How to Size a Generator for Commercial Building

To figure out the right generator size for a commercial building, you first add up the wattage needed for all important systems. Use this total to pick a generator that matches or is a little higher than what you need. You must check both starting and running loads for each piece of equipment. Getting the size right helps your building stay safe during a power outage. It also keeps important systems working. If you pick the wrong size generator, you could have problems like wet stacking, engine damage, and more maintenance costs. The table below shows what can happen if you choose the wrong generator for commercial buildings:

Key Takeaways

Add all running and starting power needs together. This helps you choose the right generator size for your building. Use real-time measurements or old data to find your building's real power needs. Add a 20-25% safety margin. This helps with power surges and future growth. Plan for starting loads, especially for motors. This stops overloads and shutdowns. Do not make mistakes like guessing too low or forgetting about future growth. This keeps your business safe and working.

Commercial Generator Sizing Methods

When you pick a commercial generator, you need to know the main sizing ways. Each way is good for different buildings or equipment.

Full Load Capacity by Measurement

You can use clamp-on ammeters to measure full load. This tool shows your building's real power needs. You check the current at the main panel when everything is running. This way, you see both running and starting loads. It helps you not pick a generator that is too small or too big. Energy audits use this method a lot, especially if your building has mixed power or tricky equipment.

Tip: Measuring in real time stops voltage drops and keeps equipment safe.

Full Load Capacity by History

You can look at your utility bills to see peak demand. Find the highest power use in the last year. Add about 25% as a safety margin. This works well if your building runs the same most of the time. It also helps with changes in seasons. But it might miss big surges when equipment starts. Using this with other ways makes sizing more correct.

Full Load Capacity with Extensive Motor Use

If your building has many motors, you must check both running and starting loads. Motors use much more power when they start. Change loads to volt-amps (VA) to include power factor. Add a 25% safety margin for motor surges and future needs. You can use load shedding to turn off non-essential loads during busy times. Diesel generators are good for life safety loads because they start fast.

• NEC codes say you need enough generator power for quick load changes.

• Energy Management Systems help control loads and make sizing better.

Square Footage Measurement

You can guess generator size by your building's square footage. For retail, start with 50 kW and add 10 watts for each square foot. This is a fast way to estimate. But it does not count all equipment or special loads. Use this as a first step, then check with better methods.

Note: Square footage alone does not promise the right generator size. Always check your real power needs.

Power Requirements

Knowing your building's power needs is very important. You must find out which systems need to stay on during a power outage. Add up the total load these systems use. This helps you avoid picking a generator that is too small or too big. If the generator is not the right size, it can cause safety problems and waste energy.

Essential Loads

Essential loads are things that must work when the power goes out. These can be:

• Emergency lighting

• Fire alarms and security systems

• IT servers and communication equipment

• HVAC for important areas

• Medical or life-safety equipment (if needed)

Make a list of each essential load. Write down how much power each one uses when running and when starting. Use the formula P = VI (Power = Voltage x Current) for every device. For example, if you have a server room, figure out the power for each server, cooling unit, and backup switch.

Total Power = 480 + 1200 + 240 + 240 = 2160 Watts

Tip: Always count the highest starting wattage for any equipment that needs extra power to start.

Non-Essential Loads

Non-essential loads are things you can turn off during a power outage. These might be:

• Decorative lighting

• Non-critical office equipment

• Vending machines

• Some HVAC zones

You can leave these out of your generator plan to save money. But if you want to back up the whole building, you must add both essential and non-essential loads. The table below shows how non-essential loads mix with essential ones to make the total system load:

Note: Adding non-essential loads makes the generator bigger and more expensive. Only add them if your business needs everything working during outages.

Total Wattage

To find the total wattage, add up the running watts for all the loads you want to power. Then add the highest starting wattage from any load. This makes sure your generator can handle both normal and extra power needs.

Follow these steps:

1. Make a list of all loads you want to power.

2. Write down running and starting wattage for each.

3. Add all running watts together.

4. Add the highest starting wattage from your list.

5. Add a safety margin (usually 20%).

For a quick guess, use these formulas:

• Retail buildings:
  Generator Size (kW) = 50 kW + (10 W x total square footage)

• Other commercial buildings:
  Generator Size (kW) = 50 kW + (5 W x total square footage)

Example: A 5,000 sq ft retail store
Generator Size = 50 kW + (10 W x 5,000) = 50 kW + 50 kW = 100 kW

You can also check the total load at the service panel. Find all appliances, write down their wattage, and add up the running watts. Add the highest starting wattage and a safety margin. This way gives you the best idea of your power needs.

Callout: If you only back up emergency loads, you need a smaller generator, usually 50-100 kW for small buildings. Full building backup needs a much bigger generator, sometimes 200-300+ kW for large places. Always pick a generator size that matches what you really need.

If you carefully figure out your power needs, your generator will keep your business working well during any outage.

