This guide explains how to plan an emergency generator that works on the day: how to size it, how to decide fuel autonomy, what the automatic transfer switch does, how to test the system without damaging it, and how standby duty differs from prime duty. It also covers what changes for hospitals and data centres, where the cost of failure is measured in lives and contracts rather than inconvenience.
What an Emergency Generator System Actually Includes
People say generator and picture an engine and an alternator in a box. The emergency system is bigger. It includes the engine and alternator, the starting batteries and charger, the cooling and ventilation path, the fuel storage and delivery, the exhaust, the control panel, the automatic transfer switch, and the distribution that feeds the loads you have decided are essential. A weakness in any one of those can stop the whole system, however good the engine is.
That is why a generator purchased on kilowatts alone often disappoints. A set can carry the load on paper and still fail because the room cannot ventilate it, the fuel line is too long for the lift pump, or the transfer switch is rated below the available fault current. Good planning treats the generator as one part of a power system and checks each interface.
Diesel remains the most common choice for emergency duty because the fuel stores on site, the engines start quickly from cold, and the technology is well understood by technicians in almost every market. Gas-fuelled and turbine-based emergency systems exist, and the right choice depends on the site, the fuel supply and the regulations that apply to it. The emergency diesel generators overview on this site sets out how USP&E approaches the diesel case.
Sizing an Emergency Generator
Sizing starts with a load list, not a catalogue. List every load the generator must carry, separate it into life-safety, critical and optional groups, and record the running power and the starting demand of each. Motors, compressors, chillers and pumps draw several times their running current when they start, and that starting surge is where undersized sets fail.
Next decide how the loads will be applied. A set that picks up everything in one step is stressed far more than one that takes load in stages, and a staged scheme lets you use a smaller machine. Staging needs controls that sequence reliably, so it belongs in the design, not as an afterthought.
Then add margin. Allow for load growth, for the effect of altitude and high ambient temperature on engine output, and for non-linear loads such as variable speed drives and uninterruptible power supplies, which can distort the voltage waveform and stress the alternator. Running a generator close to its limit every time it starts is not a reliable plan.
If you want to check your numbers, the generator size calculator on this site gives a first estimate, and a USP&E engineer can then review it against your actual load profile.
Fuel Autonomy: How Long Must It Run?
Fuel autonomy is the number of hours the generator can run at its design load without refuelling. It is a decision, not a default. The right figure depends on how long outages realistically last where you are, how quickly a fuel delivery can reach you in a storm, flood or civil disruption, and what the building does if it runs dry.
Many sites assume the tank on the base frame is enough and discover otherwise during a long event. Others install a large bulk tank and then never turn the fuel over. Fuel left for years can pick up water and microbial growth, clog filters and strand the set. A realistic plan combines enough storage, a routine to test and polish the fuel, and an agreement with a supplier who can deliver in an emergency.
Think about the whole chain. Fill points need to be reachable by a tanker. Day tanks and transfer pumps need their own power and their own testing. Containment must meet local rules. The guide to fuel storage for emergency diesel fleets covers these points in detail, and the fuel resupply plan article shows how to write a delivery plan that holds up under pressure.
The Automatic Transfer Switch
The automatic transfer switch, usually shortened to ATS, is the device that senses the loss of normal supply, signals the generator to start, and moves the load from the utility to the generator once the set is ready. When the utility returns, it retransfers and lets the generator cool down and stop. Everything the building experiences during an outage passes through it.
Specify it with care. Check the rating against the load and the available fault current. Decide whether you need an open or a closed transition, since a closed transition avoids a brief interruption on retransfer but requires synchronising equipment and utility approval. Set the time delays for start, transfer and retransfer deliberately, because the factory defaults rarely match your loads.
An ATS also needs maintenance and testing like any other piece of switchgear. Contacts wear, controls drift and manual operators are forgotten. A bypass arrangement allows the switch to be serviced without losing power to the building, and it is worth the cost wherever an outage during maintenance would be unacceptable. The article on ATS and black start for emergency diesel goes further into transfer logic and recovery from a total blackout.
Testing an Emergency Generator Properly
An untested emergency generator is a hope. Regular testing proves that the set starts, takes load and carries it, and it exposes problems while they are cheap to fix. Standards such as NFPA 110 in the United States set out expectations for emergency power supply systems, including how quickly the system must supply power and how often it must be exercised. Check the edition and local rules that apply to your site, because authorities and insurers may require specific testing regimes and records.
Do not confuse running with testing. A generator that idles briefly on no load can look healthy while accumulating unburned fuel in the exhaust, a condition often called wet stacking. Diesel engines need to be loaded enough, for long enough, to reach proper operating temperature and burn clean. Where the building load is too small to do that, a load bank provides the demand. The article on wet stacking in emergency fleets explains the problem, and load bank testing for emergency fleets shows how to apply it.
Write the test into a regime: a routine exercise on a fixed schedule, an annual test with the building load transferred, and a periodic load bank test to check full capacity. Record every test, including failures, with the date, run time, load, readings and the name of the person who carried it out. The test regime article offers a structure you can adapt.
Standby, Prime and Continuous Duty
Generator ratings describe what the machine may be asked to do. A standby rating is for emergency use during a utility outage, with limited annual running time and a variable load. A prime rating is for sites that run most of the time, such as remote locations without a grid, and allows for an overload margin for short periods. A continuous rating is for steady base load at a constant level.
