Designing Lighting, UPS Capacity and Battery Autonomy

Design Objective & Industry Benchmarks

Design lighting around visual tasks, not floor area alone. Size the uninterruptible power supply (UPS) around agreed critical loads. Battery autonomy means the required operating time without incoming power.

Typical maintained illuminance benchmarks include:

• Offices: 500 lux for writing, reading and computer work.
• Warehouses: 100 lux for general storage and 300 lux for packing.
• Industry: 300–500 lux for many assembly tasks, depending on precision.
• Infrastructure platforms: requirements vary with passenger activity, enclosure and operating conditions.

Maintained illuminance is the average light level after expected output reductions. Verify each task against the applicable EN 12464 requirements. Check uniformity, glare and colour rendering separately. Platform design also needs transport authority criteria.

Step-by-Step Sizing Methodology

1. Define spaces and critical loads.

Record task planes, ceiling heights, rack positions and surface reflectances. Reflectance describes how much light a surface reflects. Separate normal, operational backup and statutory emergency lighting.

List each critical load in watts and volt-amperes. Confirm runtime, power factor, starting demand and future capacity requirements.

2. Calculate preliminary luminaire quantity.

Use the lumen method:

N = E × A / (Φ × UF × MF)

• N = luminaire quantity, rounded upward.
• E = maintained illuminance, in lux.
• A = illuminated area, in m².
• Φ = initial light output per luminaire, in lumens.
• UF = utilisation factor, dimensionless.
• MF = maintenance factor, dimensionless.

UF represents light reaching the working plane. MF allows for expected light loss over the design period. Select both from documented room and lighting assumptions.

The result gives a starting quantity, not a finished layout.

3. Check the lighting layout.

Use manufacturer photometric files in lighting calculation software. Check average illuminance, minimum illuminance, glare and vertical lighting on racks. Uniformity equals minimum illuminance divided by average illuminance.

Adjust spacing and optics until the layout meets every criterion.

4. Calculate UPS output capacity.

P = Σpᵢ

S = Σ(pᵢ / PFᵢ)

Here, pᵢ is each load’s real power in kW. PFᵢ is its power factor, dimensionless. P is total kW; S is the conservative total kVA estimate.

Apply the agreed growth allowance to both totals. Select a UPS meeting both ratings. Check ambient derating, load steps, driver inrush and output compatibility.

5. Estimate battery capacity.

C = P × t × K / (η × V × D)

• C = preliminary battery capacity, in Ah.
• P = supported load, in W.
• t = required autonomy, in hours.
• K = combined ageing and temperature allowance, dimensionless.
• η = battery-to-output efficiency, dimensionless.
• V = nominal battery string voltage, in V.
• D = permitted discharge fraction, dimensionless.

This estimates capacity, but nominal voltage does not represent the full discharge. Final selection needs manufacturer constant-power discharge data at the specified end voltage. Confirm chemistry, temperature, ageing allowance and UPS direct-current voltage limits.

Practical Worked Example

Consider a 500 m² office requiring 500 lux. Each luminaire delivers 5,000 lumens and consumes 40 W. Assume UF = 0.65 and MF = 0.80.

N = 500 × 500 / (5,000 × 0.65 × 0.80) = 96.2

Start with 97 luminaires, then verify their layout. Lighting power equals 97 × 40 = 3,880 W.

Assume all lighting requires operational backup, alongside 2.00 kW of communications equipment. Total load is 5.88 kW. At a common power factor of 0.95, demand is 6.19 kVA.

A project allowance of 20% gives 7.06 kW and 7.43 kVA. A 10 kVA UPS rated for 9 kW satisfies both preliminary requirements.

For 30 minutes at the present load, assume 192 V, 92% efficiency, D = 0.80 and K = 1.25.

C = 5,880 × 0.5 × 1.25 / (0.92 × 192 × 0.80) = 26.0 Ah

Treat 26 Ah as an estimate, not an approved battery size. Recalculate for future load if its full runtime is required.

Cross-Trade MEP Coordination

Coordinate luminaires with ducts, sprinklers, ceilings and warehouse racks. Agree equipment heat gains and battery ventilation with mechanical engineers. Confirm battery mass, floor loading and anchors with structural engineers. Coordinate fire separation, cable routes and emergency lighting responsibilities.

Applicable Standards & Codes

Use EN 12464-1 for indoor workplaces and EN 12464-2 for outdoor workplaces. IEC 62040-1 covers UPS safety; IEC 62040-2 covers electromagnetic compatibility. IEC 62040-3 addresses UPS performance and testing. Apply local wiring, battery safety and emergency lighting requirements separately.

Core Rules of Thumb for Practice

• Use lumen calculations for quantities; verify layouts through photometric modelling.
• Reserve 20–25% UPS capacity only as a preliminary project assumption.
• Never select UPS capacity from kVA alone.
• Never approve battery capacity from nominal Ah alone.

These rules support early design. Verify project criteria before final selection.