Add your connected loads below. The calculator uses the largest single starting surge to size the peak requirement.
Use this sample schedule to test the calculator quickly.
| Load | Category | Qty | Running (W) | Start mult. | PF |
|---|---|---|---|---|---|
| Tower lights | Lighting | 6 | 400 | 1.0 | 0.95 |
| Site offices | Cabins/Office | 1 | 2500 | 1.2 | 0.90 |
| Concrete mixer | Other | 1 | 1500 | 2.5 | 0.85 |
| Submersible pump | Pumps | 1 | 2200 | 4.0 | 0.80 |
| Air compressor | Compressors | 1 | 3000 | 6.0 | 0.80 |
The calculator sizes the generator for both running and starting conditions:
- Total running load (W) = Σ (Qty × Running Watts)
- Diversified running (W) = Total running × Diversity factor
- Expanded running (W) = Diversified running × (1 + Future %)
- Largest surge add (W) = max[ (Qty × Running) × (Start mult − 1) ]
- Adjusted surge add (W) = Largest surge add × Starting method factor
- Derating multiplier = 1 / (1 − Derating %)
- Required kW = (Load W ÷ 1000) × (1/Efficiency) × Derating multiplier
- Required kVA = Required kW ÷ System power factor
Practical sizing may also require checking alternator transient response, harmonic loads, and local standards.
- List each site load with quantity and running watts.
- For motors, enter a realistic starting multiplier.
- Set diversity, future expansion, and site derating values.
- Enter system power factor if you have measurements.
- Press Calculate to see the recommended size.
- Use CSV or PDF buttons to save your report.
Construction power planning is rarely one-size-fits-all. Site loads change as trades rotate, equipment starts and stops, and temporary distribution boards are moved. A generator that is too small can stall motors, trip breakers, and create voltage dips that damage tools. A generator that is too large wastes fuel, increases rental cost, and may run at light load for long periods, which can lead to wet stacking. This calculator helps you size capacity using a clear set of assumptions.
Step one is a load schedule. Enter each load’s quantity and running watts. Treat lighting, cabins, heaters, and chargers as steady loads with a low starting multiplier. Motors, pumps, and compressors need extra care because their starting current can be several times their running current. Enter a realistic “Start mult.” for each motor-driven load based on nameplate data, experience, or supplier guidance. The calculator uses the largest single additional starting surge (not the sum of every surge) because, on many sites, the worst start event drives the peak demand.
Step two is realism and margin. Apply a diversity factor to account for the fact that not every circuit is fully loaded simultaneously. Add a future expansion allowance to cover added cabins, extra lighting, or new pumps later in the project. Then include derating for high ambient temperature or altitude, which reduces available generator output. An efficiency factor is included so the recommendation remains conservative when conditions are uncertain.
Step three is understanding kW versus kVA. kW is real power that performs work. kVA is the apparent power the alternator must supply, and it rises when power factor is low. If you do not have measurements, 0.80 is a reasonable starting point for mixed construction loads. For welding machines, VFD-driven equipment, and UPS systems, confirm harmonic limits because non-linear currents can increase alternator heating.
Example: using the sample schedule (tower lights 6×400 W, office 2.5 kW, mixer 1.5 kW, pump 2.2 kW, compressor 3.0 kW) the total running load is 11.6 kW. With diversity 0.85 and future expansion 15%, expanded running becomes about 11.34 kW. The largest surge add is from the compressor at 6× starting, adding 15.0 kW. With efficiency 0.92 and power factor 0.80, the peak requirement is about 35.8 kVA, so a 40 kVA set is a sensible standard choice. Use the CSV and PDF outputs to document assumptions for approvals, then verify breaker settings, cable sizes, and alternator transient response with the supplier. For critical loads, allow additional margin and test under site conditions.
Size for both. Running load determines continuous kW, while starting surges drive peak kVA. This tool combines diversified running load with the largest starting surge to suggest a practical generator rating.
Use 0.80 as a conservative starting point for mixed site loads. If most loads are resistive heaters and lighting, power factor may be higher. Low power factor increases required kVA.
Use nameplate locked-rotor information when available. Otherwise, typical ranges are 2–3 for small fans, 3–5 for pumps, and 5–7 for compressors. Soft starters and VFDs reduce starting demand.
Apply derating for high ambient temperature, high altitude, or restricted ventilation. These conditions reduce engine and alternator output. If you already have a manufacturer derating curve, use that value for better accuracy.
On many sites, the worst-case event is one large motor start while other loads are running. Summing all surges can oversize the set. If multiple large motors can start together, increase your multipliers or add margin.
Common allowances are 10–25%, depending on project phase and uncertainty. Early works often change quickly, so use a higher margin. For fixed, well-defined loads, a smaller margin may be acceptable.
Treat it as a strong shortlist. Confirm transient performance, harmonics, breaker coordination, and fuel consumption with the supplier. For critical operations, perform a site load test before finalizing.