Inverter for the field

Busker Alley · January 26, 2014

I need some power for my equipment in the field. Below is a list of devices I would like to run off an inverter in my car. I was wondering if 120vac/800 watts would be enough when everything is run at the same time.

Work light

120vac / 500 watts

Streetlamp

120vac / 40 watts

Handycam

120vac / 18 watts

Camera monitor

120vac / .8 A

What do you think?

Is 800 watts going to be enough?

Less? More?

Thank you

Stuart Brereton · January 27, 2014

Car batteries are designed to deliver a large amount of power over a very short period of time, not power lights over long periods. You would be better off using a leisure battery of some sort.

Mark Kenfield · January 27, 2014

Honestly, given that you can pick up a portable inverter generator for around $400-$600 off ebay, I think that's the best way to go. The freedom it gives you to recharge batteries and power a few small lights out in the field is a BIG deal when it comes to shooting remotely.

And perhaps more importantly, it can also power the kettle for a cup of tea when you're out in the wild.

Edited January 27, 2014 by Mark Kenfield

Busker Alley · January 27, 2014

I just plan on running the car engine while the inverter is in use.

Zac Fettig · January 27, 2014

Is it 800 watts intermittent or continuous? If continuous, you'll be fine. If it's intermittent, it won't be big enough.

I'd also like to point out that an alternator is only likely to produce about 50A at 12V (or 600 W). So it'll run down the battery, even if the car is running. A battery charger and a couple of deep cycle marine batteries might be a better approach.

I'd think it's probably too small. If you can, use fluorescents instead of the work light. They'll draw a lot less power. Or rent/buy a generator. A 4000W generator will be more than adequate.

Busker Alley · January 27, 2014

Well, if the little 4 banger engine is producing 600W, isn't that enough to cover me?

The units will be turned on and off as the filming progresses.

Not sure about the intermittent vs continous bit.

Now considering a little rental generator.

Thanks for the info.

Zac Fettig · January 27, 2014

Not if the inverter is drawing 800W. The engine is gonna need about 200-300 of those watts just to keep the ignition going (unless it's a diesel). So you'd be drawing 800W off a system that's only outputting maybe 300. It'll draw down the battery fairly quickly, and car batteries don't work well once they've been drawn down.

Typically an inverter will have two ratings. One for continuous (leave it on) and one for intermittent (short term, but risk burning it out).

The other big downside to an inverter is that the big ones (>150W) can sometimes cook your car's electrical system. Which gets real expensive in a hurry!

No problem for the info.

Guy Holt · January 28, 2014

The other big downside to an inverter is that the big ones (>150W) can sometimes cook your car's electrical system. Which gets real expensive in a hurry!

I also don’t recommend that you try to power lights with an inverter through the lighter socket. Car lighter sockets are only capable of handling a couple of hundred Watts at most. To run this load you would be better off, as Zac suggested, with a "Battverter" - which is a Battery/Inverter system. A "Battverter" system consists of a 12V DC power source (usually Marine Cells), a DC-to–AC True Sine Wave Power Inverter, and a Battery Charger. Wire these components into a Road Case or milk crate and you can put it on the floor in the back of the car.

Here are some production stills that show you two Battverter systems I built to run lights in vehicles at various times. The first is a 750W "Battverter" rig wired into in Calzone case.

To maximize the running time on the batteries, I made up a "jumper cable" that we attached to the leads of the pickup truck's battery. That way the engine alternator charged the batteries as they were being discharged by the light. Tie–ing the Battverter into a vehicle engine will extend the running time on your Battverter batteries so much that they may never run out of power.

The production stills below show a more elaborate 1800W Battverter system that we built to run 16 - 4’ kinos tubes inside the airport shuttle bus. Use this link for details on how we wired it into the shuttle bus.

shuttlefilmstrip4.1lg.jpg

(Kino Flo 4x4s rigged to an exo-skeletal frame of a Shuttle Bus and powered by an 1800W Battverter)

If you don’t require a lot of light, a Battverter will even enable you to use a car engine as a generator. Use the engine to run the lights through the Battverter as described above during set up and rehearsals. When it comes time to shoot a take, simply shut off the engine and continue to run the lights on the Battverter alone. Running the vehicle engine between takes charges the batteries so that they will run lights all night.

shuttlefilmstrip3.2lg.jpg

(Custom 1800W BattVerter powers 16 - 4' Kino Flo single tubes rigged

in the interior and on the exterior of an Airport Shuttle)

When building these rigs, keep in mind that when voltage goes down, amperage goes up. Wire that carries 12V DC has to be much larger than that which carries the same load at 120V AC. For instance to supply 12 volts to the 1800W inverter used on the shuttle bus required that we run 2 ought feeder to the buses' alternator. Also be sure that the alternator is large enough to take the load without burning out.

Finally, You have to be really careful when choosing a DC-to-AC inverter for film production because there are three basic types of inverters and not all of them are suitable for all types of motion picture lights. For more information on what type of inverters to use with different type of lights I would suggest you read an article I wrote about portable generators that is available online at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html. Since inverter generators use the same three types of inverters, the information in the article is applicable to stand alone DC-to-AC inverters designed for use with batteries as well.

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental & Sales in Boston.

Zac Fettig · January 28, 2014

I also don’t recommend that you try to power lights with an inverter through the lighter socket.

Good point! Although usually (hopefully?), the fuse will trip before that. Otherwise, something is gonna melt!

Busker Alley · January 28, 2014

Although I could wire that myself, I'm not going to.

I'm now looking at generators on a popular local list.

However, I'm left wondering why anyone would buy a second hand generator that doesn't include an onboard inverter.

120VDC seems pretty useless to me.

Edited January 28, 2014 by Busker Alley

Edward Lawrence Conley III · January 30, 2014

Busker, not sure what you are talking about "120v DC". The generator will output 120v and 240v AC- depends on size in the US.

If all you need is 800watts I would rent a Honda 2000eu generator- common at most rental shops.

Busker Alley · January 30, 2014

It must have been a one-of-a-kind that specifically stated it didn't have an inverter.

I saw a little used Coleman Powermate 2250W for $100.

Guy Holt · February 2, 2014

I saw a little used Coleman Powermate 2250W for $100.

For the load that Busker wants to power (incandescent lights, monitor, etc.), a Coleman Powermate 2250 will work. But, if instead of incandescent lights he were to use Flos, or HMIs, it may not. Sizing a portable generator for a lighting load can be very complicated when you use lights sources like HMIs, Kinos, CLF lamp banks, & even LEDs. On generators it matters not only what type of generator you use but also what type of ballasts the lights use. The poor Power Factor and Harmonic Noise that magnetic and non-Power Factor Corrected electronic ballasts (HMI, Kino, CFL, & LED incld.) kick back into the power stream can have a severe adverse effect on the power waveform of some generators, but not others. And, as more and more powerful LED fixtures come onto to market, there is the potential hazard that, because of the low wattage of the individual fixtures, users are lulled into complicacy. If the LED fixtures are not Power Factor Corrected, then their low wattage can create a dangerous sense of false security when it comes to sizing a portable generator for LED loads consisting of large arrays (like the ones pictured below) or in quantity on portable generators.

Besides the examples pictured here, take a hypothetical independent feature film shooting a night scene on a city street with a Canon DSLR. Because of the speed and light sensitivity of the camera and the amount of ambient light from store windows and street lamps they figure they can get away with a lighting package consisting of a 800W Joker Buglite to augment the background and four 40W 1x1 Litepanels to model their talent? It is a low budget production, so they can’t afford a grip truck but instead trick out a rental box. The rental box can’t tow a generator, so they plan instead to use a portable gas generator. Why not, after all they are using only energy efficient LEDs and a Joker 800? They opt for a conventional 1000W generator figuring it will be enough(800W+40W+40W+40W+40W = 960W). Will it work? No, because of the low wattage of the lights, they failed to consider their Power Factor when calculating the load that they will put on the generator and so they overload it. A careful analysis of the Power Factor of their lights (the Joker 800 and 1x1 Litepanels) indicates that their lighting package would in fact draw 1675W.

Why? If we look at the technical specifications for the Joker 800 Buglite, we see that it uses a non-Power Factor Corrected ballast with a Power Factor of .58. According to the K5600 website, the Joker 800 ballast draws 12.5 Amps rather than the 7 Amps you would think using Ohm’s Law (800W/110V=7.2A.) What that means is that it has an Apparent Power of 1375W (110V x 12.5A = 1375W) or draws nearly twice the power to generate 800W of light output than a quartz instrument of the same wattage. Used on wall outlets, this relatively inefficient use of power is negligible because the power draw of the Joker 800 fits easily in a standard wall circuit. However, the greater Apparent Power of the Joker 800 must be factored when using portable generators because the generator must be sized to supply the Apparent Power (1375W), even though only the True Power (800W) provides light.

The same is true when it comes to the 1x1 Litepanels. According to the manufacturer, the AC-to-DC power supply that Litepanel uses for their 1x1 fixtures has a Power Factor of .54 and so draws nearly twice the power (an Apparent Power of 75W) for it’s true power output of 40W. If you were to use this lighting package on a 1000W conventional generator, the total Apparent Power of 1675W (1375W + 75W + 75W 75W + 75W = 1675W), would overload the generator. Even though it’s power is cleaner and more stable, you would not be able to run this package on a Honda EU1000is Inverter Generator either because, with a continuous load rating of 900W, the accumulative load of 1675W would also overload a 1000W inverter generator.

Could you operate this lighting package on a conventional generator like the Coleman Powermate 2250 mentioned above? Again, the answer is “no” because the greater Apparent Power of lights with a poor Power Factor is not the only consideration when operating them on conventional generators. Of equal importance, is the Harmonic Noise that ballasts with poor Power Factor kick back into the power stream that severely limits the total amount of Leading Power Factor loads, as compared to Unity Power Factor loads, that can be reliably operated on conventional generators.

Left: Grid Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw Arri non-PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw Arri non-PFC Elec.

