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A power outage on a farm is more than an inconvenience. Cows still need milking, milk in the bulk tank must remain chilled, and heat and ammonia can rise quickly in a closed poultry house when its fans stop. Many rural farms sit at the end of long overhead lines, where outages can last longer and repairs may take more time.
This guide covers how to identify the farm loads a generator must support, what animal-welfare requirements apply to backup ventilation, how to size a generator for those loads, and how a tractor-driven PTO generator compares with a fixed standby unit. It draws on datasheet figures for the standby generators in our range and two real farm installations.
What does a farm generator need to run?
Begin with the equipment that protects animal welfare and the farm’s product, then exclude non-essential loads.
On a dairy farm, priority equipment includes the milking parlour’s vacuum pumps, milk pumps and controls; bulk milk tank cooling; cattle-shed ventilation fans; feed systems; and yard lighting and small power needs. Milk must stay cold in the bulk tank, or it may be rejected when collected.
On a poultry unit, priority equipment includes ventilation fans, automated feeding lines, lighting programmes and environmental controls, all of which rely on mains electricity throughout the day and night.
Typical running loads for common farm equipment include:
| Equipment | Typical running load | Notes |
|---|---|---|
| Milking vacuum pump | 4 to 7.5 kW | Start before other motors |
| Milk cooling compressors | 3 to 7.5 kW | Priority load, starting surge |
| Electric water heater | 3 to 6 kW | Steady load |
| Wash pump | 2 to 2.5 kW | Short surge |
| Feed auger or conveyor | 1.5 to 4 kW | Short surge |
| Grain dryer fan | 7.5 to 15 kW | Large starting load |
| Lighting, small power | 2 to 5 kW | Light load |
| Ventilation fans | Read from each fan's rating plate | Add up all fans on the circuit |
These are common ranges. For ventilation fans, use the values shown on your own equipment’s rating plates instead of depending on a general estimate.
What welfare rules require for backup ventilation
The Welfare of Farmed Animals (England) Regulations 2007 include several requirements in Schedule 1 that relate to backup power:
- Automated or mechanical equipment essential to animals’ health and wellbeing must be checked at least once daily.
- If animals’ health and wellbeing rely on an artificial ventilation system, an adequate backup must ensure sufficient air renewal if the system fails. An alarm must also function even when the main electricity supply is interrupted.
- The backup system must receive a thorough inspection, and the alarm must be tested, at least once every seven days.
The regulations do not prescribe how backup must be supplied; a standby generator is one common option. The full text is available at gensetplus.com. Wales, Scotland and Northern Ireland have their own equivalent regulations, so check the requirements for your farm’s location. This is general information, not legal advice. Your vet, assurance scheme or local authority can advise on the needs of your particular system.
Single-phase or three-phase?
Many farm buildings and smaller parlours use a single-phase supply, whereas larger parlours, grain dryers and high-powered motors often require three phase. Select a generator that matches the supply used by your essential loads.
- A three-phase set provides roughly one-third of its total capacity on each phase for single-phase loads. For instance, a 20 kVA three-phase set can supply around 6.7 kVA per phase at its prime rating.
- A single-phase set is a good fit for farms with single-phase supplies. Our range includes the Cummins C28D5-1P, rated at 22 kVA standby and 20 kVA prime at 230 V, and the Cummins C38D5-1P, rated at 30 kVA standby and 27 kVA prime.
How to size a farm generator
Follow these steps:
- List your priority loads and check each appliance’s rating plate for its running load in kW or amps.
- Identify the biggest starting load, often a vacuum pump, bulk tank compressor or dryer fan.
- Total the running loads that must operate at the same time, rather than every load on the farm.
- Keep a typical load below about 80% of the prime kW rating, leaving capacity for surges and future needs.
- Pick single or three phase to suit your electrical supply.
