load.
- Adjust efficiencies, depth of discharge, aging, and temperature derating.
- Add a reserve time to keep a safety margin for real conditions.
- Click Calculate Runtime to see results above the form.
- Use Download CSV or Download PDF for records.
Runtime planning for critical routes
Emergency lighting in garden spaces should prioritize safe egress, steps, tool sheds, and greenhouse doors. This calculator converts battery capacity into usable watt-hours, then divides by estimated battery-side watts to predict autonomy. Typical planning targets range from 90 to 180 minutes, depending on risk, occupancy, and response time. Use reserve minutes to account for unknowns such as dirty optics and aging cables.
Battery configuration and energy math
Series batteries raise pack voltage while parallel strings raise available amp-hours. The pack energy is Vpack × Ahpack, which becomes the baseline for runtime. Depth of discharge limits usable energy to protect cycle life, and aging factor reduces nameplate capacity to match real installations. Temperature derating reflects winter performance, where cold can reduce available capacity and peak output.
Load modeling for fixtures and drivers
Lamp-based entry estimates the electrical input needed to produce the lamp output. Driver efficiency increases the required input watts; lower efficiency shortens runtime. Direct watts mode is best when you measured the emergency circuit with a meter or you know the rated load of a central inverter. Keep the load realistic by including exit signs, sensors, and any always-on controls.
Efficiency, reserve, and target checks
Inverter efficiency captures conversion losses between battery and load. Reserve minutes reduce the displayed runtime to a conservative net value. The autonomy check compares net minutes to your target and flags when the design needs larger capacity, fewer fixtures, or improved efficiency. Use the downloadable CSV for quick review, and the PDF for maintenance records.
Operational data points to record
Track battery chemistry, installation date, and monthly functional tests. Note ambient temperature during tests, measured current draw, and any lamp failures. If runtime drops by more than 20% from baseline, review aging factor, DoD, and connector condition. For improved accuracy, update direct load watts after repairs and seasonal adjustments. Log charger float voltage settings, and keep replacement batteries matched by age and model.
1) What does “net runtime” mean?
Net runtime is the calculated runtime minus your reserve minutes. It is intentionally conservative, so your lighting plan still meets autonomy when batteries age, temperatures drop, or loads vary slightly.
2) Should I use lamp-based or direct watts mode?
Use lamp-based mode when you know fixture count and lamp wattage. Use direct watts when you have measured the emergency circuit, or you are sizing for a central inverter with a known load.
3) Why do efficiency values matter so much?
Inverter and driver efficiencies convert lamp output to battery-side demand. A small efficiency drop increases required watts, which reduces runtime. Enter realistic efficiencies from device labels or test data.
4) How do I choose a depth of discharge?
Choose lower DoD for longer battery life and better reliability. Many standby systems plan between 0.5 and 0.8. If you must meet long autonomy targets, increase capacity rather than pushing DoD to 1.0.
5) What aging factor should I enter for older batteries?
Start with 0.9 for lightly used batteries under two years, 0.8 for moderate aging, and 0.7 when performance is clearly declining. If you have test data, set aging factor to match measured capacity.
6) How can I improve accuracy for my site?
Measure real emergency load watts, record ambient temperature, and update factors after maintenance. Keep wiring tight, clean fixtures, and replace weak batteries promptly. Recalculate after adding fixtures or changing drivers.