A commercial generator that's too small will overload circuits, trip breakers, and shut down right when you need power most. One that's too large burns extra fuel, costs more upfront, and runs inefficiently at partial load for years. Correct sizing depends on your actual electrical load, equipment startup surges, and safety margins, not guesswork.
Get the calculation right and your backup power system keeps critical operations running through every outage. Here's a step-by-step breakdown of exactly how to size a commercial generator for your building.
What Is Commercial Generator Sizing and Why It Matters
Generator sizing is the process of matching a generator's power output, measured in kilowatts (kW) or kilovolt-amps (kVA), to the total electrical load of your building or equipment. This isn't a rough guess. It's a calculation built on your actual power draw, the type of loads you run, motor startup surges, and the safety margin your facility needs to stay protected during an outage.
Get this wrong in either direction and you'll pay for it, one way or another. An undersized generator overheats, trips offline, or damages sensitive equipment like HVAC compressors, refrigeration units, and server systems. An oversized unit runs inefficiently at partial load, burns more fuel per kWh, wears down faster from wet stacking, and costs thousands more upfront than necessary.
Most commercial facilities, including offices, restaurants, hospitals, warehouses, retail stores, and manufacturing plants, need a generator sized within 10-20% above their calculated peak load, not double or triple it. That margin covers startup surges and future growth without pushing you into an oversized, overpriced system. Accurate sizing protects your equipment, keeps operations running, and controls both installation and long-term fuel costs.
Key Factors That Affect Commercial Generator Size
Before running any formula, you need accurate data on what the generator will actually power. Skipping this step is the most common reason sizing calculations go wrong.
Here are the main factors that determine your required generator size:
- Total connected load: the combined wattage of every device, machine, and system that could run at once
- Starting (surge) load: the extra power motors, compressors, and pumps need for the first few seconds of startup, often 3-7 times their running wattage
- Power factor: the ratio between real power (kW) and apparent power (kVA), typically 0.8 for commercial equipment
- Essential vs. non-essential loads: whether the generator needs to run everything or just critical systems like lighting, security, and refrigeration
- Voltage and phase requirements: single-phase (120/240V) for smaller operations or three-phase (208/480V) for larger commercial buildings
- Altitude and ambient temperature: generators lose output capacity in high heat or high elevation, sometimes by 10% or more
- Future load growth: planned equipment additions or business expansion over the next 3-5 years
Each factor above feeds directly into the calculation in the next section, so gather this information first, preferably from an electrician-verified load list, not estimates.
Step-by-Step Process to Calculate Generator Size for Your Business
This is the core method commercial electricians and generator dealers use. Follow it in order and you'll land on an accurate, code-compliant size.
Step 1: List Every Electrical Load
Walk through your facility and write down every piece of equipment that draws power, including lights, HVAC units, refrigerators, computers, pumps, elevators, and machinery.
Step 2: Record the Wattage of Each Device
Check nameplates or spec sheets for running watts and starting watts. If only amps are listed, use this formula: Watts = Volts × Amps × Power Factor
Step 3: Add Up Running Watts for All Equipment
This gives you the continuous load your generator must supply during normal operation.
Step 4: Identify the Single Largest Motor or Compressor
This device's starting watts, not its running watts, is what you'll add on top of your total.
Step 5: Calculate Total Starting Load
Add the largest motor's starting watts to the sum of all other running watts, since motors rarely all start at the exact same second.
Step 6: Apply a Safety Margin
Add 20-25% to your total to account for future load growth, voltage drops, and derating from heat or altitude.
Step 7: Convert to kW or kVA
Divide total watts by 1,000 to get kW. If your supplier quotes in kVA, divide kW by the power factor, usually 0.8.
Step 8: Match to the Nearest Standard Generator Size
Round up to the next available commercial unit. Common sizes include 20kW, 48kW, 80kW, 150kW, 250kW, and 500kW.
Following these eight steps gives you a defensible number you can bring to a generator supplier or electrician for final confirmation.
Common Generator Sizing Formulas You Should Know
You don't need to memorize complex electrical engineering equations, but knowing these three core formulas will help you understand any quote or calculation a contractor gives you.
Formula
Purpose
Example
Watts = Volts × Amps
Convert amperage to wattage for single loads
240V × 20A = 4,800W
kW = kVA × Power Factor
Convert apparent power to real power
100kVA × 0.8 = 80kW
Total Load = Running Watts + Largest Starting Watts
Find peak demand for sizing
15,000W + 9,000W = 24,000W
For three-phase systems, the formula changes slightly to account for the extra phase:
kVA = (Volts × Amps × 1.732) ÷ 1,000
The 1.732 figure represents the square root of 3, which is standard for three-phase power calculations. Most commercial buildings above 5,000 square feet run on three-phase power, so this formula applies more often than the single-phase version.
Generator Size Chart for Common Commercial Loads
Use this chart as a starting reference point, not a final answer. It's based on typical load profiles, but your actual number depends on your specific equipment list.