The table below illustrates how this applies to sets in our range, based on their prime ratings:
| Typical running load up to | Suggested size | Example model | Prime rating |
|---|---|---|---|
| About 13 kW | 20 kVA, three phase | Cummins C22D5 | 16 kW |
| About 19 kW | 30 kVA, three phase | Cummins C33D5 | 24 kW |
| About 22 kW | 30 kVA, single phase | Cummins C38D5-1P | 27 kW |
| About 32 kW | 50 kVA, three phase | Cummins C55D5EQ | 40 kW |
| About 38 kW | 60 kVA, three phase | Cummins C66D5EQ | 48 kW |
Worked examples for dairy farms
- A small dairy. A 4 kW vacuum pump, 3 kW milk cooler, 3 kW water heater and 1.5 kW of lighting add up to 11.5 kW—around 72% of the 16 kW available from a 20 kVA set. This is a suitable match.
- A medium dairy. A 5.5 kW vacuum pump, 4 kW cooler, 3 kW water heater, 2.2 kW wash pump, 1.5 kW of lighting and 2 kW feed auger use 18.2 kW in total, roughly 76% of a 30 kVA set's 24 kW capacity.
- A larger dairy. A 7.5 kW vacuum pump, 7.5 kW cooler, 6 kW water heater, 2.2 kW wash pump, 3 kW feed auger and 2 kW of lighting total 28.2 kW—about 70% of the 40 kW available from a 50 kVA set.
- Adding a grain dryer fan. The extra 15 kW fan brings the larger dairy's total to 43.2 kW. That is above a 50 kVA set's capacity and around 90% of a 60 kVA set's 48 kW. You could schedule the fan to run after milking, or select the larger generator.
Motor starting on a farm
Vacuum pumps, bulk tank compressors, augers and dryer fans can draw several times their normal running current when starting. As a general rule, a motor with direct-on-line starting should be no larger than roughly one-third of the generator's kW rating. This works out to about 5 kW on a 16 kW set, 8 kW on a 24 kW set, 13 kW on a 40 kW set and 16 kW on a 48 kW set. Soft starters can accommodate larger motors, and alternator design also affects performance, so confirm the specifications for the exact generator.
Expert tip: Use a consistent sequence when starting equipment: switch on the vacuum pump first, followed by the bulk tank compressors, rather than allowing all equipment to restart together when electricity returns. If the farm uses several large motors, discuss staged restarting or soft starters when choosing the generator.
PTO generator or standby set?
Many farmers compare a tractor-powered PTO generator with a permanently installed diesel standby set. Each option is better suited to different circumstances.
Worked examples for dairy farms
- A small dairy. A 4 kW vacuum pump, 3 kW milk cooler, 3 kW water heater and 1.5 kW of lighting add up to 11.5 kW—around 72% of the 16 kW available from a 20 kVA set. This is a suitable match.
- A medium dairy. A 5.5 kW vacuum pump, 4 kW cooler, 3 kW water heater, 2.2 kW wash pump, 1.5 kW of lighting and 2 kW feed auger use 18.2 kW in total, roughly 76% of a 30 kVA set's 24 kW capacity.
- A larger dairy. A 7.5 kW vacuum pump, 7.5 kW cooler, 6 kW water heater, 2.2 kW wash pump, 3 kW feed auger and 2 kW of lighting total 28.2 kW—about 70% of the 40 kW available from a 50 kVA set.
- Adding a grain dryer fan. The extra 15 kW fan brings the larger dairy's total to 43.2 kW. That is above a 50 kVA set's capacity and around 90% of a 60 kVA set's 48 kW. You could schedule the fan to run after milking, or select the larger generator.
Motor starting on a farm
Vacuum pumps, bulk tank compressors, augers and dryer fans can draw several times their normal running current when starting. As a general rule, a motor with direct-on-line starting should be no larger than roughly one-third of the generator's kW rating. This works out to about 5 kW on a 16 kW set, 8 kW on a 24 kW set, 13 kW on a 40 kW set and 16 kW on a 48 kW set. Soft starters can accommodate larger motors, and alternator design also affects performance, so confirm the specifications for the exact generator.