Do not use a standby rated set for prime duty. The rating is a promise about how the engine will be stressed, and exceeding it shortens life and can void the manufacturer’s data. Equally, there is no need to pay for a continuous rated set when the real duty is an occasional outage. Ask for the rating on the data sheet for the exact model and speed, and confirm that it matches how you will use the machine.
Some sites sit between the two. A remote facility with a weak grid may run on the generator frequently and still need a reliable emergency backup. In that case, plan for more than one set so that routine running and emergency capacity are not competing for the same machine. The N plus 1 emergency diesel fleets article describes how redundancy is built.
Hospitals, Data Centres and Other Critical Sites
Hospitals, data centres, water works and telecom hubs carry a higher penalty for failure, and they are usually bound by codes that demand fast transfer, tested reliability and documented maintenance. A hospital must keep life-safety systems powered within seconds of an outage. A data centre must bridge to its generators without losing the load, normally through uninterruptible power supplies that hold the building until the engines are ready.
For these sites, redundancy is part of the design. Multiple generators in parallel, separate fuel paths, and the ability to maintain one machine while the others stay available are common requirements. Controls and communications also matter. Cyber security for generator controllers, monitoring that raises alarms to a person who will act, and clear procedures for operators all belong in the plan.
Our guide to hospital standby diesel design covers the hospital case, and the sister site on backup power for data centers covers the data centre side, where power continuity is measured against uptime commitments.
Installation, Environment and Maintenance
Installation quality decides whether a good generator behaves well. Location matters: the room or enclosure must be above flood risk, must allow enough combustion and cooling air to reach the engine and hot air to leave, and must support the weight and vibration of the set. Exhaust must discharge safely away from air intakes and meet local noise and emission rules. Cold sites need jacket heaters so the engine starts fast and takes load quickly, as set out in the cold weather start article.
Maintenance is where emergency generators are won or lost. Change oil, filters and coolant on schedule, test batteries and chargers, inspect belts and hoses, exercise the fuel system and keep the enclosure clean and dry. Keep a log and track trends, since a slow rise in exhaust temperature or a falling battery voltage often gives warning before a failure. A maintenance contract with a competent servicer, backed by spares, is worth more than a larger set that nobody looks after.
If you need service support, the commercial generator maintenance schedule on our diesel service site lays out a practical routine. USP&E’s operations and maintenance services can take on the whole programme, from routine inspection to emergency call-out.
Buying, Renting or Bridging
Not every emergency needs a permanent installation. If a set has failed, a planned shutdown is coming or a new site is waiting for its permanent power, rental or bridge units can fill the gap. Plan connection points in advance, a simple step like a pre-installed docking cabinet can turn a day of work into an hour. The rental hook-up readiness article explains how to prepare, and can you rent a generator answers the common questions.
When you are ready to buy, compare total cost, not purchase price. Include installation, switchgear, fuel storage, commissioning, training and the first years of maintenance. A reconditioned set with a documented test can serve well, provided the evidence is real. USP&E can supply new and used equipment and the engineering to install it, and we have delivered 150+ power projects in 45+ countries, with 25,000 MW contracted since 2002. USP&E is a full-service EPC and O&M contractor, not a broker.
Frequently asked questions
What size emergency generator do I need?
Start with a load list that separates life-safety, critical and optional loads, and record both running and starting power for each. Add margin for growth, altitude, heat and non-linear loads. A USP&E engineer can review your list and recommend a rating, and our online calculator gives a first estimate.
How long should an emergency generator be able to run without refuelling?
That depends on how long outages last in your area and how fast a delivery can reach you in a disaster. Some sites plan for a day, others for several. The figure should be a deliberate decision, supported by on-site storage, fuel testing and a supplier agreement, not simply the size of the base tank.
What does an automatic transfer switch do?
It detects the loss of normal supply, signals the generator to start, and moves the load across once the generator is ready. When normal power returns it transfers back and allows the generator to cool down. It must be rated for the load and fault current, and it needs regular testing and maintenance.
How often should I test an emergency generator?
Follow the codes and insurer requirements that apply to your site, such as NFPA 110 where it is adopted. In practice that means a routine exercise on a fixed schedule, an annual test with building load, and periodic load bank testing. Keep written records of every test.
What is the difference between standby and prime rating?
A standby rating is for emergency use with limited annual running hours. A prime rating suits sites that run for long periods on variable load. Using a standby set for prime duty shortens engine life, so match the rating on the data sheet to how the set will really run.
Why does a generator that starts easily still fail in an outage?
Common causes are weak batteries, stale or contaminated fuel, a transfer switch that does not operate, blocked ventilation, or a load larger than planned. Regular testing under real load and a disciplined maintenance routine catch most of these before an outage does.
Can USP&E supply and install an emergency generator?
Yes. USP&E can supply new and used diesel sets, design and install the system including transfer switchgear and fuel storage, commission it, and provide ongoing O&M. Call +27 10 822 2324 or email info@uspeglobal.com with your load and site details.
Plan Your Emergency Power With USP&E
Tell us the loads, the site and how long you need to ride through an outage. An engineer will come back with a sized, fuelled and testable plan. Look at current equipment on the USP&E diesel generator inventory, call +27 10 822 2324 or email info@uspeglobal.com.
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