Generator Sizing Calculator

Calculate generator capacity, fuel consumption, and costs for your backup power requirements

Generator Sizing Calculator

Calculate generator capacity and fuel requirements for backup power

Watts

Total power of all equipment running simultaneously

Watts

Additional load from motors, compressors during startup

hours

Hours between refueling/tank size

Advanced Options
%

Percentage of time running at full load (75% typical)

meters

Altitude above sea level (affects power output)

°C

Average operating temperature

Generator Sizing Guide for UK Installations

Selecting the correct generator size is essential for reliable backup power during electrical outages. An undersized generator won't run your equipment, while an oversized one wastes fuel and money.

Key Sizing Factors

  • Total Running Load: Add up the wattage of all equipment that will run simultaneously. Check equipment nameplates or manuals for accurate power ratings.
  • Starting Surge: Electric motors, compressors, and pumps require 2-7x their running wattage to start. Factor in the largest motor starting surge.
  • Load Type: Resistive loads (heaters, lights) are easy. Inductive loads (motors) require larger generators and have lower power factor (0.7-0.8).
  • Future Expansion: Add 20-25% capacity margin for additional equipment and to avoid running at maximum capacity.
  • Fuel Type: Diesel is most economical for frequent use. Natural gas offers unlimited runtime if mains connected. Petrol is for portable only.

Generator Sizing Formula

Required kW = [(Running Watts) + (Largest Starting Surge)] ÷ 1000 × 1.25

Power Factor = Real Power (kW) ÷ Apparent Power (kVA)

Fuel Consumption (L/hr) ≈ Generator kW × 0.25 (diesel at 75% load)

Installation Requirements

  • Location: Outdoors or well-ventilated area. Minimum 1m clearance from buildings. Protection from weather if not in enclosure.
  • Foundation: Level concrete pad. Large generators (>20kW) require substantial mounting base to reduce vibration.
  • Transfer Switch: Automatic Transfer Switch (ATS) or manual changeover switch required to safely switch between mains and generator. NEVER connect generator directly to mains.
  • Earthing: Separate earth rod required. Must meet BS 7671 (18th Edition) requirements.
  • Exhaust: Extend exhaust away from building openings and air intakes. Carbon monoxide risk - never run indoors.
  • Fuel Storage: Diesel fuel tank with bunding (double wall containment). Minimum 5m from buildings for fire safety.
  • Noise: Generators are noisy (60-85dB). Acoustic enclosures reduce noise by 10-15dB. Check local noise regulations.

UK Regulations

  • Planning Permission: Usually required for permanent generators >20kW. Check with local planning authority.
  • DNO Notification: Distribution Network Operator must be notified for generators >16A per phase.
  • Building Regulations: Part P compliance required for electrical installation. Qualified electrician must install transfer switch.
  • Environmental: Oil storage regulations apply to fuel tanks. Bunding required for tanks >200L.
  • Emissions: Large generators may require environmental permit. Check with local Environmental Health.

Professional Installation Required

Generators over 5kW should be installed by qualified electrical contractors and generator specialists. Installation must comply with BS 7671 (18th Edition), manufacturer specifications, and local regulations. Regular maintenance essential - monthly exercise and annual servicing recommended.

Generator Types Explained

🏠

Portable Generators (Petrol/Diesel)

Power Range: 1-10kW

Runtime: 4-12 hours per tank

Best For: Occasional use, camping, construction tools, emergency backup

Pros: Mobile, lower upfront cost, no installation

Cons: Manual start, must refuel, noisy, petrol limited to occasional use

Cost: £400-£3,000

🏢

Standby Generators (Diesel/Natural Gas)

Power Range: 10-150kW

Runtime: 24+ hours (diesel tank), unlimited (natural gas)

Best For: Home backup, business continuity, medical facilities

Pros: Automatic start, weatherproof enclosure, long life, quiet operation

Cons: Permanent installation, higher cost, requires maintenance

Cost: £5,000-£50,000+

🏭

Industrial/Prime Power Generators

Power Range: 100kW-2000kW+

Runtime: Continuous operation with proper maintenance

Best For: Data centres, manufacturing, remote sites, continuous power

Pros: Extremely reliable, long life, can parallel multiple units, full redundancy

Cons: Very expensive, requires specialist installation, regular maintenance

Cost: £30,000-£500,000+

💨

Inverter Generators

Power Range: 1-7kW

Runtime: 6-16 hours per tank

Best For: Sensitive electronics, camping, events, clean power required

Pros: Very quiet, clean power (THD <3%), fuel efficient, compact

Cons: More expensive than conventional, lower power output

Cost: £500-£5,000

Common Generator Applications

🏠 Home Backup Power

Typical requirements for residential backup:

  • • Essential circuits only: 5-8kW
  • • Half-house: 10-12kW
  • • Whole house: 15-20kW
  • • Natural gas or diesel fuel
  • • Automatic transfer switch recommended
  • • Runtime: 8-24 hours typical

🏢 Business Standby Power

Commercial building backup requirements:

  • • Small office: 10-20kW
  • • Retail/restaurant: 20-40kW
  • • Large commercial: 60-150kW
  • • Three-phase for larger installations
  • • Emergency lighting legally required
  • • Weekly exercise testing

🏗️ Construction Sites

Temporary power for construction:

  • • Small tools: 5-10kW portable
  • • Site huts + tools: 15-30kW
  • • Large sites: 40-100kW+
  • • Diesel fuel standard
  • • Robust construction for harsh conditions
  • • Hire often more economical than purchase

💻 Data Centres & IT

Critical infrastructure protection:

  • • Server room: 20-50kW
  • • Data centre: 100kW-2MW+
  • • N+1 redundancy minimum (spare capacity)
  • • Works with UPS - generator for extended outages
  • • Monthly testing under load
  • • Natural gas for unlimited runtime

🏭 Industrial/Manufacturing

Production facility power requirements:

  • • Light industrial: 50-100kW
  • • Manufacturing: 200-500kW
  • • Heavy industry: 500kW-2MW+
  • • Three-phase essential for motor loads
  • • High starting surge capability
  • • Prime power rating for frequent use

🎪 Events & Exhibitions

Temporary power for outdoor events:

  • • Small marquee: 5-10kW
  • • Wedding/event: 10-20kW
  • • Festival/concert: 40-150kW+
  • • Silent/super-silent models for noise limits
  • • Hire for short-term events
  • • Multiple smaller units often better than one large

Running Costs vs Downtime Costs

Consider the cost of power outages when sizing your generator:

  • • Diesel fuel cost: ~£1.50/L = £5-£15/hr for 20kW generator at 75% load
  • • Annual maintenance: £500-£2,000 depending on size
  • • Business downtime: £1,000-£10,000+ per hour
  • • Food spoilage (commercial): £5,000-£50,000 per day
  • • Data centre outage: £100,000-£1,000,000 per hour

A properly sized generator system typically pays for itself after preventing just one significant outage. For critical applications, the question isn't cost - it's "can you afford NOT to have backup power?"