Starting vs. Running Loads

When you pick a generator, you need to know about starting and running loads. Some machines use more power when they first turn on. After that, they use less power to keep working. If you do not think about this, your generator might stop working during a blackout.

Starting Loads

Starting loads are the most power your machines will ever need. When big motors start, like pumps or elevators, they use a lot of electricity. This can be three times more than what they use after starting. If your generator cannot give enough power, lights may flicker or things may shut off.

• Big motors in HVAC or elevators need lots of power to start.

• Old lighting systems can also use extra power when turned on.

• Machines like compressors turn on and off and make things harder.

Tip: Soft starters or Variable Frequency Drives (VFDs) can help lower starting loads. This means you might need a smaller generator.

Running Loads

Running loads are the power your machines use after they start. These loads stay the same while things are working. You need to add up all running loads to know how much power your generator must give.

• Running loads are things like lights, computers, and HVAC after they start.

• Add up the running watts for every device to get your total.

• Remember to count anything that stays on during a blackout.

Motor Surges

Motor surges happen when many motors start at the same time. This can make the total power go over what your generator can give. You need to plan for the highest starting and running watts so you do not overload your generator.

• Always add the starting watts of the biggest motor to your total.

• Add extra power for safety and future needs.

• If you use load management, you can start motors at different times to keep power lower.

Note: Picking the right generator size keeps your machines safe, helps your business work during outages, and saves money on repairs.

Size Generator for Future Needs

When you get a backup generator for a commercial building, think about more than just what you need now. Your business will get bigger. You might buy new equipment. You will use more power later. If you only pick a generator for today's needs, you could run out of power during an outage. It is smart to add extra power for safety and for when your business grows.

Safety Margin

Always add a safety margin when you size a generator. This extra power helps if there are power surges or if machines use more power when starting. Experts say you should add about 20% to 25% more than your highest power use. This extra space keeps your backup generator from working too hard. If it works too hard, it can get too hot and break faster.

Tip: Do not let your generator run at more than 80% of its total power for a long time. This helps your system stay safe and last longer.

Here is a table to show how much extra power you should add:

For example, if your building uses 100 kW at its busiest, you should pick a backup generator that gives 125 kW. This extra power helps with sudden jumps and lets your business grow.

Expansion

Your business will change over time. You might add more machines, make your building bigger, or hire more people. All these things mean you will need more power. When you size a generator, you should plan for these changes.

You can use a simple formula to help with this:

Future Generator Size = Current Load x 1.20 (or 1.25)

If your building uses 40 kW now, multiply by 1.25. You get 50 kW. Pick a generator that is a little bigger than this number. This way, your backup generator will work even if you add more things later.

Note: Always round up to the next biggest generator size. This makes sure you always have enough backup power.

You can also do a generator application study. This study looks at your building, your equipment, and your plans for the future. It uses computer data to guess how your power needs will change. With this, you can pick a generator that works for now and later.

Key steps for planning expansion:

• Look at your plans for growing your business.

• Write down any new machines or systems you might add.

• Add 20-25% more power to your total.

• Pick a backup generator that is at least this big.

Temporary or standby generators should always be a little bigger than your highest power use. This keeps your building safe in emergencies and stops overloads.

Callout: Planning for the future saves money and worry. You will not have to buy a new backup generator every time your business gets bigger.

By adding a safety margin and thinking about growth, your backup power will protect your business now and in the future.

How to Calculate the Size of a Generator

Method 1: Full Load Capacity

The full load capacity method is the best way to size a generator. It helps you pick a generator for now and later. First, make a list of everything you want to power during an outage. Write down both running and starting wattages for each item. Add up all the running wattages to get your total running power. Find the highest starting wattage on your list. Add this number to your total running wattage. This step makes sure you have enough power for surges. Multiply the final sum by 1.25 for safety. This extra amount lets you grow and keeps your generator safe.

Tip: If you see amps instead of watts, change it to watts. Use this formula for resistive loads:
Watts = Amps x Volts
For reactive loads, use:
Watts = Amps x Volts x Load Factor

This method gives you a good guess for generator size. It covers both normal and peak power needs. This is very important for commercial buildings. The table below shows how this method compares to others:

Method 2: Square Footage

The square footage method is a fast way to guess generator size. It works best for simple places like stores. Multiply your building's square footage by a set wattage number.

• For retail, use 50 kW plus 10 watts for each square foot.

• For other commercial buildings, use 50 kW plus 5 watts for each square foot.

Note: This method is only a rough guess. Power needs change with different tenants, equipment, and building use. If you only use square footage, you might pick a generator that is too big or too small.

• Energy rules, building type, and weather also change your power needs.

• Always do a full load check and talk to an expert before you choose your generator size.

Callout: Use the square footage method for planning costs, not for final design. Always check your numbers with a full load method.

