How do I calculate the UPS size I need?

Sum the VA (volt-ampere) ratings of all critical equipment: servers, network switches, storage, workstations, etc. If only wattage is available, divide by the power factor (typically 0.9 for IT equipment) to get VA. Add a 20-30% growth factor for future expansion. For example: 10 servers at 500W each = 5,000W ÷ 0.9 = 5,556VA. With 25% growth: 6,944VA. Select a 10kVA UPS. Never load a UPS above 80% of its rated capacity for reliable operation.

What runtime do I need for a UPS?

Runtime depends on your backup strategy: 5 minutes is the minimum — enough for a clean shutdown of servers if no generator is present. 10-15 minutes is standard — provides time for generator start-up (typically 10-30 seconds) plus fuel settling time. 30+ minutes is used when no generator is available and extended outages are expected. Longer runtime requires more batteries, more space, and more cooling. Most data centres use 10-15 minutes with diesel generator backup.

What is the difference between UPS topologies?

Online (Double Conversion): incoming AC is converted to DC (rectifier), then back to AC (inverter) — the load always runs from the inverter. Best protection: zero transfer time, complete isolation from mains disturbances. Efficiency: 93-97%. Line Interactive: a bidirectional inverter/charger handles minor voltage variations using an autotransformer. Transfer time: 2-4ms. Good for IT in stable grid conditions. Standby (Offline): load runs from mains until it fails, then transfers to battery/inverter. Transfer time: 5-12ms. Suitable for PCs, not servers.

How do I calculate battery Ah for a UPS?

Battery Ah = (Load VA × Runtime hours) ÷ (DC bus voltage × Inverter efficiency × Discharge factor × Aging factor). Example: 5kVA load, 15 minutes, 192V DC bus: Ah = (5000 × 0.25) ÷ (192 × 0.93 × 0.8 × 0.8) = 1250 ÷ 114.7 = 10.9Ah. The aging factor (0.8) accounts for capacity loss over battery life. VRLA batteries should not be discharged below 80% depth. Always use manufacturer's battery sizing tools for final design.

Do I need N+1 or 2N redundancy?

N = no redundancy — a single UPS handles the entire load. If it fails, the load transfers to raw mains. N+1 = one extra UPS module beyond what is needed. If one module fails, the remaining modules carry the load. Typically achieved with modular UPS systems. 2N = fully redundant — two independent UPS systems each capable of powering the entire load. Used in Tier III/IV data centres. The choice depends on the criticality of the load and acceptable risk level.

TradeCalculator.co.uk

UPS Sizing Calculator

Size a UPS system from critical load VA, required runtime, and redundancy level. Calculate battery Ah requirements, bypass rating, and input current for the installation circuit.

Critical Load

Total VA of critical equipment

Required Runtime

Battery backup time needed

UPS TopologyOnline recommended for servers

RedundancyLevel of fault tolerance

How We Calculate This

This calculator determines the UPS capacity and battery sizing for critical power protection applications.

UPS Sizing Formula

Design VA = Critical load VA × (1 + Growth%)

Battery Ah = (VA × Runtime_hrs) ÷ (Vdc × η × DoD × Aging)

Design Guidelines

Maximum loading: 80% of UPS rated capacity
Growth factor: 20-30% for future expansion
Runtime: 10-15min with generator, 30min+ without
VRLA life: 3-5 years at 20-25°C
Li-Ion life: 10-15 years, lighter, smaller
Bypass: Always include maintenance bypass

Frequently Asked Questions

Related Calculators

Cable Sizing Calculator

Calculate the correct cable size based on BS 7671 18th Edition regulations.

Voltage Drop Calculator

Check voltage drop complies with UK wiring regulations.

Maximum Demand Calculator

Calculate maximum electrical demand for domestic installations.

Circuit Breaker Sizing Calculator

Determine the correct MCB rating for your circuit.

Server Room Cooling Calculator

Calculate server room cooling capacity from IT equipment heat output.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.