How to Size a Generator for Facility Backup Power

By GoVets Editorial Team

Start with the outage decision: what must remain operational, and what can wait? For NYC agency facilities, fleet garages, and maintenance shops, that decision belongs with your agency's facilities, safety, and emergency-management leads before procurement begins comparing generators.

The same approach applies to federal, state, and local facility teams. To understand how to size a generator, inventory critical loads, total their running demand, evaluate motor-start demand, and allow an appropriate reserve. Then have qualified professionals validate the equipment rating, fuel plan, electrical compatibility, and connection design.

Safety first: carbon monoxide kills. Operate portable generators outdoors only, well away from windows, doors, and vents. Never operate one in a garage or other enclosed or partially enclosed space, even with doors open.

General information only: This article is not legal or engineering advice. Consult a licensed electrician, your authority having jurisdiction (AHJ), agency counsel, and procurement officials for your specific installation and purchase.

1. Decide what must stay running

Define the required level of service during an outage before choosing equipment. A generator intended to keep selected essential circuits operating is a different requirement from one intended to support an entire building or a temporary job site.

• Life-safety and code-required loads: Have qualified personnel identify applicable requirements and any dedicated emergency-power arrangements. Do not treat these as discretionary loads.
• Security and access: Review access controls, surveillance, essential lighting, and security communications.
• IT and communications: Include necessary network equipment, servers, phones, and radios. Confirm any need for uninterrupted power while a generator starts.
• Refrigeration: Identify equipment protecting temperature-sensitive materials and account for compressor starts.
• Sump and water systems: Identify pumps needed to manage water intrusion or maintain essential services.
• Heating controls: Consider controls, circulating pumps, and other equipment needed for the agency's cold-weather operating plan.
• Fleet and facility operations: Identify the specific shop equipment and maintenance activities that must continue, rather than assuming every tool or lift is essential.

Document what can wait: break-room appliances, nonessential office equipment, decorative lighting, and deferrable shop work may not need backup power. Your responsible agency leads should approve these exclusions, including any seasonal differences.

For temporary work, identify which corded tools will operate together. The Power Tools & Accessories category can support product identification, but use actual manufacturer electrical data for your load inventory.

2. Build a load-inventory worksheet

Record running and starting demand for each load using manufacturer data and qualified technical review. Running watts describe normal operating demand. Starting or surge demand describes the brief additional demand when equipment starts, particularly motors in HVAC equipment, compressors, pumps, and shop machinery.

Do not apply one guessed starting multiplier to every motor. Starting demand depends on the equipment and starting method. Ask your electrician or equipment manufacturer to resolve missing data rather than filling gaps with assumptions.

Illustrative load-inventory worksheet: All figures below are generic teaching examples, not product specifications, typical-load claims, or recommendations for your facility. Replace them with verified data.

Keep a supporting record of equipment identifiers, quantities, circuits, operating schedules, and the source of each figure. A motor designation helps your electrician see where starting demand needs particular attention.

If the nameplate gives amps rather than watts, do not guess. For a single-phase load, volts multiplied by amps gives volt-amperes (VA); actual watts also depend on power factor. Three-phase calculations require additional factors. Have a licensed electrician interpret the data and verify both kW and kVA requirements.

3. Calculate running demand, motor starts, and reserve

Calculate continuous running demand and the highest expected starting event separately. The worksheet provides a planning baseline, not a final engineering selection.

Total the critical running loads

Add only the rows marked Yes in the example:

1,500 + 500 + 800 + 3,000 + 2,400 + 1,200 + 800 + 3,000 + 400 = 13,600 running watts.

Account for the highest starting event

For this simplified example, assume only one motor starts at a time while the other critical loads are already running. Check each motor's additional starting demand: starting watts minus that motor's running watts.

• Refrigeration compressor: 7,200 − 2,400 = 4,800 additional watts.
• Sump pump: 3,600 − 1,200 = 2,400 additional watts.
• Heating circulator: 2,400 − 800 = 1,600 additional watts.
• Shop lift: 9,000 − 3,000 = 6,000 additional watts.

The largest increment is 6,000 watts. The illustrative starting-event total is 13,600 + 6,000 = 19,600 watts.

Subtracting the starting motor's running demand first prevents double-counting it. Equivalently, replace that motor's running watts in the total with its starting watts.

