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volts/amps/watts... I wish I paid more attention. [Archive] - Event Photographer Society Forum


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05-01-2011, 11:11 AM
I should have paid more attention at school...

I'm trying to work out if my little Honda 1.0i 1kva/kw generator will operate all my gear this year or if I need to buy a new 2kv genny...

My question relates to the draw under load of the DNP Ds 40/80's.

Power consumption looks like 240V 1.69A... (P29) is this right? and is this under load?

Then how do I work this out in watts such that I can add everything together (screens, computer etc) to get my answer.

It's doin' my 'ed :?

Jeremy Nako
05-01-2011, 11:20 AM

Save yourself a lot of grief by going to your nearest Maplins and buying a meter.

For about a tenner, you plug all your appliances into a gang, plug the gang into the meter and it'll tell you what the peak (usually at the time of switching on) and the running draw is.

Drew Hadwin
05-01-2011, 11:23 AM
Quick answer as a bit busy with other things.

Watts = Amps x Volts

1000 watts is 1KW

john wright
05-01-2011, 11:45 AM
For AC, to calculate accurately the size of generator required actually requires a bit more information than the manufacturers give you. The power factor of the devices to be connected is a key parameter. However, a rough and ready calculation can be done based on peak current and peak voltage.

For DC currents the calculation is simple as there is no power factor to consider and the applied voltage is constant. Power (Watts) = Volts x Amps (Current)

For AC currents the RMS (root mean squared) value of the applied AC voltage is used and the power factor of the appliance is needed to get a true average watts calculation. For simplicity and to give some margin for peak currents the VA value can be calulated using the applied AC voltage and peak current figure.

In the example given the simple VA calculation is 1.69 (Amps) x 240 (Volts) = 405.6 VA
Assuming a power factor of one then 405.6 VA = 405.6 Watts

As we have used peak values in the calculation we can (for simplicity) ignore the power factor and for the purposes of sizing use the 405.6 Watts as a base figure. I like to add a 20% margin to this to cover variations in load.

Therefore, for the figures given assume 400 Watts calculated and 500 Watts as the figure to use for sizing of the generator.

http://www.generatorguide.net/watt-acpower.html is a reasonable explanation of the above.


Mike Weeks
05-01-2011, 11:55 AM
It is not just the actual power that needs to be considered but the respnse time of the generator to an increase in load. Whilst idle it is probably less than 100 watts going to 400 as soon as it starts to print. If the generator is slow to react the increase in demand will cause a voltage drop whilst the generator speeds up - the most likely victim will be computers.


Jeremy Nako
05-01-2011, 12:02 PM
Computers, and most specifically routers are very prone to this.

We covered a couple of private functions in November in marquees where industrial generators were being used to power the evening.

Every time we fired the studio heads the router would reset due to the power drain. This caused the wireless connection to fail every time until we realised what was going on and put a small UPS in-line of the router. Netgear routers seem to be particularly sensitive.

Although most people wont use generators with studio lights, be aware that the recycle times can be particularly slow.

05-01-2011, 12:14 PM
Wow, thank you all for that quick response...
Jeremy - off the bat your idea of a trip to Maplins seems logical.

To give you all some idea, I have the two dnp printers and a laptop (with it's own battery) 5 separate screen each drawing 30 watts, 5 x U170 thin clients powered from a usb hub from the laptop and that's about it... (with the exception maybe of the card machine...)

So I know I'm hovering around the 1 kv mark under load and as John says... that's not giving me much room (the 20%).

Is it right to assume that these figures (4-500 watts) are peak loads do we know, in which case it seems I can carry on much as before, because this season I won't be using the two Dell computers as part of the equation as I did last year ...

As you can probably gather, I'm trying to avoid the 1000 for a 2 kv unit...

05-01-2011, 12:39 PM
Incidentally, can someone enlighten me as to where horse power comes into the equation... I notice some of these generators are advertised with this as part of the rating...

john wright
05-01-2011, 12:46 PM
Hi Pete

The current figure quoted on an appliance should be peak current unless otherwise stated.

This makes simplified VA calculations relatively easy as it is just Volts x quoted Amps.

Horse power is the power of the engine driving the generator. Usually, the more powerful the engine the easier it will maintain regulated output under variable loads.


Mike Weeks
05-01-2011, 01:16 PM
To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that grinded corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known. Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today.

1HP is rated to be about 746 watts but you are not allowed to sell by Hp in Europe, it is only used for equivalency.


ian griffiths
05-01-2011, 01:24 PM

No need to spend that kind of money, 500 will buy you a Honda powered 2.7Kva LPG factory converted unit from Stephill Generators in Northants.

Been using them in petrol form for 10+ years and LPG for 3 years. Serviced annually for about 25 oil, plug, filters etc.

We have about 30 of them amongst the guys, never let any of us down.

05-01-2011, 01:51 PM
Do you know, all of you make me really proud to be part of this group.

The help is fantastic and I'm sincerely grateful to you all.


John, Mike, I don't know how you knew all that, but it made great reading.

Ian, thanks, do you know if my wife can lift these machines and what the rated db is?

ian griffiths
05-01-2011, 02:52 PM
Well Pete, she threw me on the bed!



(That's the genny not me!)

http://www.stephill-generators.co.uk/products/3-se2700" onclick="window.open(this.href);return false;

05-01-2011, 04:48 PM
Thanks Ian,
I shall study it and post back when I've got a result.
I just need to test the final draw on my equipment...


ian griffiths
05-01-2011, 05:31 PM
Hi Pete,

You are welcome to come over and try out my generator if you like?

05-01-2011, 08:12 PM
Cheers, it would be good to meet up

Best wishes for now