Standby and Prime Power Generators - Elite Agri Solutions

Standby and Prime Power Generators

Jon Jon|Mar 27, 2023

Elite Agri Solutions strives to provide background information on topics which are hard to research. In cases where no reputable print resources were available for us to reference, we interviewed industry experts, so it is inevitable that the contents of this document will contain inaccuracies and bias. Use this as a resource to help you ask the right questions, not as a source of definitive answers. Elite Agri Solutions and its employee will not be responsible for the consequences of any decision made based on this guide. Where text or data has been copied directly, the sources have been noted, otherwise it can be assumed that all the information in this guide has only been curated by Elite Agri Solutions and is not our original property.

(All information in this document was gathered from generator technicians, equipment manufacturers and OMAFRA)


Self-contained – Permanently installed in a building or in their own generator shed.

  • Quiet Operation
  • Fuel Efficiency
  • Option to be set up for automatic transfer
  • Potential for natural gas or propane fueled operation.
  • Designed for continuous operation
  • Models larger than 100 kW are available

PTO – Either portable, or permanently installed with access for a tractor to be sitting outside.

  • Portable
  • Lower equipment cost because utilizes a tractor’s engine power
  • Ease of maintenance
  • Will never be rendered useless because of engine failure, just hook up a different tractor.
  • Practical capacity is limited to 100 kW

Prime Power Generation

In some applications it may be more economical for a farmer to continuously generate their own electricity. This is usually only a viable option if the cost of a grid connection and services is unreasonably high, in remote locations where there simply is no service, or if large motors with high start-up loads trigger high demand charges from the utility company (usually grain elevators).

Self-contained standby generators typically see very few hours of operation in a year, in this way the generator lifespan is unnecessarily long (if a generator is rated for 8,000 hours and you put 80 hours on it a year it will theoretically last 100 years). It then may become economical to use the generator for ‘peak shaving’ this is the standby generator would run for a few hours during the middle of the day when time of use billing is the highest. In this way, the life of the generator is still acceptable, but significant electricity saving can be garnered during the highest use and highest price times of the day. Generator or barn controllers can be set up to automatically peak shave based on the time of day and when high enough loads are engaged.

Because it is not merely an emergency application, prime power generators may be required to meet more stringent emissions regulations.

Many prime power installations utilize the waste heat created by the generator for either heating buildings or for drying grain.

Sizing to handle load is important for prime generators, a consistent load of 60% is generally accept as the rule of thumb for maximum engine life. Therefore, a prime power generator is typically sized 40% larger than a standby generator would be for the same load.

Engine life and maintenance are factors that greatly affect the profitability of prime power generation. Engine life is a hard factor to estimate, the difference of a few thousand hours of operating time can make the system profitable or not.

The need for maintenance and the inevitability of some mechanical failures makes it important to have redundancy built into a prime power system. Redundancy can be in the form of a grid connection, a standby generator or a synchronized system of generators.

A synchronized system of smaller generators is used so that the generators are always operating a nearly optimal range. This would mean that, as load is increased past a threshold, an additional generator would start up and share the load with the base generator.

For a grain handling facility, short term failure might not be critical, however in a poultry operation the grid should not be relied on as a sole back up. If your generator is being rebuilt and the grid is interrupted, you could have a barn full of dead livestock by the end of the day.

At the end of an engine’s effective life, it can either be re-built or replaced with a new engine based on cost analysis. Buying a generator that has a warranty for a certain life span can add security to the profitability calculation.


Generators must be connected to the electrical utilities power supply via a double throw switch to prevent feedback into the utilities system. A double throw switch fully disconnects from one source before connecting to the other. This makes it impossible to be connected to both the generator and the gird at the same time.

Not using a transfer switch would make it dangerous for any hydro workers who might be in contact with the line, presuming it to be dead. Your generator would also be trying to power your neighbours, and everyone connected to your line which would quickly exceed the capacity of your generator and cause it to burn out

The switch should be located at the main farm service entrance if more than one building requires emergency power.

The transfer device should be sized according to the rating of each service connected. It must be 100% of the largest service and 75% of the balance of the services


The standby generator should be large enough to start the largest motor and operate all essential portions of the service load. Electrical loads that are critical may include ventilation, watering, milking, feeding and other loads. Electrical heating loads may not be practical to operate with a generator as they use a significant amount of capacity.

Generators are rated by the amount of electrical power they generate. Power is rated by two different terms; kilovolt-amperes (kVA) and Watts (W) or kilowatts (kW); 1 kW = 1000W.

The first term, kVA is the ‘apparent power’ drawn by a motor, while kW is the ‘true power’. The ratio of   is called the power factor. k is simply a prefix on both terms that means x1000. For single motors under 1 hp, the power factor can be assumed to be 0.65, while for large single-phase motors over 5 hp the power factor can be assumed to be 0.95. A power factor of 0.80 can be assumed for motors between 1 hp and 5 hp. Resistive loads such as heaters and lights have a power factor of 1.