The one-motor-start assumption must be verified. Motors may restart together after an outage or cycle together later. If multiple starts can overlap, qualified personnel must evaluate that combined event. Do not assume that a transfer switch automatically sequences loads or that an operator can safely defer required systems.

Add an appropriate reserve and convert to kW

For illustration only, use a 25% reserve allowance to show the arithmetic. This is a planning assumption, not a universal margin or code requirement:

• 19,600 watts × 1.25 = 24,500 watts.
• 24,500 watts ÷ 1,000 = 24.5 kW.

The result is a planning discussion figure, not a recommendation to buy a particular unit. Your electrician and generator specialist should validate continuous capacity, transient motor-start capability, surge duration, power factor, voltage and frequency response, duty rating, and site-related derating.

Reserve capacity should reflect documented uncertainty and future loads without excessive oversizing. A starting-watt rating is not a continuous rating. Verify both against the actual load profile and the manufacturer's data.

Do not size on running watts alone. Do not guess from building square footage alone: buildings of similar size can contain very different electrical loads.

4. Confirm generator type, phase, and voltage

Choose portable or installed standby equipment around the approved operating requirement and connection plan, not a generic building-size rule.

Portable generators can support temporary job-site power or selected equipment when appropriate for the application. Include transport, safe outdoor placement, trained operators, connection equipment, fuel handling, and manual response in the plan.

Standby or installed generators remain at the facility and connect through a designed electrical installation. An automatic system can start and transfer loads without manual changeover; the actual configuration and required response depend on the facility's needs.

Single-phase versus three-phase describes different electrical supply arrangements. Ask a licensed electrician to confirm which phase, voltage, frequency, and distribution arrangement the building and connected equipment require. This guide does not determine those specifications for your site.

5. Resolve transfer switches and connections before buying

A transfer switch changes the supplied circuits between utility power and generator power while providing the required separation between sources. It is part of the backup-power system, not an accessory to improvise during an outage.

Never backfeed. Feeding generator power into building wiring through a wall receptacle or another improvised connection can energize utility lines and create lethal hazards for workers and occupants. Never use a male-to-male cord.

Have a licensed electrician determine the appropriate transfer equipment and whether any proposed interlock arrangement is suitable and permitted. Do not assume an interlock substitutes for a transfer switch in every installation.

Transfer-switch selection, interlocks, grounding and bonding, cable and inlet ratings, permitting, installation, and required inspection belong with a licensed electrician and the AHJ. Procurement should obtain their specifications, not design the installation from a generator's advertised wattage.

6. Compare fuels against the outage plan

Compare fuel options using your expected load, outage duration, storage constraints, cold-weather conditions, and refueling arrangements. No fuel type guarantees uninterrupted operation.

Compare the actual generator's ratings and manufacturer fuel-consumption data for the selected fuel. Do not assume equal output or runtime across fuel options.

For fleet/facility readiness planning, review the Fleet Maintenance Equipment & Supplies category alongside your existing equipment inventory. An existing fuel system is a planning input, not proof that it is suitable for a generator.

7. Plan runtime, testing, and seasonal readiness

Set runtime requirements from the outage scenarios your agency plans to manage. A short interruption, an overnight outage, and a multi-day disruption can require different staffing, refueling, and maintenance arrangements.

Use manufacturer fuel-consumption data at the expected operating load to estimate required fuel. For stored fuels, relate usable fuel quantity to the applicable consumption rate, then account for reserve, operating limits, and refueling interruptions. Tank size alone does not establish runtime.

Assign responsibility for deliveries, access, fuel checks, maintenance, and operator relief. Consider whether fuel pumps and access systems will remain usable without utility power. For natural gas, address supply interruptions and pressure limitations instead of assuming unlimited runtime.

Follow the manufacturer's exercise and maintenance instructions, your agency's emergency plan, and applicable AHJ requirements. Have qualified personnel define appropriate transfer and load testing. A no-load engine start does not demonstrate that the entire system can carry the planned facility loads.

• Record test results, alarms, fuel use, deficiencies, and corrective actions.
• Check fuel condition, starting batteries, chargers, filters, fluids, cables, and connectors before storm season.
• Confirm operator training, instructions, contact lists, and access arrangements.
• Revisit the worksheet after equipment changes or seasonal operating changes.

Coordinate the backup-power review with the NYC Winter Readiness checklist so heating-related requirements and seasonal inventory receive attention before severe weather.