Business Type
Typical Building Size
Estimated Generator Size
Small retail store
Up to 3,000 sq ft
20-30 kW
Restaurant
3,000-6,000 sq ft
40-60 kW
Office building
5,000-15,000 sq ft
60-100 kW
Medical/dental clinic
3,000-8,000 sq ft
50-80 kW
Warehouse (light equipment)
10,000-30,000 sq ft
100-150 kW
Hospital or critical care facility
Varies
250 kW and up
Manufacturing plant
Varies by machinery
150-500+ kW
These ranges assume standard commercial loads without heavy industrial machinery. If your facility runs large motors, welding equipment, or industrial HVAC chillers, your number will likely fall on the higher end, or require a custom load study.
Mistakes to Avoid When Sizing a Commercial Generator
Even experienced facility managers get this wrong. Here are the errors that come up most often, and how to avoid them.
- Ignoring starting watts: sizing only for running load causes the generator to stall the moment a compressor or motor kicks on
- Guessing instead of measuring: estimating wattage from memory instead of checking nameplates leads to sizing errors of 20% or more
- Skipping the power factor: confusing kW and kVA results in a generator that's underpowered for real-world conditions
- Forgetting altitude and temperature derating: a generator rated at sea level may lose 3-4% output for every 1,000 feet of elevation gain
- Not planning for growth: sizing exactly to today's load leaves no room for new equipment, expansion, or seasonal demand spikes
- Choosing based on price alone: the cheapest unit that meets minimum wattage often lacks the fuel efficiency or run-time rating your business needs long term
Avoiding these six mistakes alone will put you ahead of most DIY sizing attempts, and closer to the number a licensed electrician would calculate.
When to Consult a Local Generator Sizing Professional
The steps above will get you a solid working estimate, but commercial installations almost always require a final sign-off from a licensed electrician, both for accuracy and for code compliance in your area. Local electrical codes, utility interconnection rules, and permit requirements vary by city and state, so a generator dealer or electrician near you can confirm your calculation matches local standards.
You should bring in a professional when:
1. Your building has three-phase power or exceeds 10,000 square feet
2. You're powering medical, refrigeration, or life-safety equipment
3. Local code requires a licensed load calculation for permitting
4. You're unsure how to read nameplate data or convert units correctly
5. Your business plans to expand within the next few years
A local generator supplier can also run a professional load study, recommend fuel type (diesel, natural gas, or propane), and handle installation and permitting, saving you from costly resizing later.
Final Thoughts
Calculating commercial generator size comes down to one core principle: add up your real power needs, account for startup surges, and build in a safety margin. Skip the guesswork, use actual nameplate data, and round up rather than down. The formulas and chart in this guide will get you a reliable estimate, but for any facility running critical equipment or three-phase power, a final check from a licensed electrician or local generator dealer is worth the cost. A properly sized generator protects your equipment, your operations, and your bottom line for the next 15-20 years of use.
FAQs
How Many Kw Does A Commercial Generator Need To Be?
Most small to mid-size commercial buildings need between 20kW and 150kW, depending on square footage and equipment load. Larger facilities like hospitals or manufacturing plants often require 250kW or more.
What Is The Difference Between KW And KVA In Generator Sizing?
kW measures real power that does actual work, while kVA measures apparent power including reactive load. Multiply kVA by the power factor (usually 0.8) to get kW.
How Do I Calculate Starting Watts For A Generator?
Check The Equipment Nameplate For Starting Or "Locked Rotor" Amps, Then Multiply By Voltage. Starting Watts Are Typically 3-7 Times Higher Than Running Watts For Motors And Compressors.
Do I Need A Generator Sized For 100% Of My Building's Load?
No. Most businesses only need to power essential circuits, such as lighting, refrigeration, security, and critical equipment, not the entire building, which reduces the required generator size significantly.
What Size Generator Does A Small Office Building Need?
A small office between 5,000 and 15,000 square feet typically needs a 60kW to 100kW generator, depending on HVAC load and equipment count.
How Much Extra Capacity Should I Add For Safety Margin?
Add 20-25% above your calculated peak load to account for future growth, voltage drops, and generator derating from heat or altitude.
Does Altitude Affect Generator Sizing?
Yes. Generators lose roughly 3-4% of rated output for every 1,000 feet above sea level, so facilities at higher elevations need a larger unit than the base calculation suggests.
What's The Formula For Three-Phase Generator Sizing?
Use kVA = (Volts × Amps × 1.732) ÷ 1,000. The 1.732 factor accounts for the three-phase power configuration common in commercial buildings.
Can An Oversized Generator Cause Problems?
Yes. Running a generator well below its rated capacity for long periods causes wet stacking in diesel units, reduces fuel efficiency, and shortens engine life.
Should I Hire A Professional To Size My Commercial Generator?
For any building with three-phase power, life-safety equipment, or square footage over 10,000 sq ft, a licensed electrician or local generator dealer should verify your calculation before purchase and installation.