Calculating the cost of lighting

[Techie-v2]

Hi all,

I want to get some rough figure of how much is costs electricity wise to run our lighting but things like this always confuse me.

If I had 1 1000w parcan running at 100% for 1 hour, would this mean it would be 1kwh x electricity unit cost (which is usually given in kW per hour isn't it)?

If it was only running at 50% for 1 hour would that be 0.5 x electricity unit cost?

If an LED fixture was rated at 148w, would that be 0.148 x electricity unit cost?

Just trying to calculate a rough cost for electricity usage, not total cost of ownership ie maintance/servicing etc.

Thanks in advance

 

[dbuckley]

With the dimming of incandescent fixtures, the relationship between brightness and power consumed is not linear.  So 50% brilliance (or indeed 50% fader position, which may or may not be the same thing) may not be 50% power consumption.

Yes, a 1K parcan on for an hour is 1 KWH of electricity consumed.  One KWH is 1000 watts for one hour, so a 148W LED is indeed 0.148 KWH.

And yes, cost of power per KWH times the sum of KWH for the period involved will be the cost of power consumed.  So one needs to take a reasoned estimate of how much of the rig is on for a general situation.  So if it's "most" of the rig and an average of 75% on the faders, then use those numbers multiplied by the max theoretical load to get the estimated load, but be prepared to be a fair way out.  But for an estimate it may well do.

Silly (but can be useful) rule of thumb; if electricity is about 0.25 pounds (or dollars), then the cost to run that thing for a year continuously is about twice the wattage in pounds (or dollars). 

[Don Allen]

One point to take into account is that LED fixtures have a power supply, which may have a fan, so there is always a small amount of power always used when they are on.

There were some good spreadsheet calculators put out by suppliers to show how much saving can be made when switching to led fixtures.

[Techie-v2]

Thanks both, it's useful to know I was on the right lines. Like I said its just rough numbers in my head to get an idea of what power we actually use going forward and that I wasn't over thinking the calculations.

[timsabre]

Also bear in mind, LED fixtures would only use their rated power with all colour emitters on full, which rarely happens in normal use. For a simple RGB par can showing full red, the power usage would be roughly a third of the rated power (plus a small amount of static power draw as Don says). So a realistic calculation for LED fixture power draw would use a figure somewhat less than the rated power.

[adam2]

For a tungsten lamp operated at full power, simply multiply the load in kilowats by the operating time in hours to get the total energy consumed in KWH. For example 12 kw of lamps used for 3 hours will be 36 kwh consumed. Multiply this consumption by the price that you pay per unit, in order to get the  expense, for example 36 kwh consumed at 40 pence per unit will be 1440 pence, or £14-40.

This ignores the standing charge which is reasonable since that is payable whether or not the lighting is used.

Lamp replacement costs are extra.

For lamps used on a dimmer the position is more complex, I would calulate asabove, and then deduct say 25%

 

[Andrew C]

You know, the simplest method might be to read your meter immediately prior to, and after a performance.  OK, there will be other loads (catering, public space lighting etc) but those figures can probably be cancelled out by doing a similar exercise on a non-event day.

[alistermorton]

47 minutes ago, Andrew C said:

the simplest method might be to read your meter immediately prior to, and after a performance.

Indeed at one our local halls this is precisely how they operate - the stage lighting has it's own sub-meter(s) and costs are calculated directly from that.

[indyld]

When I was a kid, my dad put on some fairly high quality productions in our small village hall. 

Two major management items needed attending during performances. 

The first was to tie up the ballcock to the header tank that fed the Gents toilet, as this otherwise noisily refilled during the show. It was released for the interval. 

The second, much more important, was keeping the electric meter fed with 50ps to avoid unwanted blackouts in the middle of the show. 

[Andrew C]

17 minutes ago, indyld said:

The second, much more important, was keeping the electric meter fed with 50ps to avoid unwanted blackouts in the middle of the show. 

Been there, done that, AND failed (once).

[sunray]

2 hours ago, alistermorton said:

3 hours ago, Andrew C said:

You know, the simplest method might be to read your meter immediately prior to, and after a performance.  OK, there will be other loads (catering, public space lighting etc) but those figures can probably be cancelled out by doing a similar exercise on a non-event day.

Indeed at one our local halls this is precisely how they operate - the stage lighting has it's own sub-meter(s) and costs are calculated directly from that.

A local church hall specify that the meter readings will be taken and the cost of all energy used will be paid for when stage lights are used, this includes the kitchen, 4 hand driers, a little heating, external lights etc. All of the things normally expected and included in a hire we have to pay extra for.

Bearing in mind the stage lighting runs on a B32 MCB (Peak lighting was only 5KW before my involvement but I added about the same each time) and peak current for the whole building reached 92A for one show it felt a little unfair, but in reality I don't think it had ever been more than about £10 for a show, £2 for rigging/focusing and £4 for rehearsal (before covid) it isn't really much of an issue. The drama group always budgeted £30 which I imagine covered it.

When I started helping the AmDram I spoke with the admin and rather than book the hall (£12/hour) for rigging they allowed me to use vacant time for a £5 donation to cover power used... amazing what a box of Quality Street does.

[sunray]

2 hours ago, Andrew C said:

Been there, done that, AND failed (once).

Y e a h 1986 and the meter was in a locked room and the keyholder who let us in and locked up after was not available during the hire. Luckily one of the guests was able to get a generator so we were blacked out for only about 20 minutes.

Edited November 8, 2023 by sunray

[AncientLight]

3 hours ago, indyld said:

When I was a kid, my dad put on some fairly high quality productions in our small village hall. 

Two major management items needed attending during performances. 

The first was to tie up the ballcock to the header tank that fed the Gents toilet, as this otherwise noisily refilled during the show. It was released for the interval. 

The second, much more important, was keeping the electric meter fed with 50ps to avoid unwanted blackouts in the middle of the show. 

A similar issue occurred several times in schools and village halls to my knowledge.

All lighting works fine for rehearsals, but about fifteen minutes before the first interval, pop goes the power as two or three tea urns are switched on.

[J Pearce]

The best I managed on that front was many years ago on a rather dire amdram production of Sound of Music in a church, where the MD had decided to use the organ for some of the numbers. By the nature of amdram we never managed to rehearse both the organ and the lighting at the same time, and so on the first show the organist issued forth a large chord, the blower responded to demand, and the whole place blacked out.

(They also used a real blindfold and the actor got lost and fell off the stage landing through the violin player’s violin - I was altogether rather happy when the show finished…)

[Techie-v2]

Thanks for the replies people. Yeah reading the meter is an option but as we're a school there could be a lot else on at the same time. Lots of classroom lighting, computers etc although we could take reading one day over a time period when we're not using the lighting then another day when we are and look at the difference as Andrew C says.

Yeah I've had the tea urn issue, 15 minutes before curtain up, not fun, didnt knock out the lighting, but sound, projection, everything else!