Include post-storm cleaning supplies in facility recovery planning where relevant. Backup power and safe recovery operations need separate, coordinated requirements lists.

8. Make portable-generator safety part of the specification

Carbon monoxide kills. Portable generators belong outdoors only. Follow OSHA's portable generator CO guidance, manufacturer instructions, and your agency's safety procedures.

• Keep generators well away from windows, doors, vents, and air intakes. Direct exhaust away from occupied spaces and follow required clearances.
• Never operate in a garage, basement, shed, or enclosed or partially enclosed space. An open door does not make indoor operation safe.
• Provide working CO alarms under your facility's safety plan and alarm-manufacturer instructions. Alarms do not make unsafe generator placement acceptable.
• Use a dry, level, shaded location with adequate ventilation. Weather protection must not enclose the unit, trap exhaust, or obstruct cooling. Keep it away from flooding and standing water.
• Shut down and allow the generator to cool before refueling. Keep fuel handling away from ignition sources.
• Use undamaged, grounded, outdoor-rated extension cords specified for the load and length. Protect connections from water and damage; do not overload cords or receptacles.
• Use only the professionally specified building connection arrangement. Have qualified personnel address grounding, bonding, and required protective devices.

If anyone develops headache, dizziness, nausea, confusion, or other possible CO symptoms, move to fresh air immediately and seek emergency medical help. Do not re-enter until trained, properly equipped personnel determine the area is safe.

9. Avoid common generator sizing mistakes

• Undersizing motor starts: Verify starting demand and overlapping starts, not just the steady running total.
• Forgetting seasonal loads: Review winter heating controls, summer cooling requirements, and storm-related pumping with responsible personnel.
• Oversizing excessively: Chronic light loading can create operating and maintenance problems, particularly wet-stacking in diesel engines. Confirm the manufacturer's suitable loading range rather than applying a universal percentage.
• Ignoring future loads: Identify planned equipment additions explicitly and distinguish them from an arbitrary reserve.
• Leaving connections until outage day: Resolve transfer equipment, permits, installation, cables, operator procedures, and testing before the generator is needed.

Pre-purchase checklist for facilities and procurement

Give this checklist to your facilities lead and procurement office before requesting a final quote.

• Critical loads and deferrable loads approved by facilities, safety, and emergency-management leads.
• Worksheet completed with verified running and starting data, quantities, and seasonal requirements.
• Starting scenarios, reserve, future loads, kW/kVA, duty rating, and site conditions reviewed by qualified personnel.
• Portable versus installed configuration, phase, voltage, and equipment compatibility confirmed by a licensed electrician.
• Transfer/connection design, permits, installation scope, and AHJ review identified.
• Fuel type, usable storage, planned runtime, refueling access, and outage staffing documented.
• Safe placement, CO alarms, cords, and operator procedures addressed.
• Testing, maintenance, service responsibility, and required documentation included.
• Purchase method, item-level contract scope where applicable, approvals, and total project scope verified by procurement.

Prepare the purchasing requirement with GoVets

Once technical requirements are approved, use the GoVets product categories to begin browsing generators, backup-power accessories, and facility equipment. Confirm specific equipment suitability and sourcing rather than treating a category listing as a technical recommendation.

Include the GoVets capability statement in your supplier review where appropriate.

GoVets serves NYC agencies under NYC contracts RC #4387223 (Fleet Maintenance Equipment & Supplies) and RC #4387305 (Power Tools & Accessories).

Review the New York capabilities information and confirm that each specific requirement is within current contract scope. Neither a general catalog listing nor this article establishes that a generator, accessory, or installation service is eligible under either contract.

NYC buyers can contact NewYork@GoVets.com.

For eligible purchases using that workflow, consult the NYC PASSPort PunchOut buying guide. Follow your agency's approval and purchasing procedures.

GoVets holds GSA Schedule Contract GS-35F-301GA.

Federal buyers can review the GSA Schedule capabilities information as part of purchasing-path research.

Use the GSA Advantage contract search to investigate specific listings. Confirm current contract scope and item eligibility with procurement; holding a Schedule contract does not establish coverage for every product or service.

Before buying, hand the completed worksheet and outage scenarios to your facilities lead for technical validation, then give procurement the approved specification and connection scope. Keep electrical design, safety review, and purchase authorization with the responsible professionals.