Introduction
The Generator Size Calculator totals the wattage of every light and piece of equipment in your location setup, applies a safety headroom factor (typically 20–30%), and returns the minimum generator capacity needed in kilowatts. It also flags whether a single generator is sufficient or whether a split power distribution with two generators is recommended. Enter individual fixture wattages along with the number of units, and the tool builds a running total with a generator size recommendation from standard rental tiers (2kW, 5kW, 10kW, 20kW, 30kW, 60kW).
What This Tool Calculates
Total load (W) = sum of (Wattage × Quantity) for all fixtures and equipment. Generator capacity required = Total Load / Power Factor × Safety Headroom. Power factor for resistive loads (tungsten, heaters) is 1.0. For inductive loads (HMI ballasts, motors) it is typically 0.85. Safety headroom of 80% running load is the industry standard — a 10kW generator should carry no more than 8kW continuous load. Required generator kW = Total Load / (Power Factor × 0.8). Worked example: 2 × 2000W HMI (inductive, PF 0.85) + 4 × 600W LED (resistive, PF 1.0) + sound cart and monitor at 500W. Total = 4000 + 2400 + 500 = 6900W effective. With PF correction on HMI: 4000 / 0.85 = 4,706W apparent + 2,900W = 7,606W. Required generator = 7,606 / 0.8 = 9,507W → rent a 10kW.
The Formula and How It Works
A corporate video shoot with a 1200W HMI key, two 600W LED panels, camera, monitor, and sound cart calculated a total of 4,100W effective. Required generator: 4,100 / 0.8 = 5,125W → a 5kW generator with 19% spare headroom. For a car commercial with six 2.5kW HMI Jokers, six 1kW LED panels, grip equipment, and 2kW of camera and support gear, the total was 23kW effective. Required: 23,000 / 0.8 = 28.75kW → two 20kW generators in split distribution, each carrying under 75% capacity. A location feature film set running 30kW of lighting continuous plus intermittent crane and dolly motors at 5kW peak specified a 60kW generator with a power distribution board handling both the lighting and grip circuits separately.
Real-World Examples
Generator Size Rental Tiers
2kW (2000W): small documentary and corporate, up to 1 × 1200W HMI plus camera. 5kW (5000W): standard documentary and ENG, up to 2 × 1200W HMI and LED fill. 10kW (10,000W): commercial and small narrative, up to 3–4 × 2500W HMI or equivalent LED. 20kW (20,000W): mid-range narrative and large commercial, up to 6 × 2500W HMI. 30kW (30,000W): large commercial and TV drama, full HMI package with practicals. 60kW (60,000W): feature film, large-scale exterior, and multi-unit location work. Always add 25% above calculated minimum to allow for morning startup surges and last-minute additions.
Pro Tips and Common Mistakes
| Detail | Value |
|---|---|
| HMI ballasts draw more current at startup than during steady-state operation. | |
| Never assume you can run every HMI simultaneously from a cold start without a staggered power-up sequence. | |
| Stagger HMI ignition by 30 seconds each to avoid tripping the generator's overload protection. | |
| Factor in the production's non-lighting draw: craft services hot plates, laptop chargers, walkie-talkie chargers, and on-set monitors add up to 1–3kW on a typical commercial. | |
| The most common mistake is sizing for lighting wattage only and discovering on the first morning that the generator cannot power the total production load.. |
Pro Tips and Common Mistakes
Pro Tips
- An undersized generator on a remote location shoot is one of the most expensive production surprises — it means lost time, emergency rental costs, or compromised lighting.
- Running the numbers the day before the shoot, before the generator trucks are locked, costs nothing..
Common Mistakes
Frequently Asked Questions
What is power factor and why does it matter for generators?
Power factor is the ratio of real power (watts) to apparent power (volt-amperes) drawn by a load. Resistive loads like tungsten lights have a power factor of 1.0. Inductive loads like HMI ballasts and motors have a power factor below 1.0, typically 0.85. A generator must supply the apparent power, which is higher than the watt figure, so inductive loads require a larger generator than their wattage alone suggests.
What is the 80% rule for generators?
The 80% rule states that a generator should carry no more than 80% of its rated capacity as a continuous load. This provides headroom for startup surges, added loads during the shoot, and prevents thermal stress that reduces generator reliability and lifespan. Size your generator so the expected load is no more than 80% of its rated output.
Can I run two generators in parallel?
Yes, if both generators have compatible synchronization electronics and the same voltage and frequency rating. Parallel generators require a synchronization panel and should be managed by a qualified generator operator. For most production shoots, separate distribution boards fed from two independent generators is simpler and safer than true parallel operation.
What fuel does a typical production generator use?
Most production generators use diesel. Diesel is more energy-dense, more fuel-efficient at continuous load, and safer to store on a location than petrol. Propane generators are used on stages and in enclosed locations where diesel exhaust is not acceptable. Electric battery generators are increasingly used for quiet documentary work with loads up to 5kW.
Start Calculating
Use the calculator above to run your numbers before your next production.
