How to Calculate Solar Street Light Battery Capacity

Key conclusion

Battery capacity cannot be selected from luminaire wattage alone. A defensible calculation starts with the measured nightly energy load, multiplies it by the required autonomy period, and then accounts for usable depth of discharge, discharge efficiency, temperature, aging and auxiliary loads.

Keep watt-hours and ampere-hours separate. Watt-hours describe energy. Ampere-hours describe charge at a stated voltage. A battery labeled only in Ah cannot be compared until its nominal voltage and usable operating window are known.

Inputs required

• Measured system input power at every lighting level.
• Hours at each level during the 12-hour schedule.
• Controller, sensor and communications energy.
• Required autonomy in nights, defined by the project.
• Battery nominal voltage and usable voltage window.
• Allowed depth of discharge for the selected cells and warranty conditions.
• Discharge-path efficiency.
• Temperature and aging capacity factors.
• BMS current, low-voltage and temperature limits.

Step 1: calculate the 12-hour load

For a dimming schedule:

Nightly lighting energy (Wh) = sum of [measured power at each dimming level (W) x time at that level (h)]

Add the controller, sensor and communications loads:

Total nightly energy = lighting energy + auxiliary energy

If a motion sensor is used, calculate a conservative event profile. Do not reduce the battery from a claimed saving percentage without a stated traffic assumption.

Step 2: convert the load into required battery energy

A transparent preliminary equation is:

Required nominal battery energy (Wh) = nightly energy x autonomy nights / [usable depth-of-discharge fraction x discharge efficiency x temperature-and-aging capacity factor]

Each factor must be tied to the proposed cell, pack, environment and warranty. For example, a usable depth-of-discharge limit is not the same as the BMS emergency cutoff. Normal operation should preserve the agreed reserve and avoid relying on protective shutdown.

Step 3: convert watt-hours to ampere-hours

After determining the required nominal energy:

Required capacity (Ah) = required nominal energy (Wh) / nominal battery voltage (V)

Use the pack’s nominal voltage for preliminary comparison, then verify the controller and LED driver across the actual battery voltage range.

Worked example

This is a method example, not a universal product specification.

• 30 W for 4 hours: 120 Wh
• 15 W for 4 hours: 60 Wh
• 9 W for 4 hours: 36 Wh
• Controller and sensor: 12 Wh
• Total nightly energy: 228 Wh
• Autonomy requirement: 2 nights
• Usable depth-of-discharge fraction: 0.80
• Discharge efficiency: 0.95
• Temperature-and-aging capacity factor: 0.85

Required nominal energy = 228 x 2 / (0.80 x 0.95 x 0.85) = approximately 706 Wh

For a nominal 12.8 V pack:

Required capacity = 706 / 12.8 = approximately 55.2 Ah

This result is a starting point. The proposed commercial pack must be checked for cell configuration, actual tested capacity, current limits, temperature range, enclosure, BMS settings, available charging energy and recovery time.

Temperature and aging are not optional margins

Lithium-ion battery life and available capacity are affected by temperature, charge voltage, depth of discharge, calendar age and cycling conditions. A supplier should state the cell and pack assumptions behind any cycle-life or calendar-life statement. Do not convert a laboratory cycle number directly into a guaranteed number of field years.

For hot climates, review cell temperature inside the installed battery compartment rather than using ambient temperature alone. For cold climates, confirm charge restrictions and controller behavior below the cell’s permitted charging temperature.

Why a larger Ah label may still be misleading

• The stated Ah may be measured at a different voltage or discharge rate.
• The pack may reach the BMS cutoff before delivering the claimed usable energy.
• Cells may be mismatched, aged or not traceable to the stated grade.
• Parallel cell count may not match the physical pack and cell capacity.
• The controller may use only part of the nominal voltage window.
• High enclosure temperature may reduce available capacity and accelerate aging.

Capacity verification before approval

1. Record cell manufacturer, model, chemistry and traceability.
2. Check series and parallel configuration against nominal voltage and capacity.
3. Review BMS current, voltage and temperature settings.
4. Perform a controlled charge and discharge capacity test under stated conditions.
5. Record delivered Wh as well as Ah.
6. Compare the tested result with the design’s usable energy requirement.
7. Retain the report with serial or batch identification.

Battery, panel and autonomy must be checked together

A battery sized for two nights is not useful if the panel cannot restore the consumed energy within the required recovery period. The energy model should show:

• nightly load under the approved control profile;
• usable battery energy and reserve;
• low-solar sequence and autonomy definition;
• daily charging energy under the design solar resource;
• recovery time after the low-solar sequence;
• low-state-of-charge control behavior.

Sources and further reading

• U.S. Department of Energy: factors affecting lithium-ion battery life
• U.S. Department of Energy: Energy Storage Cost and Performance Assessment
• U.S. Department of Energy: Storage Cost and Performance Characterization

Also review the solar street light battery guide and the panel sizing method.