Low power bulbs just make economic sense. Just.

Image by Jeffrey Simms Photography via Flickr

Been looking at my energy bills in the wake of some startlingly high ones and am struggling to do the maths around low-cost bulbs.

The basics. Our gas supply is much cheaper than electricity, weighing in 3.6p per kWh for the bulk of our usage compared to 9.7p per kWh for the bulk of our electricity. Of our not inconsiderable bill we run 65:35 in gas’ favour. Gas powers heating, hot water and the bulk of cooking so no surprise there. A quick calculation suggests gas generates six times more of our power than electricity.

We have abnormal electricity use. I run an a freakish amount of networking equipment for a non-geek. Two routers, two wifi networks, Mac servers, hard drives, laptops. We have halogen spotlights everywhere (12 in the kitchen alone). And despite that electricity represents less than a sixth of our total energy consumption.

Getting more efficient on the gas side is simple enough: lower the thermostat, cut back on hours of heating, wear more layers. The economics speak for themselves. I have already bought socks and jumpers, the wear and tear for keeping them on while in the house is minimal.

On the electrical side the trade off just isn’t as clear. Say (and this is a big assumption), I run 12 x 50W lights, 8 hours a day, (this would be the top end of my estimate; we generally turn lights off as we move round), I am looking at a 4.8kWh daily lighting budget. This translates into 46p a day or about £180 a year.

Say each bulb has a lifespan of 2,400 hours. It would mean each bulb needs replacing once a year. Assume each bulb costs £3. The total replacement cost is £36.

So over a year my lighting costs me £216. And over five years it costs £1030, but discount it back for time (say a 5% annual time discount, deflationary times after all), we have a net present cost of £977.

Now, energy efficient LED lights are now available from great stores like this, but boy are they expensive.

A typical LED light replacing a 50W GU10 halogen is the beast pictured.. It runs at six watts but has the brightness of a 50w halogen. Unfortunately, it’ll set you back around £20.

What are the economics of the LED bulbs?

Now assume the lighting needs I discussed about. 12 x 50W equivalent lights, 8 hours a day. These bulbs run to 6W, which equates to a 576kWh daily lighting budget running to 5.3p per day or about £20 a year.

Each bulb has a lifespan of 40,000 hours or about 15 years. At £20 a piece it’ll cost me £240 to pick up the bulbs. So my year one cost is £260. And over five years it will cost £340, or discounted for time £330.

So the economics look sensible–except for two things:
(a) The payback period of a year looks long for a . . . light bulb. Even though my model tells me so, I still need to persuade my heart that this is an investment with a positive pay back.
(b) Eight hours a day of continuous usage, 365 weeks a year is a really aggressive assumption even from the depths of the British winter. If I evened it out at say 4 hours per day, the payback period runs into nearly two years. And then I have to consider the forty seven 50W GU10s I have in the house–a seven hundred pound replacement cost just to get started.

Which makes me think: I am paying too little for my electricity. Or rather, energy efficiency is being under priced by the market, most likely because we aren’t pricing energy waste correctly.

Any ideas?

Reblog this post [with Zemanta]


  1. Posted December 11, 2008 at 7:58 am | Permalink

    Hi Azeem,

    The simple answer is that it’s a no brainer to upgrade. LEDs are coming along very fast – and the price is dropping accordingly. B&Q have 1.2W halogen replacements for £4.50 that last up to 30,000 hours. By the time you need to replace them I imagine the cost will be < £1.

    All you need to factor in now is increasing your heating to compensate for the 2.4kW of halogen you currently have, and will reduce to 56W as a result of using LEDs!

    Also – if you install solar, you could look at running everything apart from your high-current appliances (cooker, washing machine, etc) on DC instead of AC.


  2. Posted December 11, 2008 at 9:42 am | Permalink

    Hey Gavin

    Thanks for this. I couldn’t see the B&Q halogen replacements on their websites. In any case, these would be equivalent of 12W, I am guessing, so not enough to replace my brighter spots.

    I’ve adopted a slightly different plan: low-energy bulbs where we tend to run the lights for longer (e.g. staircase). For the one spot in the hall where we have GU10s, I have ordered a pair of 6W LED GU10 replacements. I’ll run them for a while, make sure we like the light quality, and then look for a case-by-case replacement of others bulbs.


  3. Pierre Far
    Posted December 11, 2008 at 11:55 am | Permalink

    I’ve been thinking about this too and here is one interesting twist to the math: could you redesign your house life to require less lighting?

    For example: can all desks (“offices”) in the house be put in one room? It’s quite likely more than one person will be working at the same time.

    Take it more micro: I tend to work with the room lights off but I have powerful desk lights. A powerful neon or halogen light doesn’t get too hot and can be positioned far away enough so as not to cause a headache. I currently have 2 shade lamps with halogen bulbs in them, one on each side of the desk. It even looks nice 🙂

    And where you don’t need that much light, put it on a dimmer; keep it dimmed by default and crank it up when needed.

    Another thing: the biggest electricity drains in a computer are the screens, CPUs, and hard disks. For screens, try dimming them a couple of levels. For CPUs, run them (always!) in power saving mode as that decreases power consumption and might just increase their lifetime.


  4. Posted December 11, 2008 at 7:01 pm | Permalink

    Hi Azeem

    http://tinyurl.com/6evtuy 1.2W – I just bought a load of them. They’ve got a pretty interesting range of bulbs and the prices are as cheap as I’ve seen.

    I do completely agree there’s a way to go on the colour temperature.

    Pierre – dimmers and LEDs/CFLs unfortunately don’t yet mix. I hope they do soon.

    Hoping that Philip’s £1bn spending spree on lighting helps move things along quickly…


  5. alemunr
    Posted May 17, 2010 at 1:03 pm | Permalink

    Homewatt claim to have 50W equivalent GU10 LED bulbs for less than £6 each. It might just be a case of “You get what you pay for”, though, but it’s worth checking out.


    Click on GU10 Low Energy and the first couple of products should be 50W equivalent GU10 bulbs.

  6. Posted May 20, 2011 at 9:02 am | Permalink

    Great article, very interesting and thought provoking. I recently purchased some GU10 LEDs from here:


    with a nice multipack discount. We’ll see how this affects my next electricity bill. fingers crossed 🙂

Post a Comment

You must be logged in to post a comment.

%d bloggers like this: