Generator Sizing Calculator
Accurately calculate the required generator capacity for your commercial facility. Our tool accounts for power factor, starting surges, and future expansion to ensure 100% power reliability.
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Comprehensive Guide to Generator Sizing
Selecting the correct generator size is the most critical decision in power systems engineering. An incorrectly sized generator does not just lead to power interruptions; it causes long-term mechanical damage, excessive fuel consumption, and potential safety hazards. Proper generator installation begins with accurate sizing.
Why Accurate Sizing is Mandatory
In commercial environments like manufacturing plants or telecommunications hubs, power is the lifeblood of operations. If a generator is too small, it will struggle to handle the initial current surge when heavy motors start. This leads to voltage dips that can trip sensitive electronics or cause motors to overheat and fail.
Conversely, an oversized generator is equally problematic. Diesel engines are designed to operate under load. When a generator runs at less than 30% capacity for extended periods, it suffers from "wet stacking"—a condition where unburnt fuel and carbon accumulate in the exhaust system.
Worked Example: Sizing for a Small Manufacturing Facility
Say a small manufacturing facility measures its steady-state running load at 80kW, and its equipment operates at a typical power factor of 0.8. Dividing kW by power factor gives a base apparent load of 100kVA (80 ÷ 0.8). That figure alone is not what you size a generator against — it only reflects normal running, not the moment machinery switches on.
Applying a 25% starting-surge safety margin (the same "startingLoadFactor" the calculator above uses) to account for motor inrush current brings the adjusted demand to 125kVA. Adding a conservative 10% allowance for future expansion — a new production line, additional cooling, or a second shift — brings the design target to roughly 137.5kVA of prime power. Because standby generators are conventionally rated about 10% higher than their prime-power equivalent, the generator actually specified for this facility should be in the 150kVA range, not the 100kVA figure the running load alone would suggest. This gap — between "what we normally draw" and "what we should buy" — is exactly where undersized installations go wrong.
Critical Technical Factors
1. Starting vs. Running Load
Electric motors require significantly more power to start than they do to run. This is known as Locked Rotor Amps (LRA). A motor might draw 10 Amps while running but require 60 Amps for the first few seconds of starting.
2. Altitude Derating
In high-altitude areas like Nairobi, the air is thinner, which reduces cooling efficiency and oxygen for combustion. A generator might lose 3% of its power for every 300 meters of altitude above 1000 meters.
Next Steps After Sizing
Once you have determined the correct size, it is vital to establish a diesel generator maintenance schedule to protect your investment. Failure to maintain a correctly sized unit can still lead to a generator not starting when you need it most.
Frequently Asked Questions
What is the difference between Standby and Prime power?
Standby power is for emergency use during a utility outage. Prime power is for continuous use where utility power is unavailable. Prime units are typically rated 10% lower than standby.
Can I run a 100kVA load on a 100kVA generator?
Technically yes, but it is not recommended. The "sweet spot" for efficiency and longevity is between 50% and 80% load.
Do I need to account for altitude if I'm outside Nairobi?
Yes — altitude derating applies anywhere above roughly 1,000 metres, which covers most of Kenya's major towns. Eldoret and Nakuru sit even higher than Nairobi, so their derating allowance should be larger, not smaller, than the capital's.
Professional Audit
Calculators provide estimates. For mission-critical facilities, a physical load audit is mandatory to ensure 100% capacity accuracy.