Generators for Farms: How to Size Standby Power

A power cut on a farm is not an inconvenience. Cows still need milking, milk in the bulk tank has to stay cold, and in a closed poultry shed heat and ammonia can build up quickly once the fans stop. Rural farms are often at the end of long overhead lines, so outages tend to be longer and repairs slower.

This guide explains how to work out what a farm generator needs to run, what the welfare rules say about backup ventilation, how to size by load, and where a tractor-driven PTO generator fits against a fixed standby set. It uses datasheet figures from the standby generators in our range and two real farm installations.

What does a farm generator need to run?

Start with the loads that protect the animals and the product, and leave out everything else.

On a dairy farm the priority loads are the milking parlour (vacuum pumps, milk pumps and controls), bulk milk tank cooling, ventilation fans in the cattle sheds, feed systems, and lighting and small power across the yard. Milk in the bulk tank must stay cold, or it may be rejected at collection.

On a poultry unit the priority loads are ventilation fans, automated feeding lines, lighting programmes and environmental controls, which all run on mains electricity around the clock.

Typical running loads for common farm equipment:

These are typical ranges. For ventilation fans in particular, take the figures from the rating plates on your own equipment rather than relying on a generic number.

What the welfare rules say about backup ventilation

The Welfare of Farmed Animals (England) Regulations 2007 set out several conditions in Schedule 1 that bear on backup power:

• Automated or mechanical equipment that is essential for the animals' health and wellbeing must be inspected at least once a day.
• Where the animals' health and wellbeing depends on an artificial ventilation system, you must provide an appropriate backup system to guarantee enough air renewal if the system fails, and an alarm that will operate even if the principal electricity supply has failed.
• The backup system must be thoroughly inspected, and the alarm tested, at least once every seven days.

The regulations do not say how to provide the backup, and a standby generator is one common way to do it. You can read the text on legislation.gov.uk. Wales, Scotland and Northern Ireland have their own equivalent regulations, so check the rules that apply where your farm is. This is general information, not legal advice. Your vet, assurance scheme or local authority can confirm what your specific system needs.

A weekly inspection routine fits naturally with a weekly generator test. Our guide to how often generators should be tested sets out a sensible routine.

Single phase or three phase?

Many farm buildings and smaller parlours run on a single-phase supply, while larger parlours, grain dryers and bigger motors often need three phase. Choose the generator to match the supply that feeds your priority loads.

• A three-phase set gives each phase only about a third of its total for single-phase loads. A 20 kVA three-phase set, for example, carries about 6.7 kVA per phase on its prime rating.
• A single-phase set suits farms with single-phase supplies. Our single-phase range includes the Cummins C28D5-1P, rated 22 kVA standby and 20 kVA prime at 230 V, and the Cummins C38D5-1P, rated 30 kVA standby and 27 kVA prime.

Our guide to single phase vs three phase explains how to decide.

How to size a farm generator

Work through these steps:

1. List the priority loads and find each running load on its rating plate, in kW or amps.
2. Find the largest starting load, usually a vacuum pump, bulk tank compressor or dryer fan.
3. Add up the running loads you need at the same time, not everything on the farm.
4. Aim for a typical load under about 80% of the prime kW rating, which leaves headroom for surges and growth.
5. Choose single or three phase to match your supply.

The table below shows how that works for sets in our range, using prime ratings:

For more detail on each size, see our guides to what a 20 kVA generator can run, the 30 kVA generator range, what a 50 kVA generator can run and what a 60 kVA generator can run.

Worked examples for dairy farms

• A small dairy. A 4 kW vacuum pump, a 3 kW milk cooler, a 3 kW water heater and 1.5 kW of lighting total 11.5 kW, about 72% of a 20 kVA set's 16 kW. That fits.
• A medium dairy. A 5.5 kW vacuum pump, a 4 kW cooler, a 3 kW water heater, a 2.2 kW wash pump, 1.5 kW of lighting and a 2 kW feed auger total 18.2 kW, about 76% of a 30 kVA set's 24 kW.
• A larger dairy. A 7.5 kW vacuum pump, a 7.5 kW cooler, a 6 kW water heater, a 2.2 kW wash pump, a 3 kW feed auger and 2 kW of lighting total 28.2 kW, about 70% of a 50 kVA set's 40 kW.
• Adding a grain dryer fan. An extra 15 kW fan takes the larger dairy to 43.2 kW, which exceeds a 50 kVA set and is about 90% of a 60 kVA set. Either stage the loads so the fan runs after milking, or choose the larger set.

Motor starting on a farm

Vacuum pumps, bulk tank compressors, augers and dryer fans draw several times their running current as they start. As a rough guide, a motor started direct-on-line should be no more than about a third of the generator's kW rating. That is around 5 kW on a 16 kW set, 8 kW on 24 kW, 13 kW on 40 kW and 16 kW on 48 kW. Soft starters allow larger motors, and the alternator's design matters, so check the exact set.

