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Choosing the right generator size is one of the most important decisions when purchasing or hiring a generator. A generator that is too small may struggle to meet demand, leading to overload and power interruptions. A generator that is too large can result in unnecessary fuel costs, reduced efficiency and higher purchase or hire expenses.
Whether you’re looking for backup power for a home, a generator for a construction site, emergency power for a business or a temporary power solution for an event, understanding your power requirements is essential. The good news is that generator sizing doesn’t need to be complicated.
This guide explains how to calculate your power requirements, understand generator capacity and choose the right generator for your needs.
Generators are designed to provide reliable power when mains electricity is unavailable. To do this effectively, the generator must be able to handle the total electrical load placed on it.
If a generator is undersized, it may struggle to start larger equipment or cope with sudden increases in demand. This can lead to overheating, reduced performance and potential equipment damage.
An oversized generator can also create problems. Larger generators typically consume more fuel and cost more to purchase, hire and maintain. Selecting the correct size generator helps maximise efficiency while ensuring you have enough power available when needed.
By accurately assessing your requirements before purchasing or hiring a generator, you can achieve the best balance between performance, reliability and running costs.
The first step in selecting a generator is identifying the equipment you need to power.
Start by creating a list of all electrical devices, tools, machinery or appliances that will be connected to the generator. Each item will have a power requirement measured in watts (W), kilowatts (kW) or kilovolt-amperes (kVA).
Once you’ve identified the equipment, add together the total power requirements to calculate your expected load.
When calculating generator size, it’s important to account for:
Many motors, pumps and compressors require significantly more power when starting than they do during normal operation. Failing to account for this additional demand is one of the most common generator sizing mistakes.
If you’re unsure about the difference between generator ratings, our guide explaining what is the difference between kVA and kW can help.
While every application is different, the following guide provides a useful starting point when selecting a generator.
Application | Typical Generator Size |
Home Backup Power | 5kVA – 20kVA |
Small Office | 10kVA – 30kVA |
Small Business | 20kVA – 50kVA |
Farm & Agricultural Use | 20kVA – 100kVA |
Construction Site | 50kVA – 150kVA |
Manufacturing Facility | 100kVA – 500kVA |
Data Centre | 250kVA+ |
Large Events | 100kVA – 500kVA+ |
These figures provide general guidance only. Actual generator requirements will vary depending on the specific equipment being powered.
Construction sites often require larger generators due to the high starting currents of power tools, compressors, lighting systems, and site welfare facilities.
A small construction site may operate effectively with a generator in the 30kVA to 50kVA range, while larger developments often require 100kVA, 150kVA or even larger units.
Typical equipment requiring power may include:
Because site requirements often change throughout a project, many contractors choose generator hire solutions that allow capacity to be adjusted as required.
Understanding what different generator sizes can typically support can help narrow down your options.
Generator Size | Typical Applications |
10kVA | Small tools, lighting, domestic backup |
20kVA | Site offices, workshops, small businesses |
50kVA | Farms, commercial premises, larger construction sites |
100kVA | Manufacturing equipment, larger offices, events |
250kVA | Industrial facilities, distribution centres |
500kVA+ | Data centres, large industrial operations |
The actual number of devices a generator can support depends on the total load and operating conditions.
Another important consideration is whether you require a single-phase or three-phase generator.
Single phase generators are commonly used in domestic and smaller commercial environments. They are ideal for powering lighting, appliances and standard electrical equipment.
Three phase generators are typically required for industrial applications, larger commercial facilities and equipment with higher power demands.
Choosing the wrong phase configuration can limit performance and create compatibility issues, so it’s important to understand your equipment’s requirements before making a decision.
The fuel type you choose will also influence generator selection.
Petrol generators are often suitable for:
Diesel generators are generally preferred for:
Diesel generators often offer greater fuel efficiency and durability for demanding applications, making them the preferred option for many businesses.
One of the most common mistakes is only considering running power requirements and ignoring starting loads.
Other common mistakes include:
Taking time to properly assess your requirements can help avoid costly mistakes and ensure reliable performance.
Choosing the right generator size can feel overwhelming, particularly when dealing with specialist equipment, industrial machinery or multiple power requirements.
At Bellwood Rewinds, we help customers across the UK identify the most suitable generator for their specific application. Whether you’re purchasing a diesel generator, exploring generator hire options or planning a standby power installation, our experienced team can provide expert advice and practical recommendations.
Contact Bellwood Rewinds today to discuss your requirements and let our specialists help you find the right generator size for your project, business or property.