99% Waste: The Unexpected Energy Consumption of Smoke Alarms

I thought I'd kick off this series of 'Hidden Standby Loads' with a bit of an obscure one: smoke alarms. It's not a device normally considered for its energy consumption.

Battery powered smoke alarms

Many smoke alarms which you may buy to add to your existing set-up are solely battery-powered. Obviously, if you have this sort of smoke alarm they are not contributing to your household electricity bill.

They do of course still use power, typically supplied by a nine volt battery. The best way to save costs with these is to buy yourself a nine volt battery charger and a set of nine volt rechargeable batteries. At least this way you'll save some money in the long-term and reduce your contribution to toxic landfill.

Mains powered smoke alarms

What I'm really interested in is the power consumption of mains powered smoke alarms. These are hard-wired in to your electricity supply. They too have a battery, but only for back-up. So it's still worth getting  a rechargeable as above for these.

Anyway, most homes will have at least two smoke alarms. It you follow some guidelines you could end up with one in almost every room of your house. Which is probably a good idea, but gives us even more reason to be thinking about their energy consumption.

From doing some analysis with a wireless energy monitor I have discovered that the apparent power draw of these units can be significant (up to around 8 VA). But, we don't pay for apparent power, we pay for real power, so I need to dig a little deeper.

I found this surprisingly comprehensive report on the federal government's energy rating website. In summary:

  • There are two main types of smoke alarm used domestically: ionisation units and photoelectric units
  • Units installed under the Building Code of Australia (BCA) must be connected to the consumer mains power.
  • A battery operated ionisation smoke alarm typically draws less than 100 μW (or 0.1 mW).
  • The background power requirement for photoelectric units is similar to ionisation smoke alarms (typically around 200 μW or 0.2 mW).
  • The problem for smoke alarms is providing DC power to the system in an efficient way.

The verdict

As summarised in the report:

Given the smoke alarm circuit itself uses negligible power (much less than 1 mW), most of the power consumption for these devices (>99%) is associated with the conversion of AC power to a suitable, safe and reliable DC supply. Small amounts of additional power may be consumed by indicator LEDs (that show power is connected) and interconnection to other smoke alarms (if present).

The range of power consumed by these units range from a low of 250mW to 500mW with an average of 405mW.


Most mains powered smoke alarms draw about 0.4 watts. This means a total power consumption over one year of 3.5kWh costing you around $0.70 per unit per year.

So, as an individual there is no pressing need for improvement. But, as the report points out, the smoke alarm industry can still do its bit to improve.

If every home in Australia had two smoke alarms, that means 5,600 kW of continuous and largely wasted energy consumption. I don't mean wasted in purpose: smoke alarms are essential. The wasteful part is the fact that 99% of the power going into smoke alarms goes to converting AC power to DC power.

As far as I'm concerned, that's 99% scope for improvement.

- Ryan McCarthy

By Ryan McCarthy |

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