Sizing an Agricultural Generator: A Practical Guide for Greenhouses, Irrigation and Cold Storage

Nothing exposes a bad energy specification faster than a hot summer afternoon in a commercial greenhouse. Grid drops, the automatic transfer switch calls the generator, the fogging pump tries to start, the voltage collapses and the alarm panel lights up before the crop even notices. By the time the operator understands what happened, the inside temperature has climbed several degrees and a day's yield is at risk. In almost every case the root cause is not the generator brand — it is the sizing method.

Sizing an agricultural generator is not the same as sizing a residential standby set. Farms have large single-motor loads (fogging pumps, chiller compressors, irrigation boosters, ventilation fans), harsh ambient conditions, dust, altitude and — on prime-power sites — thousands of hours per year on load. A specification that ignores any of these produces a genset that looks fine on the datasheet and fails in the field. This guide walks through the five factors that actually decide the rating: continuous load, largest-motor inrush, altitude derate, ambient temperature derate and future headroom.

Step 1: Build an honest continuous load list

The starting point is a load list of everything the generator must supply simultaneously on the worst day of the year. On a high-tech greenhouse that typically includes: climate computer and controllers, HAF fans, heating circulation pumps, boiler burner, dehumidifiers, CO₂ dosing pumps, screen motors, fogging pumps, main irrigation pump, fertigation dosing station, LED grow-lights (if supplemental lighting is used), pack-house lights and the network switch stack. Add nameplate kW for each item, apply a realistic simultaneity factor (rarely below 0.7 on a modern greenhouse) and record the result as the continuous load.

The single most common sizing error is to build this list from memory instead of from the panel schedule. Every hidden load — the emergency lighting inverter, the perimeter cameras, the office HVAC — reappears at the worst possible moment. If in doubt, add 5–10% to the continuous total as a documented allowance rather than pretending those loads do not exist.

Step 2: Add the largest single motor's starting current

Direct-on-line motors draw four to seven times their nameplate current for the first few seconds of a start. On a 15 kW fogging pump, that is 75–105 kW of transient demand on top of whatever else is already running. The genset must supply that inrush without letting the voltage or frequency fall outside the range the climate computer and drives will tolerate.

Three starter types dominate agricultural sites. Direct-on-line (DOL) is the cheapest and worst for the generator: assume an inrush factor of 6. Soft starters cut the peak to roughly three times nameplate. Variable-frequency drives (VFDs), now standard on any modern fogging or irrigation pump, ramp the current up smoothly and keep the effective inrush around 1.5–2× nameplate. The sizing method is always the same: continuous load plus (largest motor × inrush factor) is the starting demand the generator must survive.

Step 3: Convert to kVA using a realistic power factor

Generators are rated in kVA, not kW. Divide the starting demand by the plant's power factor to get the required kVA. On a modern greenhouse with VFDs and correctly-specified motors, a power factor of 0.8 is a safe design assumption; heavy resistive heating pushes it higher, while old motors with no correction pull it lower. If your electrical designer has calculated an actual site power factor, use that number — but do not use the motor nameplate PF without a plant-wide check.

Step 4: Derate for altitude and ambient temperature

Every generator manufacturer publishes derating curves for altitude and ambient temperature, and every specification has to apply them. A conservative rule for a first-pass estimate is 1% capacity loss per 100 m above 1,000 m of elevation, plus 2% per °C of ambient above 40 °C. A greenhouse at 1,600 m altitude with a design ambient of 45 °C loses roughly 6% for altitude and 10% for temperature — 16% total. A genset that looks like 100 kVA on the datasheet will deliver about 84 kVA on that site. Ignore this and the generator hits its full-load limit long before the load actually reaches nameplate.

Step 5: Add explicit headroom for the future

Agricultural projects grow. A second glasshouse span is added, a supplemental LED lighting system goes in, a pack-house is bolted on, a cold room appears. A generator sized exactly to today's load leaves no margin for any of that. A 20–30% headroom is not padding; it is the difference between a genset that survives the project's second phase and one that has to be replaced. Document the headroom in the specification so the manufacturer's proposal is comparable across bidders.

Prime, standby or emergency — pick the duty class deliberately

Manufacturers rate the same physical genset differently depending on how it will be used. Emergency Standby Power (ESP) assumes very few hours per year on load; Prime Power (PRP) tolerates unlimited hours at variable load; Continuous Power (COP) is a base-load rating for full-time generation. On a farm with a stable grid and rare outages, ESP is enough. On a farm where the grid drops for hours daily, or on an off-grid site, PRP or COP is mandatory — and the same kVA number costs meaningfully more. Choosing the wrong duty class is one of the fastest ways to void a warranty.

Fuel: diesel, natural gas or biogas

Diesel remains the default for backup and remote agriculture: highest energy density, fastest start, easiest to service. Natural gas is cheaper per kWh where pipeline supply is available and produces significantly lower particulate emissions — attractive for greenhouses close to urban markets. Biogas from on-farm digesters is compelling on livestock or fruit-processing sites where the feedstock already exists; the payback is often measured in months once the digester is amortised. On a hybrid site, the fuel choice usually follows the payback model rather than the other way around.

Do not forget ATS and UPS

A generator without an automatic transfer switch (ATS) is a manual generator. On a commercial greenhouse, an outage at 14:00 in July does not wait for the operator to walk to the switchgear. A properly specified ATS restores power in 8–15 seconds; a UPS bridges that transfer window so the climate computer, controllers, alarms and network stay online continuously. Every commercial protected-agriculture project should specify genset + ATS + UPS in one integrated brief, not as three separate purchases.

Hybrid solar-diesel is the modern default

For prime-power sites — off-grid greenhouses, weak-grid irrigation projects, remote nurseries — a hybrid solar + battery + generator system almost always beats pure diesel on total cost of ownership. Solar covers the day, the battery covers evening and morning transitions, the generator only starts when demand exceeds solar and battery. Payback is typically 3–5 years, and green trade finance is often available specifically for this configuration. If the project has any daytime-heavy load — irrigation, cooling, fertigation — a hybrid design should be quoted alongside the pure-diesel option, not instead of it.

From load list to RFQ

The output of this exercise is not a purchase order. It is a specification: a load list, a starting-demand number, a chosen power factor, an altitude and ambient derate, a headroom margin, a duty class, a fuel and a switching philosophy. That specification is the input to a vendor-neutral RFQ. Opened to qualified global manufacturers, it produces comparable proposals — same duty class, same derate assumptions, same warranty and after-sales scope — which is the only reliable way to compare a Perkins, a Cummins, a Volvo and an FG Wilson head-to-head.

Key takeaways

Sizing starts with an honest load list, not a memory of the last project.

The largest single motor's inrush usually decides the kVA number.

Altitude above 1,000 m and ambient above 40 °C impose real derates — apply them explicitly.

Document the headroom margin so proposals are comparable.

ATS and UPS are part of the same specification as the generator, not separate purchases.

Hybrid solar + battery + diesel almost always beats pure-diesel prime power on TCO.

Related hubs and tools

- Energy & Backup Systems — https://seedmatchgroup.com/energy-backup-systems

- Energy Calculators — https://seedmatchgroup.com/energy-backup-calculators

- Solar Greenhouses — https://seedmatchgroup.com/solar-greenhouses

- Irrigation Center — https://seedmatchgroup.com/irrigation-center

- Climate Control Systems — https://seedmatchgroup.com/climate-control-systems

- Project Financing — https://seedmatchgroup.com/financing

- RFQ Builder — https://seedmatchgroup.com/rfq-builder