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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.
Total VA of critical equipment
Battery backup time needed
0.9 typical for IT loads
Future capacity allowance
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
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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 = (Load_W × Runtime_hrs) ÷ (Vdc × η × DoD × Aging)
where Load_W = Critical load VA × power factor (a battery delivers real power, so the load is taken in Watts, not VA).
Design Guidelines
- Maximum loading: aim for ≤80% of the UPS rated capacity at full design load. Because the UPS is sized to the design VA (present load plus growth), the present-day loading shown can sit above 80% until that growth materialises.
- 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
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. As a target, keep the UPS loaded to no more than about 80% of its rated capacity at full design load to leave headroom for inrush and battery recharge. For upstream supply sizing, see our Transformer Sizing Calculator.
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.
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.
A battery is a DC source that delivers real power (Watts), so the load is taken in Watts, not VA: Load W = Load VA × power factor. Battery Ah = (Load W × Runtime hours) ÷ (DC bus voltage × Inverter efficiency × Discharge factor × Aging factor). Example: 5kVA load at 0.9 power factor = 4,500W, 15 minutes, 192V DC bus: Ah = (4500 × 0.25) ÷ (192 × 0.93 × 0.8 × 0.8) = 1125 ÷ 114.3 = 9.8Ah. The aging factor (0.8) accounts for capacity loss over battery life. VRLA batteries should not be discharged below 80% depth. Always use the manufacturer's battery sizing tools for final design.
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.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.