Wanted: power on/off recorder

Google "data logger".

You may already have one. Using a suitable spare computer (say a laptop running XP) plugged into a good supply, connect it to a hub/switch plugged into the supply that you want to monitor. Set the correct level of audit for system events. You will then get TCP entries in the event log (Events 4201/4202 with XP) every time the power fails on the hub/switch and is restored. Filter the log for those events and you will have a complete history of on/off supply transitions.
You can set traps on those events, to carry out actions such as generating messages, emails, start other systems, sound alarms, etc. You can monitor the events remotely - eg over the internet..
A very cheap and effective way of, say, monitoring when and for how long something runs (eg a water pump) - even wirelessly, if the hub/switch is wireless..
Set the log file appropriately and you can record for years and years..
Another of my little secrets given away.... ;)

It sure helps to know the right search string. Thanks for the tip. It yielded a gold mine of useful info.

Thank you for sharing this secret. I think this is what's needed. Would you share another secret with this neophyte? Could you provide a bit more detail about "hub/switch," what it is and how I would connect it to the XP computer?

This is an example:
It connects via an ethernet cable, such as this:
Notice that the total cost is opnly a few pounds.
The hub/switch is a device normally used to connect computers together into a local area network. However, once connected, if it loses power and stops working, this is detected by the computer(s) connected to it. When it gets power back, this is also detected by the computer(s).
In this case, the hub/switch is not being used to connect computers, but is being used purely to give the computer a way of sensing when the power supply (into which the mains adapter of the hub/switch is plugged), is interrupted and restored. Many operating systems, Windows XP included, log events such as the loss of connection mentioned above. It is then fairly easy to extract the times of all the power losses and power resumes, from the event log. Windows XP gives tools for doing that. Plus allows you to trigger things, such as sending an email, whenever an event (or particular event) happens..

OK. I understand. Thank you for all the help.

I don't know if this can be any useful, but I know of 12v mechanical counters that step up 1 digit every time they receive a 12v dc pulse at their input. I would connect a little delay relay circuit, like those used for obtaining a couple of seconds delay before connecting the speakers to a power amplifier. Of course I'd use it in reverse mode, connecting in line the relay contact that is ON when the power is off. So, when the 110v or 220v line comes back, it will power a transformer (with a 12v dc output) to generate a pulse for the counter, that ends when the relay switches on (after 2 seconds). And when the mains power goes off, it all starts again...

An alternative would be to use a little LCD digital counter, but it must stay always powered on (e.g. battery-powered). The count pulse could be easily generated with a 110v or 220v ac relay, using one of its contacts to send the appropriate voltage reference to the counter input...