The Sizing Decision That Most Facilities Get Wrong

I remember sitting across from a frustrated facility director last year. They’d just commissioned a 750kW diesel generator—impressive hardware, state-of-the-art controls. Problem: their actual peak load was 240kW. They’d over-sized by more than 200%, were burning fuel inefficiently on every operating hour, and faced a potential capital loss of $200,000+ when they discovered they could have deployed a 300kW unit with equal reliability. “Nobody told me there was a right size,” she said. “I thought bigger was always better.”

This scenario repeats itself constantly across hospitals, manufacturing plants, data centers, and emergency services. Most facility managers approach generator sizing like insurance—bigger feels safer. In reality, generator sizing is an engineering problem, not a risk-management free-for-all. The right size is not the biggest size. It’s the fit-for-purpose size: one that meets your actual load, respects your budget, and operates in the efficiency sweet spot.

If you’re standing where that director stood—paralyzed by the sizing decision—this guide walks you through the proven methodology I’ve used across 50+ engagements. By the end, you’ll understand exactly what size generator you need, why it matters financially, and how to avoid the costly mistakes that most facilities make.

Why Sizing Matters: The Financial Reality

Let me start with the money. Generator sizing decisions ripple through your facility’s economics for 10–15 years.

Oversizing locks you into excess capital expense and chronic fuel waste. That 750kW unit burning fuel at 300kW load is operating at 40% capacity—far below the 60–80% “sweet spot” where generator efficiency peaks. Running lean burns more fuel per megawatt-hour of useful output. Over 10 years of continuous or semi-continuous duty (think hospitals, data centers, telecommunications hubs), that efficiency penalty compounds into tens of thousands in wasted diesel. You’re also paying for capacity you’ll never use.

Undersizing is rarer but catastrophic. It forces the generator to run continuously at or near its rated limit, which causes excessive heat stress, accelerates bearing wear, and risks shutdown during transient spikes (like motor startup). Worse, an undersized generator can’t meet peak demand, and your facility goes dark—the precise failure mode you installed backup power to prevent.

The financial sweet spot sits at 60–80% of generator capacity. In this band, diesel engines operate efficiently, waste heat dissipation is manageable, and transient margin exists for unexpected load spikes. Most facilities undershooting this band operate above 90% continuous load; most overshooting operate below 40%. The gap between “too small” and “too big” is surprisingly narrow, which is why diesel generator sizing matters.

Understanding Your Electrical Load: The Foundation

Before you can size a generator, you need to know what you’re powering. This sounds obvious, but it’s where most mistakes start.

Generator load calculation requires thinking in three categories:

1. Continuous Load (Running Watts)

This is your steady-state electrical demand. Start by listing every device that runs during normal operation—lighting, HVAC, pumps, conveyors, computing equipment. You’ll pull nameplate power (in watts or kilowatts) from equipment nameplates, electrical schematics, or utility bills. Real power draw often runs 70–90% of nameplate, but use nameplate as your conservative estimate.

2. Peak Load (Demand Factor)

Peak load isn’t constant; it’s the spike that happens when multiple systems energize simultaneously. Motor-driven compressors, fans, and pumps all draw extra current during startup, potentially making your peak load 30–50% higher than continuous demand. The exact variance depends on equipment coincidence and duty cycle—something you’ll refine as you dig deeper.

3. Motor Starting Wattage (Inrush Current)

Here’s where most facility managers stumble. Motors don’t draw their nameplate power instantly. Instead, they draw 3–6 times their running current during the first few electrical cycles of startup—a transient spike called motor starting wattage. A 10 HP motor running at 7.5 kW might spike to 45 kW for just 50 milliseconds. If your generator isn’t sized for this transient, voltage sags, the motor fails to start, and nearby equipment glitches.

Walking Through the Calculation: Say your facility has 100 kW of continuous load plus three 15 HP motors (11 kW each). Motor inrush multipliers mean one motor at full spike could be around 45 kW. Planning realistically, one motor at full inrush plus the other two running gives you roughly 167 kW (45 + 22 + 100 = 167 kW). That’s your peak demand scenario—the spike you must accommodate.

The Diesel Generator Sizing Methodology – Step by Step

Alright, let’s build the actual sizing calculation. I use this five-step process on every engagement.

