A commercial electrical load calculator works out how much electrical demand a business property really draws, in volt-amperes (VA), kilowatts (kW), and kilovolt-amperes (kVA). It applies the demand factors of the National Electrical Code (NEC) to the building's lights, outlets, HVAC, motors, and equipment, and returns the number every downstream decision hangs on.
The code calculates in volt-amperes, not watts. Mixing watts into a motor or transformer load undersizes your conductors.
The actual usable power your equipment consumes. Generators are rated in kVA at a 0.8 power factor, so the translation is direct.
The total capacity your generator must provide. Demand load in kVA is the starting point for backup power sizing.
The difference between connected load and demand load is the whole game. Connected load is every watt of everything installed. Demand load is what the building actually draws at its peak, after demand factors, because nothing runs at full capacity at the same time. Code-based demand factors turn the first into the second, and that's the number that sizes your service and your generator.
What is my building's total electrical demand? The calculator returns the real number in VA, kW, and kVA — not a guess.
What service size in amps do I need? The calculator divides demand VA by your system voltage to give you the next standard service size.
What size generator do I need for standby or prime power? The calculator returns both recommendations mapped to real models.
What will that generator cost to run? The calculator shows estimated fuel cost per hour, linked to the fuel consumption tool.
Enter your building type, square footage, voltage, and equipment. The calculator applies NEC Article 220 demand factors and returns your demand load in kVA, service size in amps, and a diesel generator recommendation.
Select your building type and fill in the values. Equipment defaults are pre-loaded for each occupancy.
Edit values to match your building. Lighting is auto-calculated from square footage.
The chart below is a planning reference. The lighting figures are the NEC minimum unit loads for service sizing, and the generator ranges are rough planning bands for typical buildings.
| Building Type | Lighting Load (VA/ft²) | Typical Standby Generator |
|---|---|---|
| Office | 3.5 | 20–250 kW |
| Retail / Store | 3.0 | 30–150 kW |
| Restaurant | 2.0 | 40–120 kW |
| Warehouse | 0.25–1.5 | 50–400 kW |
| Hospital | 2.0 | 200–2,000 kW |
| Industrial / Loft | 2.0 | 100–4,000 kW |
| Auditorium / Assembly | 1.0 | 100–500 kW |
Article 220 gives the prescriptive minimum for a compliant service. Real facilities often need more for harmonics, power factor, and growth — which is where an engineer and a load study come in. Use the calculator as your starting point, then confirm with a licensed professional.
To calculate electrical load for a commercial building, total the lighting at the NEC table value per square foot, add 180 VA per receptacle, add your HVAC at the larger of heating or cooling, and add motors at 125% of the largest unit. Apply the demand factors, then divide by your system voltage to get the service size in amps.
Start with occupancy and square footage. Lighting is calculated at the minimum unit load from NEC Table 220.12 — the value applies even with LED fixtures installed.
Add 180 VA per outlet, then apply the demand factor: the first 10,000 VA at 100%, the remainder at 50%.
Show windows count at 200 VA per linear foot. Sign lighting at 1,200 VA minimum.
Add HVAC from nameplate ratings — count only the larger of heating or cooling. Add motors at 125% of the largest motor; all others at 100%.
Mark continuous loads (3+ hours) at 125%. Total the demand in VA, then divide by voltage (or voltage × 1.732 for three-phase) to get the service size in amps, rounded up.
1. Occupancy & square footage → lighting at Table 220.12
2. Receptacles: 180 VA each → first 10kVA at 100%, remainder at 50%
3. Show windows: 200 VA/ft • Signs: 1,200 VA min
4. HVAC: larger of heating or cooling • Motors: 125% of largest
5. Continuous loads at 125% • Total VA ÷ voltage = service amps
Your building's demand load in kVA is the starting point for backup power. Generators are rated in kVA at a 0.8 power factor, so the translation is direct: a building with 63 kVA of demand needs a generator whose kVA rating covers that demand with margin.
A generator that runs at 80% of its rating handles start-up surges and seasonal peaks without strain. A building with 63 kVA of demand lands in the 80–100 kVA class.
The largest motor — usually the AC compressor or a pump — draws 2–8× its running load at start. A soft starter or VFD needs far less headroom than a direct-on-line starter.
Standby sets run only during outages and can be sized closer to the load. Prime sets run continuously and carry extra margin for load swings. The calculator returns both.
Every class maps to a real model. A typical store lands on a 100 kVA silent diesel set in a soundproofed enclosure, with transfer switch and base tank, factory-direct.
A 100 kVA silent standby set with transfer switch and base tank runs roughly $15,000–$40,000 delivered, depending on site work and enclosure. A factory-direct quote lands your exact number.
Use the calculator results as your starting point. Our engineers confirm the load and deliver a factory-direct quote with your numbers pre-filled — no markup, no surprises.
Our engineers see these five errors on commercial jobs more than any others. Avoid them and your service and generator will be sized right the first time.
Mixing watts into a volt-ampere calculation undersizes conductors and breakers for inductive loads. Always use VA for code calculations.
Adding every connected watt with no diversity oversizes everything. Demand load, not connected load, is the number that matters for service and generator sizing.
Heating and cooling don't run together — counting both inflates the service and the generator by a full class. Count only the larger of the two.
Buildings add equipment. A 15–25% reserve protects against a service that's obsolete the day you add an EV charger or a second AC zone.
The AC compressor's start-up draw, plus altitude and temperature on a high site, both change the generator size. A VFD or soft starter shrinks it; a direct-on-line motor grows it.
Not sure your building's number is right? Our engineers will review your load and site and confirm the service and generator size before you spend anything. Talk to our engineers.
Our engineers will review your building’s load calculation, confirm the service size, and deliver a factory-direct price — no obligation, no hidden fees. Every quote includes the generator, automatic transfer switch, and soundproofed enclosure for a complete commercial standby solution.
A: Total the lighting at the NEC table value per square foot, add 180 VA per receptacle, add HVAC at the larger of heating or cooling, and add motors at 125% of the largest unit. Apply the demand factors, then divide by the system voltage to get the service size in amps.
A: The NEC minimum lighting load is 3.5 VA/ft² for offices, 3.0 for retail, and 0.25 to 1.5 for warehouses. Total building demand, including HVAC and equipment, typically runs much higher, and the calculator returns your actual number.
A: A demand factor is the percentage of connected load you count for sizing, because not everything runs at once. Commercial demand factors typically run 0.6 to 0.8, and the NEC sets specific values by load type.
A: Take your building’s demand load, add about 25% headroom, and pick the generator class that covers it. A store with 63 kVA of demand lands in the 80 to 100 kVA standby class. Run the calculator for your building.
A: Connected load is every watt of everything installed. Demand load is what the building actually draws at peak after demand factors. Sizing runs on demand load, because nothing runs at full capacity at the same moment.
A: No. Heating and cooling don’t run together, so the code counts the larger of the two. Counting both overstates the load by a full class.
A: Standby power runs during outages and can be sized closer to the load. Prime power runs continuously and carries the building’s everyday demand, so it needs more margin. The calculator returns both recommendations.
A: Yes. We build standby and prime diesel generators from 8 to 4,000 kVA for offices, retail stores, restaurants, warehouses, hospitals, and telecom sites, with soundproofed enclosures and factory-direct quotes.