VA = Volts (V) x Amps (A)]

W = VA x Power Factor

The generator must provide power at the same voltage and frequency as that delivered by the utility’s grid, in Ontario this is 120/240V and 60hz AC.

Electric motors can require up to 12x the regular operating power on start up. Typically, a value of 4 times the operating watts is used for start-up. Therefore, generators often carry two ratings, a continuous rating and a peak rating. Resistive loads (lights and electric heaters) draw nearly the same power for start-up and operation.

Standby Generators that are setup to engage automatically, must be sized to have capacity enough to handle any loads that would be left in the on position. For example, if several ventilation fans are controlled by mechanical switches and the power goes out on a hot summer’s day the generator will have to deal with a load much greater than if the fans are engaged individually and allowed to get up to speed.

Many electronic ventilation controllers have a boot up period that will take a few seconds after power is transferred to the generator. Controllers can also be programmed to re-engage fans in stages if it is known that the generator doesn’t have enough capacity for a maximum start up draw. If overloading occurs, both the generator and the loads could be damaged.

Part electrical load systems are those typically used with PTO driven generators or portable self-contained generators. In these systems the generator is only intended to be used to power the essential loads. A person must detect the outage, hook up the generator and throw the transfer switch before power is live to the system. It is then necessary to turn on loads one at a time to ensure that the generator is operating within its capacity.

Table1. Motor output in horsepower (hp) vs. start-up and operating watts for single-phase motors at 240 V. For start-up, four times operating levels are assumed, although it can be two to twelve times as much. See electrical contractor for details. * (OMAFRA)

 Motor Output (hp)  Startup Watts (W) (assuming 4 times operating)  Typical Operating Watts (W)
 ½  2,300 W  575 W
 ¾  3,200 W  800 W
 1  4,300 W  1,075 W
 2  7,400 W  1,850 W
 3  12,300 W  3,075 W
 5  18,200 W  4,550 W
 7 ½  27,000 W  6,750 W
 10  36,000 W  9,000 W

*Data is typical for capacitor start, capacitor start/capacitor run, and permanent split capacitor motors used for agricultural use (compiled by Enertech Solutions Inc. for the Ice Storm Recovery Assistance Program 1999).


OMAFRA Steps for Generator Sizing If Using a Full Electrical Load System

  1. List operating and start up watts of all equipment that must be operated on standby power. Check equipment name plates or use table 1 to estimate the loads. Note that a 120V motor and a 240V motor will both use the same wattage. Note that lights and heaters have the same load on start up as when running.
  2. Check your list and determine which combination of motors will not run at the same time. Use the value for the combination that has the highest watt requirement. For example, if you have a 4hp hammer mill and a 5hp silo unloader it is likely not necessary to be running both at the same time.
  3. Add 20% for future expansion then round up to the nearest 5 kW.
  4. For a PTO generator, to size the tractor, accordingly, allow for 2 brake horsepower per 1kW of electrical output by the generator.

OMAFRA Steps for Generator Sizing If Using a Part Electrical Load System

  1. List operating and start up watts of only critical motors that must operate on standby power in order of motor size.
  2. Determine the peak power required as each of the loads is added in sequence, starting with the largest load. i.e. The first stage in the sequence is the starting wattage of the largest motor. The second stage is the operating wattage of the largest motor plus the starting wattage of the second largest motor. Continue this sequence for each of the loads. Then find the sequence where the wattage requirement is the greatest and use that as the peak level.
  3. Add 20% for future expansion, then round to the nearest 5 kW.
  4. For a PTO generator, to size the tractor, accordingly, allow for 2 brake horsepower per 1kW of electrical output by the generator.

You will note that the generator size needed using the part electrical load calculation is much smaller than the size needed full electrical load. A partial load system will still cover most of the farm’s needs and will be significantly cheaper, however, it is not well suited to situations where more than a few minutes without power could be critical. For example, a poultry barn with mechanical ventilation would not be a good fit for a partial load system.

Alternative Method of Sizing


A very accurate way to size a prime power generator or a standby generator is to log actual usage with a power monitor. If deadlines in the building process won’t allow enough turnaround time for monitoring to happen on the barn, a barn with very similar equipment could be monitored instead. This power monitoring will be able to show the size of large load steps much more accurately than theoretical methods. This is the best method for sizing a prime power generator, as it could assess the potential of having multiple stages of generator capacity. If the primary load on the generator is for ventilation it might be beneficial to have a larger generator operate during the day when the largest loads are and have a smaller generator operate at night as the temperature cools and ventilation in reduced.

Fuel Choice

Typically, gasoline, diesel or propane tanks for standby generators should be sized to supply three days of continuous operation. If considering prime power, consider the total fuel cost including delivery and storage. Storage size for prime power should be based on delivery availability and the specific consumption of the generator. Natural gas has the benefit of needing no storage, however it is not available at all sites and pipeline installation cost increases with distance.