Step 1: Inventory and Total Your Running Load

Walk through your facility with a spreadsheet. List every major electrical consumer—lighting circuits (estimate kW by breaker capacity), HVAC units (kW from nameplate), pumps, compressors, motors (nameplate kW), computing equipment and servers (kVA from specs), and process loads like kilns or ovens. Sum the running watts. That’s your baseline generator load calculation—call it RW (running watts).

Pro tip: Don’t include equipment that won’t run during an outage, like EV chargers or non-critical lab equipment. Zero in on critical loads only.

Step 2: Identify Motor Loads and Calculate Starting Surge

Flag every motor rated 3 HP or larger. For each, estimate motor starting wattage as 3–6 times its running power. Industrial motors have starting code letters (A–E) on their nameplate—code “E” means 3× running current, “B” means 5–6×. Realistically, motors don’t all start at once; expect staggered start sequences over 2–5 seconds. Plan for one motor at full inrush plus the others running. That formula becomes: Peak Motor Scenario (kW) = [Max Motor Inrush] + [Sum of Other Running Motors] + [Continuous Load minus motors already counted]

Step 3: Apply kW-to-kVA Conversion Using Power Factor

Generators are rated in kVA (kilovolt-amperes), not kW. The bridge between them is power factor: kVA = kW ÷ Power Factor. Power factor (PF) typically sits at 0.8 for mixed industrial loads and 0.9 for modern commercial facilities with good power conditioning. So a 100 kW facility at PF 0.8 actually needs 100 ÷ 0.8 = 125 kVA generator. This step trips up facility managers constantly. Many specify “100 kVA” thinking they’ll get 100 kW. Reality: at PF 0.8, that same 100 kVA unit delivers only 80 kW.

Step 4: Add Safety Margin

Add 10–20% overhead to your calculated kVA. Why? Several reasons. Your load estimate might be conservative because you didn’t measure everything. Your facility evolves—new equipment, expanded operations, process scaling. You need transient margin for unexpected step loads and voltage sag tolerance during motor inrush. So: 125 kVA requirement plus 15% margin equals roughly 144 kVA. Round to the next standard generator size: 150 kVA (or 120 kW at typical 0.8 PF).

Step 5: Account for Generator Derating

Environmental conditions reduce generator output, so apply corrections. High Altitude (above sea level): add 2–3% to kVA requirement per 1,000 meters. High Ambient Temperature (above 25°C): add 3–5% per 5°C rise. Humidity (very high or coastal): typically minor, but note it.

Example walkthrough: A facility at 1,500 meters elevation in a 38°C climate needs 150 kVA baseline. Apply altitude derating: 1.5 × 2.5% = +3.75%. Apply temperature derating: +4%. Adjusted requirement: 150 × 1.0775 ≈ 161.6 kVA. Your final spec: 175 kVA generator.

Selecting the Right Generator Capacity: Tools and Frameworks

Once you’ve calculated your requirement, how do you choose the actual unit? Diesel generator size calculator tools (both online and in software like CAD) can automate these steps. But the real decision sits here: Do you match the calculated requirement exactly, or do you add margin?

Most professionals add 20–30% margin above the calculated peak load. Here’s why: Your facility evolves. New equipment, expanded operations, and changed duty cycles add load. Running at 80–90% capacity leaves room for transient spikes without exceeding the generator’s emergency overload rating (typically 110% for 15 minutes, once per day). Generators operate most efficiently at 60–80% continuous load. Adding margin doesn’t hurt efficiency if you stay in band.

Now, for backup power planning, your sizing strategy depends on duty cycle. Standby Power (10–100 hours annually, emergency-only use) lets you size closer to calculated peak—less concern about chronic over-efficiency. Prime Power (24/7 continuous operation, cost-critical) needs larger margin because any efficiency loss compounds daily. Peak Shaving / Load Leveling (daily peak hours) requires staying in the 60–80% band, so plan ample margin over daily peak.

