Formula Used
Effective running watts = Quantity × Running watts × Duty percent.
Apparent power = Effective running watts ÷ Load power factor.
Usable generator watts = Generator kW × 1000 × Derating percent.
Safe target = Usable generator watts × (1 − Reserve margin) × (1 − Shed buffer).
Required shed load = Total running watts − Safe target.
Peak starting watts = Total running watts + Selected surge extra watts.
Estimated runtime = Fuel tank liters ÷ estimated fuel use per hour.
How to Use This Calculator
Enter the generator rating, voltage, phase count, derating, and reserve margin. Add each load with quantity, running watts, starting watts, power factor, duty percent, phase, and priority. Press the calculate button. Review the result above the form. Shed loads marked as low priority first. Export the CSV or PDF for records.
Example Data Table
| Load | Qty | Running W Each | Starting W Each | Power Factor | Duty % | Priority |
|---|---|---|---|---|---|---|
| Essential lighting | 12 | 18 | 18 | 1.00 | 100 | 1 |
| Water pump | 1 | 1100 | 3200 | 0.78 | 60 | 2 |
| Air conditioner | 1 | 1800 | 5400 | 0.82 | 70 | 5 |
Generator Load Shedding Planning Article
Planning Generator Load Shedding
Generator load shedding is a practical plan for keeping vital circuits alive during an outage. It compares the generator rating with the real demand from lights, pumps, refrigerators, fans, tools, chargers, and controls. The goal is not only to total watts. The goal is to decide what stays online when capacity becomes tight.
Running Demand and Starting Demand
A strong plan starts with running load. This is the power used after equipment is operating normally. Motors and compressors also need starting power. That short surge can be much higher than normal demand. A generator that looks large enough on running watts may still trip during a motor start. This calculator estimates both values, then adds derating, reserve margin, and shed buffer.
Priority Based Control
Priority matters during load shedding. Essential circuits should have low priority numbers. Comfort or delayable circuits should have higher numbers. When the calculated load exceeds the safe target, the tool suggests shedding lower priority loads first. It also keeps each circuit visible, so the final choice stays clear.
Power Factor and Phases
Power factor helps convert watts into apparent power. This matters because generators are rated by both power and current limits. Low power factor loads can consume more capacity than their watt rating suggests. Phase balance is also important for larger systems. A heavy single phase load can create stress even when total demand seems acceptable.
Fuel Runtime Review
Fuel runtime is estimated from tank size, full load fuel use, and the active load percentage. It is an estimate, not a factory test. Real fuel use changes with engine condition, altitude, maintenance, and fuel quality. Still, it gives a useful planning number for outage schedules.
Final Planning Notes
Use this calculator before an outage plan is finalized. List every connected circuit. Enter realistic demand values. Check the suggested shed list. Then compare the output with site rules, equipment labels, and qualified electrical advice. The result helps teams reduce overload risk, improve uptime, and document a clear generator operating plan.
Good records also help future reviews. Export the summary after testing several scenarios. Compare the kept load with the shed load. Save the phase balance and runtime notes. During drills, update loads that changed. Small additions, such as chargers or heaters, can quietly reduce reserve. Regular reviews keep the plan reliable under outage pressure.
FAQs
1. What is generator load shedding?
It is the planned removal of selected loads when generator demand rises above a safe limit. It protects the generator from overload and keeps essential circuits running.
2. Why is starting watts important?
Motors and compressors may need extra power for a few seconds during startup. This surge can trip a generator even when running watts look acceptable.
3. What does reserve margin mean?
Reserve margin is unused capacity kept for safety. It helps handle measurement errors, added loads, voltage dips, heat, altitude, and future changes.
4. How should I set load priority?
Use priority 1 for essential circuits. Use priority 5 for loads that can be turned off first during an outage or overload condition.
5. Why does power factor matter?
Power factor changes the apparent power drawn by a load. Low power factor equipment can use more generator capacity than simple watt totals show.
6. What is phase imbalance?
Phase imbalance happens when one phase carries much more load than others. It can cause heating, voltage problems, and poor generator performance.
7. Is the runtime result exact?
No. Runtime is an estimate. Real fuel use depends on generator model, maintenance, temperature, altitude, fuel quality, and changing load patterns.
8. Can this replace an electrician?
No. It supports planning and documentation. Always confirm final generator settings, wiring, breakers, and transfer systems with qualified electrical professionals.