Low-energy bulbs

Low-energy light bulbs split the nation down the middle. Half of us believe that these bulbs will save energy, bring down our energy bills, and reduce carbon emissions. The other half believe that there are all sorts of things that haven’t been considered, and that if we look at the whole picture then the traditional incandescent bulbs are actually better. I have a lot of sympathy with the second point of view. You really do have to look at the bigger picture to get the truth. Trouble is you actually have to look, not just raise a new talking point.

Low-energy bulbs take more energy to manufacture than incandescent bulbs.

Compact fluorescent lightbulbs (CFLs) are much more complicated than incandescent bulbs. Each CFL bulb contains phosphor, mercury vapour and a microchip (the ballast). This compared with a simple tungsten filament in an incandescent bulb. Of course it takes more energy to produce a CFL than a regular bulb.

In fact, it takes over 9 times as much energy. However CFLs last about 8 times as long as an incandescent bulb (around 8000 hours against around 1000 hours). That means that in the long run, the CLFs only come out slightly worse on embodied energy. This page (cached version as the site seems to be down) shows that it only takes 50 hours of operation for the CFL to save that additional energy.

Just as you shouldn’t only look at the operational energy, you shouldn’t only look at the embodied energy. Both will give you a distorted view. You need to look at the whole life costs – both the capital costs (although they’re fairly non-existent in this case) and the running costs, as well as the benefits or savings.

Incandescent light bulbs help to heat the house.

I came across this idea at the weekend in a letter to the Times science supplement, Eureka (I would link to the online version but it’s paywalled). It asked the question, “What about the heating you lose when you switch from an incandescent bulb to a low-energy one?” In essence this is suggesting that the wasted energy is not truly wasted, as it is useful as heat.

I’m glad I came across it as gives us a chance to explain one of the common misconceptions about electricity.

It’s actually a good question. All the wasted energy from an incandescent bulb does turn to heat. This means that if you assume that all the heat given off by the bulb is useful (which of course it isn’t – not on a warm summer night when you’re tucked up in bed reading for example) then there would in fact be a slight increase in energy use as measured at the meters. This is because the boiler will not be 100% efficient.

The big thing that this misses out is the fact that electricity has to be generated in a power station. No power station is 100% efficient – or even 60% efficient – so a large proportion of the energy in the fuel is lost as heat up the cooling towers.

This wasted energy is reflected in the relative costs of gas and electricity, with gas being about 1/3 of the price of electricity. Because of this, even though there may be a slight increase in units of energy used in the home, the overall energy bills will certainly go down. As is usually the case, saving money shows that when the whole system is considered, energy is also being saved.

To prove the point I ran some numbers on this and came up with the graphs below for the annual change. The numbers include costs of bulbs, cost of electricity and cost of heating.

Assumptions: Light bulbs is on 3.12 hours per day (13% of the time), the boiler in the house is 90% efficient and 75% of the heat from the bulbs is useful heat.

This set of graphs shows the big difference between the energy savings and the other two measures. This is because we are only looking at the energy saving as measured by the meter in the house. If we looked at the energy bills of the power station where the electricity is generated there would be a much closer match.

So are low-energy light bulbs worth it?

So now we have figures for the investment in low-energy bulbs, and for the savings from them, we can calculate some payback ratios.

I’ve not been able to find any figures for the embodied carbon of light bulbs so this analysis makes the assumption that there is a 2:1 split between gas and electricity used in the manufacturing process, and that the bulbs are all made in China with a carbon factor of electricity of around 0.9 kg CO₂/kWh. If anyone knows of a better source than this guess, please let me know and I’ll be happy to update the analysis.

See this post for an explanation of what all these numbers mean.

The chart above shows that low energy light bulbs are one of those special beasts that save energy, carbon and money. That gives us a negative cost of energy and carbon, and a saving of carbon for every kWh saved. This is not uncommon for energy efficiency projects. Looking at marginal abatement cost curves (MAC curves) you will find a lot of the energy efficiency measures lie below the zero line, meaning that the cost of carbon is negative.

This type of analysis is a great way of looking at the whole life cost of a project, whether it be a virtually capital-free project such as deciding to use low-energy bulbs rather than incandescent ones, or whether it is a capital-intensive project such as installing solar panels. The numbers in the diagram above can help you come to a decision on whether your idea is worth it.