Hiring a diesel generator in London sounds straightforward until you start comparing quotes with different kVA ratings, hire terms, fuel arrangements and delivery charges. This guide cuts through the confusion and gives you a clear framework for making the right decision for your site, your budget and your timeline.
We cover the factors that affect hire cost, how to choose the correct kVA rating, what fuel consumption to plan for, the noise and access regulations that apply to London sites, and what to look for when choosing a hire supplier. By the end, you will know exactly what questions to ask before signing a hire agreement.
What Affects Generator Hire Cost?
Generator hire rates are not standardised across the industry. Two quotes for the same kVA rating can differ by 30% or more depending on several variables that are worth understanding before you start comparing prices.
kVA rating is the primary cost driver. A 20 kVA unit costs significantly less per week than a 500 kVA set. But hiring an undersized unit to save money is a false economy if it trips under load and causes a programme delay. Always size correctly first, then compare like-for-like quotes.
Hire duration has a significant effect on the weekly rate. Daily hire rates are the highest per day. Weekly rates are lower, and monthly contract rates are lower still. If your programme is likely to run longer than originally planned, negotiate a monthly rate from the outset rather than extending a daily hire.
Fuel is often excluded from headline hire rates and quoted separately. Check whether the initial fuel fill is included, and what the arrangement is for top-ups. Unmanaged fuel can result in an emergency shutdown if the tank runs dry on a weekend when no one is watching the level.
Delivery and collection charges vary by distance and access. London sites with restricted access, low bridges or congestion charge zones may attract a higher delivery fee. Confirm the full delivered price, not just the weekly hire rate, before committing.
How to Choose the Right kVA Rating
The kVA rating of the generator you hire must be sufficient to carry your total connected load plus a margin for motor starting currents. Oversizing slightly is acceptable. Undersizing causes overload trips and, at worst, equipment damage.
The most accurate way to size a generator is to produce a load schedule listing every piece of equipment that will be connected, its running kW demand and its starting kVA demand (if it has a motor). Our engineers carry out this calculation as part of every free site survey. But as a rough guide, the ranges below cover most common applications.
- 20 to 40 kVA: Small welfare unit, site lighting, power tools and a few sockets
- 60 to 80 kVA: Medium welfare cabin with heating, hot water and a canteen
- 100 to 150 kVA: Small construction site with plant, offices and welfare combined
- 200 to 400 kVA: Medium construction project, outdoor event with stage and catering, temporary hospital facility
- 500 kVA and above: Large construction site, data centre backup, major event infrastructure
One important note: the power factor of your connected load affects the relationship between kVA and kW. Most generators are rated at 0.8 power factor, which means a 100 kVA set delivers 80 kW. If your load is predominantly resistive (heaters, lighting), the set can carry more in kW terms. If your load is predominantly inductive (motors, transformers), the kVA rating is the binding constraint.
Fuel Consumption: What to Expect
Diesel fuel is a significant operating cost for any hire. UK diesel generators in the 100 to 500 kVA range typically consume between 20 and 110 litres per hour at 75% load, depending on the engine make and age (Cummins Power Generation, 2023). At current commercial diesel prices, this adds up quickly on a long-duration hire.
The key to managing fuel cost is running the generator at the right load factor. Sets running below 30% load consume a disproportionate amount of fuel relative to their output, and they are also at risk of wet stacking. If your actual load is consistently below 30% of the set's rated output, consider downsizing to a smaller unit.
For hires longer than two weeks, we recommend arranging a managed fuel service so that tank levels are monitored remotely and deliveries are scheduled automatically. This removes the risk of an unplanned shutdown due to fuel exhaustion, which is one of the most common and most avoidable causes of site power loss.
Site Access and Noise Regulations
London construction sites and outdoor events are subject to noise restrictions that affect generator placement and specification. The relevant standard for assessing environmental noise from industrial and commercial sources in the UK is BS 4142:2014+A1:2019 (Methods for rating and assessing industrial and commercial sound). Where a generator is likely to be audible at a nearby residential boundary, a noise assessment may be required before work begins.
Local authorities can issue noise abatement notices under the Environmental Protection Act 1990 where generator noise causes a statutory nuisance. In practice, this risk is highest on sites near residential properties and for hires that run overnight. Acoustic canopies reduce generator noise by typically 10 to 20 dB(A) and are available as standard on most modern hire sets. If your site has a noise-sensitive boundary, confirm the set's noise rating in dB(A) at 1 metre and at 7 metres before accepting delivery.
Access constraints in central London — including low bridges, weight restrictions and narrow streets — can affect the size of delivery vehicle that can reach your site. Confirm access details with us before booking, including any road closure permits required for the delivery and installation of large sets.
How to Choose a Hire Supplier
Not all generator hire companies are equal. When comparing suppliers, the following questions will help you identify who is likely to deliver reliably and who is likely to leave you without power at the worst possible moment.
Does the supplier own its fleet or broker from a third party? A supplier that owns its fleet controls availability, maintenance standards and dispatch logistics. A broker may be cheaper on paper but has less control over what arrives on site.
What is the emergency breakdown response time and is it staffed by engineers? A call centre that logs a ticket and promises a callback is not a breakdown service. A generator engineer who answers the phone and dispatches from local stock is.
Does the supplier carry out a load assessment before recommending a unit, or do they ask you to specify the kVA? A supplier that asks the right questions before quoting is more likely to send the right unit. A supplier that just asks "how many kVA?" is more likely to send what is convenient rather than what is correct.
To discuss a diesel generator hire requirement, call 0800 000 0000 or get a free site survey online.
Frequently Asked Questions
What is the cheapest way to hire a generator in London?
The most cost-effective approach to generator hire in London is to choose the correct kVA rating for your actual load, hire for the full expected duration at a monthly rate rather than a daily rate, and bundle fuel management into the agreement from the start. Avoid undersizing to save on hire cost, as a set that trips under load costs more in downtime than the saving on the weekly rate.
Can I hire a generator for a single day?
Yes. Daily hire is available for construction, events and one-off power requirements. Daily rates are higher per day than weekly or monthly rates. For very short hires, it is worth asking whether a weekly rate with a minimum hire period works out cheaper overall, as this is often the case when delivery and collection are factored in.
What documentation do I need to hire a generator?
For commercial hires, you will typically need to provide a purchase order or hire agreement, proof of company registration, and site access details. For hires on regulated sites such as hospitals or airports, additional documentation such as contractor approval forms may be required. We guide you through the paperwork requirements when you enquire.
Is red diesel still legal for generator hire in London?
Red diesel (rebated gas oil) remains legal for off-road commercial generators in the UK, including construction sites and events, as of 2025. The rules changed in April 2022 to remove the rebate for some uses, but off-road generation for construction and non-commercial events remains eligible. We supply red diesel with all hire sets and confirm current HMRC guidance at the point of hire.
How do I know if I need a 50 Hz or 60 Hz generator?
The UK mains supply frequency is 50 Hz, so virtually all equipment used on UK sites is designed for 50 Hz operation. A 60 Hz generator would run UK motors and some electronic equipment at incorrect speed and may cause damage. All of our hire fleet operates at 50 Hz, 230/400 V, which is the standard for UK commercial and industrial applications.