Expert tip: Use a consistent sequence when starting equipment: switch on the vacuum pump first, followed by the bulk tank compressors, rather than allowing all equipment to restart together when electricity returns. If the farm uses several large motors, discuss staged restarting or soft starters when choosing the generator.
| PTO generator | Diesel standby set | |
|---|---|---|
| Needs a person to start | Yes, hitch up, connect and run the tractor | No, with an automatic changeover it can start on its own |
| Ties up a tractor | Yes, for the whole outage | No |
| Output | Depends on tractor engine speed | Governed engine at a steady speed (1,500 rpm on our Cummins sets) |
| Best for | Occasional use where someone is always on hand | Unattended failures, nights and weekends |
Runtime estimates are based on nominal tank capacity, so leave a practical reserve. If an outage could last longer, make a safe refuelling plan. Our guide to calculating generator runtime shows how to estimate your requirements, while our guide to generator fuel contamination covers keeping stored fuel in good condition.
Noise and siting
Noise matters around livestock, and many farms are near neighbouring properties. A silent, canopied generator helps reduce noise and shields the engine from the weather. Our guide to how quiet a silent generator really is explains how to interpret dB(A) ratings, while our silent generators range outlines the available options.
Place the generator where it remains accessible in poor weather, with adequate airflow and a safe exhaust route. Diesel exhaust contains carbon monoxide, so never operate a generator indoors. The HSE's electrical safety guidance explains general principles for working safely with electricity. A qualified electrician should install a changeover arrangement for any generator connection.
Testing, maintenance and permits
A standby generator is useful only if it starts when needed. Test it regularly under load, follow the service schedule, and inspect the fuel and batteries. Our backup generator maintenance guide outlines a routine.
For intensive pig or poultry farms operating under an environmental permit, the standby generator may need to be included in the permit application. The Environment Agency's guidance on emergency backup diesel engines states that testing must be below 50 hours per year. In permit correspondence, the Agency has indicated that it can accept intensive farming operators generally testing for around an hour a week. Confirm the requirements for your permit with the Environment Agency.
Two real farm installations
A dairy farm in Northern Ireland. After several mains outages the previous winter, a dairy farm served by a long overhead line chose a Cummins C55D5EQ from stock. This three-phase silent generator is rated at 55 kVA standby and 50 kVA prime, and has a 200-litre tank. It supplies the milking parlour, bulk tank cooling, shed ventilation, feed systems and lighting. Read the full dairy farm case study.
A poultry farm near Lincoln. Concerned about winter storm outages, a poultry unit installed a Cummins C66D5EQ silent generator. It was sized to cover essential loads—including ventilation, feeding, lighting and environmental controls—rather than the entire site.
Conclusion
Choose a farm generator based on the loads that safeguard your animals and milk, keep normal demand below roughly 80% of its prime rating, and match its phase to your power supply. Consider automatic starting if no one can be there to operate it. Review applicable animal-welfare rules and environmental permit requirements, and test the generator regularly.
FAQ
What size generator does a dairy farm need?
The right size depends on the milking setup and the other equipment you need to power. Many small and medium-sized dairies need between 20 and 50 kVA. Total the loads for the vacuum pump, milk cooling, water heating, ventilation and lighting; aim to keep normal demand below roughly 80% of the generator’s prime kW rating, and account for the largest motor’s starting load.
Do poultry farms need a standby generator?
Regulations require a backup arrangement that ensures sufficient air renewal if mechanical ventilation fails, together with an alarm that operates during a mains power failure. A standby generator is a common backup option, although the regulations do not prescribe a specific method.
Can I use a tractor PTO generator instead?
It can work for occasional outages if someone is always available, but a person and tractor must remain on hand throughout the outage, and output varies with engine speed. For failures that may go unattended, a fixed standby generator with automatic starting is a better fit.
Should a farm generator be single phase or three phase?
Choose a generator that matches the supply serving your essential loads. Single-phase units are suited to single-phase farm supplies, while three-phase units are appropriate for larger parlours, grain dryers and bigger motors. With a three-phase generator, single-phase loads are limited to around one-third of the total on each phase.
How long will a farm generator run on one tank?
At full prime load, estimated running time is about 32 hours for a 20 kVA unit with a 165-litre tank, 23 hours for a 30 kVA unit, 16.7 hours for a 50 kVA unit and 14.3 hours for a 60 kVA unit. Farm loads are often below full capacity, which can extend operating time.
How often should I test a farm generator?
Test it regularly under load. Welfare rules require backup ventilation to be inspected and its alarm tested at least every seven days, making a weekly generator check a practical routine.