> Expert tip: Start the loads in a fixed order, with the vacuum pump first and the bulk tank compressors next, rather than letting everything restart at once when the power returns. If a farm has several large motors, ask about staged restart or soft starters when you specify the set.

PTO generator or standby set?

Many farmers weigh up a tractor-driven PTO generator against a fixed diesel standby set. They suit different situations.

A fixed standby set has its own engine, fuel and controls. The Cummins PowerStart 0600 controller supports automatic mains failure operation, so with a suitable automatic changeover in place the set can start and take the load with nobody on site. Our guides to automatic versus manual transfer switches and generator ATS installation explain the switching side.

Not every generator is suited to unattended use. The Pramac GBW22P in our range is a manual key-start set, so it would not start by itself in a mains failure without extra equipment. For a farm, an automatic-start model is usually the safer choice, and our guide to generator remote monitoring systems covers ways to be alerted when something goes wrong.

Fuel use and run times

The table shows run time at full prime load on a full tank. Most farm loads sit below full load, so real run times are longer.

Run times use nominal tank capacity, so allow a reserve in practice. If the farm could be cut off for longer, plan how you will refuel safely. Our guide to how to calculate generator runtime shows how to work out your own figures, and our guide to generator fuel contamination explains how to keep stored fuel in good condition.

Noise and siting

Noise is a genuine consideration around livestock, and many farms sit close to neighbouring properties. A silent canopied set reduces noise and protects the engine from weather. Our guide to how quiet a silent generator really is explains how to read the dB(A) figures, and our silent generators range lists the options.

Position the set where it can be reached in bad weather, with good airflow and a safe route for exhaust. Diesel exhaust contains carbon monoxide, so never run a generator inside a building. The HSE's electrical safety guidance covers the general principles of working safely with electricity, and any generator connection should be made through a changeover arrangement fitted by a qualified electrician.

Testing, maintenance and permits

A standby generator only helps if it starts. Test it regularly on load, keep to the service schedule, and check fuel and batteries. Our backup generator maintenance guide sets out a routine.

If your farm is an intensive pig or poultry unit with an environmental permit, the standby generator may be part of the permit application. The Environment Agency's guidance on emergency backup diesel engines says you must test for less than 50 hours a year. In permit correspondence, the Agency has said that intensive farming operators generally test for about an hour a week, which it can accept. Check the position for your own permit with the Environment Agency.

Two real farm installations

A dairy farm in Northern Ireland. After losing its mains supply several times the previous winter, a dairy farm on a long overhead line took a Cummins C55D5EQ from stock. It is a three-phase silent set rated 55 kVA standby and 50 kVA prime, with a 200-litre tank. The set protects the milking parlour, bulk tank cooling, shed ventilation, feed systems and lighting. Read the full dairy farm case study.

A poultry farm near Lincoln. A poultry unit worried about winter storm outages took a Cummins C66D5EQ silent generator, sized around the farm's essential loads of ventilation, feeding, lighting and environmental controls rather than the whole site. Read the poultry farm case study.

Frequently asked questions

What size generator does a dairy farm need?

It depends on the milking system and the other loads you want to run. Many small to medium dairies fall between 20 and 50 kVA. Add up the vacuum pump, milk cooling, water heating, ventilation and lighting, keep the typical load under about 80% of the generator's prime kW rating, and check the largest motor's starting load.

Do poultry farms need a standby generator?

The regulations require a backup system to guarantee enough air renewal if an artificial ventilation system fails, plus an alarm that works if the mains has failed. A standby generator is one common way to provide the backup, but the regulations do not specify the method.

Can I use a tractor PTO generator instead?

Yes for occasional use where someone is always on hand, but it needs a person and a tractor for the whole outage, and its output depends on engine speed. A fixed standby set with automatic start suits unattended failures better.

Should a farm generator be single phase or three phase?

Match it to the supply feeding your priority loads. Single-phase sets suit single-phase farm supplies, while three-phase sets suit larger parlours, grain dryers and bigger motors. On a three-phase set, single-phase loads are limited to about a third of the total per phase.

How long will a farm generator run on one tank?

At full prime load, about 32 hours on a 20 kVA set with a 165-litre tank, 23 hours on a 30 kVA set, 16.7 hours on a 50 kVA set and 14.3 hours on a 60 kVA set. Most farm loads are lower, so run times are longer.

How often should I test a farm generator?

Regularly, and on load. The welfare rules require backup ventilation to be inspected and the alarm tested at least once every seven days, so a weekly generator check is a sensible routine.

Conclusion

Size a farm generator around the loads that protect your animals and your milk, keep the typical load under about 80% of the prime rating, match the phase type to your supply and choose automatic start if nobody will be on hand. Check what your welfare rules and any environmental permit require, and test the set regularly.

> Planning standby power for your farm? Global Generators is an authorised Cummins reseller and an authorised Grupel and Pramac dealer, with new diesel generators in stock for UK delivery and export. Browse the stock list, then contact our team on +44 1302 700200 or at enquiries@global-generators.com with your loads, supply type and location, and we will help you size a set.