⚡ Electrical

Standby Generator Sizing Calculator

Size standby diesel generators (DG sets) for buildings. Calculate required kVA based on connected loads, demand factors, and motor starting kVA per NFPA 110.

📐 Standard: NEC
✅ Free to use
📄 PDF export
📱 Mobile friendly
⚡
Standby Generator Sizing Calculator Calculator
Reference: NEC
⚡ Electrical
Size standby diesel generators (DG sets) for buildings. Calculate required kVA based on connected loads, demand factors, and motor starting kVA per NFPA 110.
Inputs
Results
Calculation confidence: Planning-levelSuitable for early design, estimating and feasibility checks. Verify the final design against the governing code and a licensed engineer before construction or procurement.
Was this helpful?
💬 Suggest an improvement

ℹ About This Calculator

Correctly sizing a standby generator (DG set) is critical for reliable backup power. Under-sizing causes the generator to overload and shut down; over-sizing wastes capital and fuel. This calculator determines the required generator kVA rating by summing critical loads with demand and power factor corrections, and adds a motor starting kVA component for the largest motor per NEC.

NEC covers diesel generator sets for standby power in buildings. NEC specifies which loads must be on backup power (emergency lighting, lifts, fire pumps, critical ICU loads). EPA emission norms set limits on DG set exhaust. DG sets require a stacking / acoustic enclosure above 5 kVA in residential areas. Monthly test run (minimum 30 minutes under load) is mandatory for maintained reliability.

Generator Sizing Formula

NEC

Step 1: Running kW kW_running = Σ(P_load × DF_load × FF_load) Where DF = demand factor, FF = future expansion factor Step 2: Running kVA kVA_running = kW_running / PF_weighted PF_weighted = Σ(kW_load) / Σ(kW_load / PF_load) Step 3: Motor Starting Allowance kVA_start = Largest motor kW × motor start factor (4–8 × FLA for DOL) kVA_step = kVA_start - running kVA of that motor Step 4: Generator Rating kVA_gen = (kVA_running + kVA_step) × 1.15 (15% derating for altitude/temp) Select next standard rating above kVA_gen

Worked Example

Example: A commercial building has critical backup loads of 60 kW lighting (DF 1.0), 40 kW lifts (DF 0.6) and one 30 kW fire pump motor (DF 1.0). Running kW = 60×1.0 + 40×0.6 + 30×1.0 = 114 kW. At a weighted PF of 0.85, running kVA = 114/0.85 = 134.1 kVA. The fire pump motor starts DOL at 6× FLA, adding a starting kVA step of about 25 kVA above its running contribution. Total = 134.1 + 25 = 159.1 kVA; applying the 15% derating factor gives 182.9 kVA, so the next standard 200 kVA generator is selected.

Frequently Asked Questions

What demand factor should I use for generator sizing? +
Lighting loads: 1.0 (assume all on during a power failure). Computers/UPS: 0.9. Air conditioning: 0.6–0.8 (not all zones at peak simultaneously). Motors (pumps, fans): 0.7–0.8. Lifts: 0.6 (not all lifts running simultaneously). Fire pumps: 1.0 (critical safety load, always size at 100%). Total connected load × appropriate demand factors = running load for generator sizing.
Why does motor starting require a bigger generator? +
When a motor starts (DOL – Direct On Line), it draws 5–8× its full-load current for 2–5 seconds. This sudden surge causes a voltage dip on the generator. If the voltage dip exceeds 15–20%, other connected equipment (especially electronics and other motors) may trip. The generator must be sized so that adding the largest motor start kVA does not cause more than 15% voltage dip – this is the "motor starting" sizing criterion.
What is the difference between prime power and standby power ratings? +
Standby power: maximum power for emergency use when grid fails, not more than 200 hours/year, no continuous overload permitted. Prime power: continuous power generation as main source, 80% average load, 100% rated power available continuously. An generator rated "100 kVA standby / 90 kVA prime" means: 100 kVA only for standby duty (grid-parallel or grid-failure backup), 90 kVA if operating continuously as prime power.
What size fuel tank should I provide with a DG set? +
EPA regulations and NEC: minimum 8 hours' fuel storage at full load. Typical diesel consumption: 0.28–0.35 litres/kWh (about 0.3 L/kWh at 70% load). For a 100 kVA (80 kW) generator at 70% load for 8 hours: 80×0.7×0.3×8 = 134 litres minimum. EPA allows maximum 2,000 litres above ground without petroleum licence; larger tanks need petroleum storage licence.
Does the DG set need to be synchronized with the grid? +
For pure standby (grid fails, DG starts, grid returns, DG stops): no synchronisation needed. For parallel operation (DG supplements grid supply or export surplus): synchronisation equipment (auto-synchroniser, synchronising breaker, protection relays) is mandatory. Parallel operation requires prior approval from the DISCOM. For most buildings with backup-only DG sets: transfer switch (ATS – Automatic Transfer Switch) is sufficient, no synchronisation needed.

Related Calculators

⚡
Generator Critical Load Calculator
🔧
Diesel Fuel Tank Calculator
⚡
Transformer Sizing Calculator

🧮 110 Free MEP Calculators

Browse all HVAC, Electrical, Plumbing, Fire, Gas and Mechanical calculators - IS/IBC/ASHRAE compliant, free PDF export.

Browse All Calculators →

⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →