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Electricity Cost Calculator

Why I created the Electricity Cost Calculator

At the time I was replacing light bulbs around the house. I wanted to know if I replaced light bulbs, how much energy I'd save and how long it would take to pay back the new light bulb.

For example, I had two lights in the family room area both with two 40W lights. When a light was on it used 80W. Now I don't know exactly how much time the lights are on each night so I estimated their usage at about two hours a night. During the winter months that could increase.

Using the Electricity Cost Calculator this made it easy for me to make the calculation for how much energy and the cost of the energy per year. The cost for using a light worked out at about $11.74 a year for electricity.

The next question I had was, if I bought two new low energy fluorescent 8W globes (total 16W), what was the saving and how long would it take to recover the money spent on the globes?

If I recall correctly the cost of the globes was around $7 each. I enter $14 for the cost and 16W as the wattage. This gave me a payback of 18 months. One of the globes had already died, so I needed to either buy a new globe, or get two low energy lights. When replacing a blown globe it is only the price difference between the cheaper globe and the more expensive globe and not the full price which is the cost. In other words you were going to replace a glove anyway, so how much extra does it cost for the low energy globe. At a guess the normal screw globe may have been around a $1. I was going to keep the second globe as a spare for the other light, but over time I just ended up replacing all the lights. After 18 months they'll have paid for themselves.

Supposedly the low energy lights last longer, which means there should be additional savings.

The good thing is with the calculator I could now go into a lighting shop and using my iPhone, I was able to do the calculation whilst I was checking out lights.

For example in our bathroom we have two lights. An single light in the three-in-one unit (light, fan, heater) and a light above the mirror with three halogen lights. When we purchased the light we never thought about energy usage, which in hindsight is a real shame. The halogen lights are 50W each so when the light is on, it is using 150W. The light is on most mornings for around an hour.

Replacement low energy lights with the GU-10 connection are expensive at around $15 each. The low energy lights also aren't very attractive.

The Energy Calculator shows us using the lights for 1 hour a day costs $11 a year. If I replaced the three lights with low energy lights ($15 each for 11W lights) the cost is $45 for 33W. The payback period of 63 months, which is over five years.

Whilst the low energy lights supposedly last longer, I've found a few blowing in a short time, so I really don't have much confidence in the life expectancy figures. I've also found halogen and normal globes blowing sooner than they should, which is another reason I don't put much faith in the life expectancy figures.

As it turned out I decided to purchase a lower energy light for the three-in-one unit. I bought a suitable incandescent 40W light as it needed replacing. In hindsight, I could have bought a dearer low energy light. Now I use the lower energy light most of the time. It was a bit tricky at first as I tend through habit to turn on the light above the mirror with the three halogen lights. But after a while it became second nature to turn on the three-in-one light. I can't remember the cost of the light, but I estimate it to be around $5. Just by changing which light I use I now expect to save around $6 a year.

Rather than use the government light replacement program, which to me means someone is making around $10 for each light globe which is coming from tax payers pockets, I opted to spend my own money and gradually replace lights with low energy lights. There are savings on buying low energy lights. I also avoid the single light fits all approach of the government scheme. The scheme provides lights which are 15W (equivalent to 75W normal globes) and in many cases I've used lower energy lights (down to 8W) to replace lower wattage incandescent lights I previously had. I've also heard a number of people complain of the slow start up of the lights provided by the government scheme. The lights I've used start up with no noticeable delay.

I've also found supermarkets often have lights on special. I recently purchased six lights for $5 at a clearance sale. OK. I purchased six boxes of the lights which will end up lasting me for years. I couldn't go past lights for less than a $1 each.

Now whilst my initial focus was on lights, I've since moved on to other devices around the house. Using the power meter shown on the Electricity Cost Calculator page, I worked out the energy being used by various appliances around the house and some of the results amazed me.

  1. The TV/DVDs around the house when turned off were in standby and the power used in standby added up to around 10% of our electricity bill. I now use a combination of remote control, wall and foot switches to turn off appliances rather than leaving them in standby.
  2. Computers with screens when turned off consume $30 to $40 in standby. Turning them off at the wall saves power and thus money.
  3. Some notebook power charges keep consuming considerable power even when the computer is fully charged. Once the computer is charged turn off the charger.
  4. Whilst it is preferable to turn off power chargers for devices like mobile phones when not being used, I actually found their power use to be very small.
  5. We had a small freezer in addition to our fridge/freezer. It was very convenient to have a good range of food on hand. We made the decision to not store as much food. Really we didn't need to have food stored for weeks since we go to the shops every week. Turning off the small freezer was estimated at saving around 7% of our power.
  6. I now use an iPhone to check things like my email rather than turning on a computer. The iPhone uses much less energy. I then use a netbook and only when I'm doing a solid amount of work use my main computer. I use a notebook as my main computer as it gives me the option of portability. But now I realise using the notebook saves me around two thirds the energy of using a desktop computer.
  7. We decided to stick with our 32" LCD TV rather than purchasing a larger screen. Whilst the larger screens look fantastic, the power consumption is significantly more.
  8. One thing I found very interesting is I worked out the cost of making a cup of coffee in the microwave versus boiling a kettle. A kettle is very efficient, but the problem is with most kettles we fill them up to around half full and end up boiling far more water than we need. I now boil a single mug of water in the microwave for 1 minute and 45 seconds at 1200W. The kettle would take 3 minutes to boil at 2400W. I'm now using 30% of the energy I previously used to make a cup of coffee. Kettles are good when you want to make tea or coffee for a group, but for one person the microwave saves energy and thus money. My next kettle will be one which will allow me to boil enough water for a single cup.

    I have about three cups of tea or coffee a day. Using a kettle that works out at 3 minutes times 3 cups at 2400W. In other words 9 minutes (0.15 hours) at 2400W. The Electricity Cost Calculator shows the energy cost works out to be $26 a year for coffee. Using the microwave means I save around 70% or around $18.

The electricity savings are usually quite small and often it feels like it isn't worth the effort. So far based on our last electricity bill our savings look like they are in the order of 20%, which over the year will be a few hundred dollars. If it was just for the money I'd find it hard to justify the extra effort, but really, the thought that in some way what I do helps a little to reduce my impact on the planet, which in some way might help future generations, to me, makes it worthwhile.

I hope that by sharing my experience it may help you in some way.

- Kelvin Eldridge


  • This article shows the efficiency of a kettle is around 79%. I thought this article was interesting as it shows the maths behind the energy usage. I performed a similar calculation to determine the time it would take to boil a mug of water in a microwave.

  • I've found the easiest method to determine an approximate cost of an appliance is to measure the watts being used by a device using the power meter and the time taken. Electricity retailers state their prices in kWh which is kilowatt hours.

    For example a normal kitchen kettle (which is the kettle we used until recently) uses 2400W (or 2.4kW). I found it took about 3 minutes to boil about 1.5 litres of water which is the level I'd fill the kettle up to without thinking much about it. To work out the cost then, the kettle took 3 minutes which as a fraction of an hour is 0.05 hours (3/60). That gives us 2.4kW for 0.05 hours which equals 0.12kWh. The price of electricity is roughly 20 cents per kWh. Thus the cost to boil the kettle is about 2.4 cents. I estimate that people boil a kettle about 6 times a day (but it could easily be more) which is 14.4 cents per day in electricity. Over a year that works out to be $52.56. For us that would be about 5% of our electricity usage.

    I've since purchased a new kettle for $39.95. This enables me to boil water for a single mug and now takes 45 seconds. This is one sixth of the time it previously took and thus a sixth of the cost. That means I'll save around $43 per year and have paid off the kettle in less than a year. Every year after that I'll be saving around $43.

    I have to say buying a new kettle when the old kettle still worked felt like a waste. But given the new kettle will pay for itself in under a year and be saving money after that, it really is hard to keep using the old kettle once you know.

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