Regulators for propane fueled engines will need periodic service as a black tar will build up over time. Natural gas is a cleaner fuel and will not need regular service on its regulators.

Gasoline and diesel should be replaced every 24 months or have stabilizing chemicals added annually. It is always important to keep the tank reasonably full. Less air space in the tank means less moisture related issues.

For a rough cost comparison, operating a generator on natural gas would cost 1/3 compared to diesel. Operating on propane is about 1/2 the cost of diesel.

Diesel generators generally have quick response to load change and can handle a full loading in one step, while a natural gas or propane engine will have to take the load over multiple step.

Because of the nature of exhaust gases, different regulation applies for diesel fueled engines if they are going to be installed inside a building.


Generators should not be exposed to any harsh conditions, particularly any corrosive livestock environment.

Maintenance for a PTO generator includes storing it in a dry place, test running it every few months, and greasing the PTO shaft and bearings.

If buying a new self-contained generator set, make sure to ask for training in the manufacturer’s recommended maintenance regime. A general rule of thumb is to change engine oil annually, every 250 hours or to sample the oil and send it away to a lab. Sampling oil is penny-wise on large engines, not only to avoid replacing hundreds of litres of oil pre-maturely, but to identify any abnormal wear that could indicate further maintenance is needed.

For gaseous fuel generators, training could be provided to the farmer for regulator maintenance.

PTO Operation Considerations

Ensure that the generator is securely lagged into 6 inches of concrete or mounted on a sturdy trailer or three-point hitch that will not allow it to be flip under heavy loading.

Ensure that the PTO shaft is aligned as straight as possible, greased regularly and is rated for the horsepower required.

Ensure that all electrical loads are shut off before the generator is started. Once the generator is running at its rated speed slowly re-apply one load at a time. It is useful to clearly label all circuits at the panel box so that you know you are bringing the correct circuits online.

If the breaker on the generator trips, go to the panel box and make sure that your 120V loads are balanced, the generator breaker will trip based on the highest loading, not the average across the two sides of the panel.

Air inlet sizing

OMAFRA says to provide half a square foot of inlet and outlet opening for each 1kw of generator capacity and to keep exhaust pipes 6” away from combustible materials. This is a rule of thumb and should be checked with the municipal building official to ensure compliance to applicable law.

Generator Building Requirements

If using a gaseous fuel, a licenced gas fitter must install all fuel related equipment and TSSA rules will apply. To install the electrical equipment related to the generator, a licensed electrician must be employed, and ESA rules will apply.

Note that gas lines cannot be placed under a concrete slab, if the line were to leak gas would be trapped by the slab and funneled into the building. Therefore, all gas lines must be above ground when they enter a building.

Farm buildings with a human occupancy of less than one person per 4mof floor area are exempt from the requirements of the Ontario Fire Code. The National Farm Building Code applies in this scenario and says: Fire resistance Ratings

1) Except as provided in Articles and, fuel-fired appliances in farm buildings of low human occupancy shall be

  1. a) Located in a service room or service space designed for that purpose, and
  2. b) separated from the remainder of the building by a fire separation having a fire-resistance rating of not less than 30 min.

In Appendix A of the National Farm Building Code a table is given for the fire resistance ratings of various assemblies.

Table A-

Estimated Fire-Resistance Ratings for Assemblies

Structure Membranes Fire Resistance, Min
38 mm x 89 mm wood studs 

400mm O.C.

11.0mm Douglas Fir Plywood, OSB or waferboard (both faces) 


14.5mm Douglas Fir plywood, OSB or 15.5mm waferboard (both faces)


4.5 mm asbestos cement board over 9.5 mm gypsum wall board (both faces)


12.7 mm gypsum wall board (both faces)


8.0mm Douglas Fir plywood or 9.5 mm OSB or waferboard (both faces) with stud spaces filled with mineral wool batts
















38 mm x 89 mm wood studs 

600mm O.C.

11.0mm Douglas Fir plywood, OSB or waferboard (both faces) with stud spaces filled with mineral wool batts 


4.5 mm asbestos cement board over 9.5 mm gypsum wall board (both faces)


12.7 mm Type X gypsum wallboard (both faces)











Steel studs 400 mm o.c. 4.5 mm asbestos cement board over 9. Mm gypsum wallboard (both faces) 50
Wood floor and roof joists (38 mm thickness) 400 mm o.c. 


Open web steel joist floors and roofs with ceiling supports 400 mm o.c.

12.7 mm Type X gypsum wallboard ceiling 


4.5 mm asbestos cement board on 9.5 mm gypsum wall board ceiling


26 mm Portland cement and sand or lime and sand plaster on metal lath ceiling










90 mm hollow concrete blocks (normal weight aggregate) 


140 mm hollow concrete blocks (normal weight aggregate)


190 mm hollow concrete blocks (normal weight aggregate)








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