Generator Sizing Factors

Power Phases

You need to pick single-phase or three-phase power for your building. The phase type changes how much power your generator gives. It also affects how well it works. Single-phase is good for small offices or stores. Three-phase is better for big buildings and factories. It gives steady power for heavy machines. Look at the table below to see the differences:

Tip: Talk to an expert before you choose the phase type. Make sure it fits your building's needs now and later.

Voltage

Voltage is very important when picking a generator. You must check your building's voltage and phase setup first. The generator's voltage should match your system. This stops equipment from breaking and keeps things working well. Wrong voltage can cause problems and expensive fixes. Safety rules like OSHA and NEC say what voltage and generator you can use.

• Check your building's voltage and phase setup.

• Make sure the generator's voltage matches your system.

• Medium voltage generators are good for big buildings. Low voltage ones work for small or sensitive loads.

• Pick a generator voltage that fits your building's needs.

• Take care of your generator and check voltage often to make it last longer.

Note: Always follow local safety rules when you put in a generator.

Site Conditions

Site conditions change how you size and install a generator. Small spaces in cities can make it hard to find a spot for the generator. Local rules and permits tell you how to install, store fuel, and control emissions. Noise laws may mean you need soundproof covers or walls. Environmental rules say you must meet emissions standards, especially in busy or old areas.

• Check space and access before you install the generator.

• Get all permits for electrical, building, mechanical, and environmental needs.

• Use ways to lower noise so you follow local rules.

• Pick a generator that meets emissions rules.

• Have a pro check your load to plan for starting surges and future growth.

Callout: If you balance site needs, local rules, and power needs, your generator will fit your building and follow all rules.

Common Mistakes in Commercial Buildings

Sizing a generator for a commercial building is not always easy. People often make mistakes that can cost a lot of money or cause safety problems. You can stop these problems if you learn from what others have done wrong.

Underestimating Loads

Sometimes, you might guess your building uses less power than it really does. This happens a lot when people forget about the big power needed to start large motors, like fire pumps or elevators. If you do not count these power surges, your generator might not start important equipment. You could also overload the generator when there is an emergency.

Tip: Always check both running and starting loads. Use a sizing calculator to add up all power needs, including the biggest motor surges.

Here are some common mistakes:

• Not counting when more than one pump or motor starts together.

• Forgetting the locked rotor current for big machines.

• Not giving both kW and kVA numbers to the generator company.

• Not thinking about how temperature and altitude change generator output.

• Not using a sizing calculator or expert tools.

If you guess too low on your loads, you might have:

• Equipment that stops working during power outages.

• Drops in frequency and voltage.

• Needing to turn off some loads, which can cause downtime and money loss.

Ignoring Expansion

Your business will get bigger over time. If you only pick a generator for what you need now, you might not have enough power later when you add new equipment or space. Many people forget to plan for more loads or changes in how the building is used.

Note: Add a 20-25% safety margin to your total load. This extra amount helps you with future growth and surprise power surges.

You should:

• Look at your business plans to see if you will grow.

• Talk to experts early when you design your system.

• Use a sizing calculator that lets you plan for more power later.

• Work with fire and mechanical engineers to follow all safety rules.

Common mistakes to avoid:

• Not thinking about load shedding or crash transfers.

• Not checking if you have enough fuel for longer use.

• Forgetting about local and national safety and installation rules.

If you use the right tools and ask experts for help, you can size your generator the right way. This keeps your building safe and your business working well.

Conclusion

Sizing a generator for your commercial building means following some important steps. First, check how much power you use when everything is on. Next, look at your old utility bills to see your highest power use. Then, figure out how much power motors need to start and use the square footage method to help. Think about where the generator will go and add 20-25% more power as a safety margin. Always talk to experts and look at what the manufacturer says. If you size your generator the right way, your business can keep working during outages. This also stops your equipment from breaking and keeps everyone safe. By doing these things, you make sure your backup power works well and protects your money.

FAQ

How do you know if your generator is too small?

If your generator is too small, it cannot start everything. You might see lights blink or machines turn off. Too much load can break the generator and your devices. Always add up all starting and running loads before picking a size.

Can you use one generator for multiple buildings?

You can use one generator for more than one building if you connect them the right way. The generator must be big enough for all the buildings together. Always follow local rules and ask a licensed electrician before setting this up.

What is the difference between kW and kVA in generator sizing?

kW shows real power. kVA shows total power used. Most things use kW, but some motors need you to check kVA too. Always look at both numbers when picking a generator. This helps stop overloads and keeps your system safe.

How often should you test your commercial generator?

Test your generator at least once every month. Run it with a load to make sure it works. Testing often helps you find problems early. It also keeps your backup power ready for emergencies.

Do you need a permit to install a commercial generator?

You need a permit to put in most commercial generators. Local rules say you must have safety checks. Always ask your city or county before you begin. Permits help you follow the law and meet insurance rules.