It was extremely effective in two of our previous homes and we're hoping for the same savings on the heating bills here.
Cavity wall insulation typically pays for itself in about 3-5 years. Perhaps even quicker if the price of fuel increases. While loft and floor insulation are DIYable, cavity wall insulation isn't.
We've also started measuring the electricity consumption of some of our appliances. To do this we bought the "power calculator" pictured on the right. This purchase was prompted by a recent visitor who commented that our firewall PC would cost more to run for a year than a cheap off the shelf replacement would cost to buy.
Our firewall is actually quite frugal. It's built around a half length ISA embedded 486SLC card on a passive backplane in an old 286 case with 60W power supply. It runs freesco, a tiny linux distribution, and consumes only around 20W. This adds up to about 8 cents of electricity per day for the 16 hours or so that it's usually switched on - around €29 per year. We could buy a replacement box for €20, so our guest is right. The consumption of the plugin appliance style firewall ought to be no more than half this amount, so it would pay for itself in about 18 months which is worthwhile.
I then went on to other things. It turns out that the firewall is a relatively small user of energy. The PC on which I'm typing this (a 1GHz Athlon based machine dating from 2001 - someone else's cast off but fast enough for us because we use Linux) gets through 100W on its own, and the 17" monitor another 55W. It amounts to about 35 cents per day or over €120 of electricity a year. That's nearly a sixth of our entire year's electricity bill !
As a result, I'm also looking into ways of reducing this monster's consumption. A laptop might consume less, but the cost of them makes it unlikely that they'd pay for themselves ever. The same seems to be true for ITX and other smaller form factor PCs.
Replacing the CRT monitor with an LCD of a similar size would probably not help a lot as while it would reduce consumption by 15-20 W when it's on (they typically seem to be quoted as consuming 40W), it would add 5W or so of consumption when it's off. Our current monitor has a proper off switch so consumes nothing when its off. What I have done is to make sure the auto-power off feature of the computer is enabled. If we leave it on and walk away, the monitor goes into low power mode after 10 minutes and the computer goes into standby after an hour.
Read further posts about energy conservation on my other blog.
This being a post from 2008, some parts of it are outdated. We no longer run the same firewall nor any CRT monitors. The argument which I made then still makes sense, but the availability of extremely inexpensive second hand LCD monitors made the CRTs no longer make sense even on the basis that I wrote about above. What's more, I now also have a switch installed to completely disconnect the computer from the electricity network when its not in use. That made a drastic improvement.