Home of Professional Cinematography since 1996

Generator Needs

Published : 31st January 2004


I am new to the list and hope that this is the correct place to post this question. I am working for a photographer and am trying to get lighting set up for his studio, and on-site locations. My question is what is the smallest generator that can be used for still photos? Do we need to look at amps or wattage? I am a newbie at this, so please bear with me.

Thanks,

Kalli Ward
Tom Bridges Photography



>My question is what is the smallest generator that can be used for still >photos? Do we need to look at amps or wattage?

Kali,

Welcome to the list!

The most important thing to remember when considering lighting or electrical needs is West Virginia! That is... it's an easy way to remember the basic electrical formula Watts=Volts x Amps or W=VA. West Virginia get it. I know it sounds silly but it is a godsend when you need to do quick electrical calculations.

What you do is figure that every 1000 watt light at 120 volts will draw 8.3 amps in US electrical systems. Or another way to look at it is that 8.3 amps are needed for a 1000 watt light in 120 volt systems. The same formula works with any voltage (in other countries) just adjust the numbers.

To figure out your needs for a generator you must figure out how many lights at what wattages and add them up. Remember that when looking at portable generators the specs are usually in watts. Larger commercial generators used on film sets are more often rated in amps.

Look at the generator specs and see how it is set up. Most are setup for single phase systems which means that a 4000 watt generator will have 2 x 2000 watt circuits. This is important when considering what lights you will use on the generator and what plugs will work with the generator.

Another consideration is that the calculation I provided is for use with tungsten or incandescent lights ONLY. Not for HMI's or other discharge lamps like florescent which have their own specific specs of amperage and voltage needs. You must look into that as well if you have needs for those type of lights. Most HMI's over 1200 watts use 220/240 volt power. This will require you to use specifically wired generators to be able to match your plug needs. Under 1200 watt HMI's will use a standard Edison plug which is typically provided on small generators. All HMI's use more amperage on start up and this can bog down a generator is you are near maxing out it's amperage rating.

The other major consideration for HMI or discharge use is to have a generator with crystal synch to maintain the generator speed to produce 60 Hz (cycles/second) in the US at 24 fps or any HMI windows. There is the ability to use a non-synch generator but then the HMI's or discharge lights MUST BE FLICKER FREE. And the generator must be relatively stable. It gets tricky using a generator with any HMI discharge lights so user beware! Best to hire a good qualified film electrician for those needs to avoid film
flicker problems and such.

In most electrical situations it is best to give yourself a 10-15 % headroom in ratings. A 2000 watt generator will be max'd out using 2 x 1000 watt lights on it. Get the next higher Geni so you have some headroom in the wattage ratings. Things will run better and smoother with less heat problems overall on the generator.

Hope this helps.

Jim Sofranko
NY/DP