Given the large sub-transient impedance of conventional generators even a small degree of harmonic noise being fed back into the power stream will result in a large amount of distortion in its’ voltage (see oscilloscope shots above.) Add to that, the fact that the original supply voltage waveform of conventional generators is appreciably distorted to begin with, and you have a situation where the return of any harmonic currents by a non-PFC HMI, Fluorescent, or LED ballast will result in significant waveform distortion of the voltage at the power bus and operational problems with the generator voltage and frequency regulation. This is graphically illustrated in the You-Tube video, “Compact Fluorescent verses The Generator", by Lighting Designer Kevan Shaw’s (available on-line at http://www.youtube.com/watch?v=LeCqreRMzKM), when he is not able to operate an equivalent Apparent Power of CFLs, as he could incandescent light, on his small 850W generator.

For the same reason that Kevan Shaw was not able to operate more than 270 Watts of CFL bulbs (15–18W bulbs) on his little 850W generator, our hypothetical indie feature will not be able to operate their lighting package on a 2000W conventional AVR generator like the Coleman Powermate 2250 mentioned above. After all CFLs have the same Power Factor as the Litepanel 1x1 power supplies (.54.) The adverse effects of the harmonic currents that non-PFC ballasts generate, so graphically demonstrated in Kevan’s video, limits the total amount of Leading Power Factor loads, as compared to Unity Power Factor loads, that can be reliably operated on conventional AVR generators.

In fact, Kevan Shaw’s You-Tube video illustrates the old math that it is not possible to load conventional generators beyond roughly 65% of their rated capacity for more than a short period when the load consists of lights with a poor Leading Power Factor (Max Apparent Power of 540W/850W Generator = .64) Which translates to a maximum load of 1300W on a 2000W conventional generator. Where the total Apparent Power of our lighting package consisting of a Joker 800 and a couple of 40W 1x1 Litepanels is 1525W, it will overload even a 2000W conventional generator.

Will our lighting package operate on a 2000W inverter generator like the Honda EU2000is? The oscilloscope shots above indicate that it would. Even though the non-PFC ballasts of our lighting package kick back the same harmonic currents, the voltage waveform of inverter generators retain an over all sinusoidal shape because of their lower system impedance and purer original power waveform. The appreciable difference in voltage distortion created here by the same light demonstrates that an inverter generator will provide cleaner power, and operate more reliably, regardless of the type of load.

Left: Grid Power w/ 1.2Kw P-2-L PFC Elec. Ballast. Center: Conventional AVR Power w/ 1.2Kw P-2-L PFC Elec. Ballast. Right: Inverter Power w/ 1.2Kw P-2-L PFC Elec. Ballast

As the oscilloscope shots above illustrate, Power Factor Correction can be of tremendous benefit when operating HMIs, Kinos, and LEDs on portable gas generators because a PFC circuit realigns voltage and current, eliminates the generation of harmonic currents, and induces a smoother power waveform at the distribution bus. PFC circuits successfully increase the power factor to as much as .98, making ballasts with it near linear loads. As a result, the ballast uses power more efficiently with minimized return current and line noise and also reduces heat, thereby increasing their reliability. For instance, if you were to replace the Joker Ballast with a Power-2-Light 800W PFC HMI ballast instead, the same head would draw 8 Amps at 110 Volts (instead of 12.5) and have an Apparent Power of only 880 Watts. If you were also able to replace the non-PFC AC power supplies of the 1x1 Litepanels with Power Factor Corrected ones, the oscilloscope shots above also indicate that you would likely be able to operate the whole package on a 1000W inverter generator (880W + 40W + 40W = 960W.)

What is true of small lighting loads on small generators is also true of larger lighting loads on larger generators. For instance, before reading this post, you would have thought that you could reliably operate a 4k HMI with non-PFC ballast on a conventional 6500W generator. But, where a non-PFC 4k electronic ballast will draw 58A at 120V it will overload a 6500W. To understand why, simply compare its’ Apparent Power of 6960W (58A x 120V = 6960W), to the continuous load capacity of a conventional 6500W generator after de-rating it for a load with Leading Power Factor of .58 (6500W x .65 = 4225W.) Likewise, if you were to replace the non-PFC 4k electronic ballast with a Power Factor Corrected one, the light would only draw 38A at 120V and have an Apparent Power of 4560W. And since, the ballast has a near Unity Power Factor, the 6500W generator would not have to be de-rated, and so could operate the 4560W Apparent Power load without a problem.

These power generation issues have been vexing set electricians for years. Use this link for an article that explains the electrical engineering principles behind these issues and how to resolve them. Even though the poor Power Factor of an individual Litepanel will not have a disastrous effect on a portable generator, poor Power Factor should be eliminated wherever possible because its’ accumulative effect can be severe. Where just about every piece of production equipment used on set today (including video cameras, field monitors, hard-drives, lap-tops, and battery chargers) generates harmonics a viscous cycle can get started with unpredictable results like those evidenced in Kevan Shaws You-tube video.

Guy Holt, Gaffer, ScreenLight & Grip, Lightng Rental & Sales in Boston