Here’s a backup power planning framework I use:

• For standby (emergency only): add 10–15% margin; match calculated peak plus 10%
• For prime power (24/7 ops): add 25–30% margin; match calculated peak plus 25%
• For peak shaving (daily peaks): add 30–40% margin; size for continuous load plus margin

Real-World Case: How Right-Sizing Saved One Facility $80,000

Let me give you a concrete example from a recent engagement. A mid-sized pharmaceutical manufacturing facility in New Jersey had deployed a 500 kVA diesel generator for critical production lines and HVAC. Their actual measured load, averaged over two years, was 140 kW continuous with peaks of 185 kW during shift changes when three large compressors cycled on.

My analysis showed: 140 kW continuous load, 210 kW peak with one motor inrush, adjusted for PF 0.8 gives 262.5 kVA, and with 20% margin they need roughly 315 kVA. They’d installed 500 kVA—59% oversized.

The financial toll: Capital overexpenditure around $120,000 (extra hardware, installation, upgrades). Annual fuel waste at 30% load factor versus ideal 70%: roughly $45,000 in excess diesel burn over 5 years. Maintenance overkill: 500 kVA requires heavier cooling, larger fuel tanks, more frequent overhauls—add $8,000 annually.

Eventually they replaced it with a 350 kVA unit—still over calculated need, but with growth capacity and efficiency gains. They recovered $85,000 in capital value and now run at 62% average load (in the efficiency sweet spot). Payback on the engineering study: 6 months. This is why backup power planning isn’t a commodity task—it’s precision engineering.

Optimizing Ongoing Operations: Monitoring and Continuous Improvement

After you’ve sized correctly, operational optimization is where real savings happen. Tesla Power provides real-time monitoring dashboards that track fuel consumption rate, load profile, and efficiency metrics continuously. Instead of guessing whether your generator operates optimally, you see instant kW output and load factor, running-hour history and maintenance triggers, fuel consumption rate per hour versus baseline (with alerts on efficiency drift), and predicted runtime given current burn rate and tank level.

Real-time fuel consumption rate monitoring catches early maintenance issues—degraded injectors, air-filter restriction—that would otherwise waste 5–15% extra fuel unnoticed. Tesla Power‘s load-matching features let facility managers schedule non-critical loads during low-load windows, keeping the generator in the 60–80% efficiency band. For a facility running 8,000 hours annually on backup power, this alone saves 12–18% of fuel costs—thousands of dollars monthly.

Frequently Asked Questions

1. What’s the difference between kW and kVA?

kW is real power (actual work being done). kVA is apparent power (kW divided by power factor). At PF 0.8, 80 kW = 100 kVA. Generators are rated in kVA; your load is measured in kW. Always convert.

2. How much safety margin should I add above my calculated requirement?

For standby/emergency: 10–15%. For prime/continuous power: 25–30%. For peak-shaving: 30–40%. The margin protects against load growth, transient spikes, and operating efficiency.

3. Does altitude really affect generator sizing?

Yes. Every 1,000 meters above sea level reduces air density, cutting engine intake oxygen by ~3%. Add 2–3% to your sizing per 1,000 meters elevation. At 2,500 meters, add 7.5%+.

4. Can I upgrade my generator later if my facility expands?

Technically yes, but it’s expensive ($40,000–100,000+ for replacement/installation/testing). Better to size with 20–30% growth margin upfront, even if you don’t use it immediately.

5. What’s the cost impact of oversizing by 50%?

Over 10 years at typical use: roughly $120,000 in excess capital cost plus $50,000–80,000 in wasted fuel burn plus $15,000–25,000 in unnecessary maintenance. Proper sizing pays for itself in 12–24 months.

Conclusion: Get the Size Right, Watch the Costs Fall

Generator sizing isn’t complicated, but it’s precise. Oversizing feels safe until your spreadsheet reveals the cost. Undersizing feels risky until your engineer explains the margin you’ve built in.

The approach that works: inventory your loads honestly. Calculate your peak (including motor inrush). Apply power factor. Add sensible margin. Account for your site conditions. Then—critically—monitor your generator’s ongoing operation with real tools like Tesla Power to catch efficiency drift before it bleeds your budget.

Get this right, and you’ve funded years of facility reliability strategy through fuel savings alone. Get it wrong, and you’re watching money burn—literally in diesel exhaust—for the next decade. Ready to right-size? Start with your load inventory this week. You’ll have the answer by Friday.