Recently I've been playing with the Microchip MCP79412 RTC. It records power down and power up times in internal registers. If you're only interested in logging power outages after the fact, as opposed to knowing about them while they are occurring, then no external backup battery, timer, etc. would be needed. As part of the initialization (setup), the sketch would check the RTC to see if an outage had occurred, and if so, save the power down and power up times. The MCP79412 has on-board battery-backed SRAM and also EEPROM. Like most, the RTC itself does require a coin cell to keep it running during outages; as CrossRoads says, these are usually good for quite a few years.
I've had a few breakout boards made to facilitate development, and am currently working on a library. If the MCP79412 RTC sounds like it would fit your application, I'd be willing to send you a board (pictures below) for the price of testing the library and letting me know how it works for you.
I'm liking the MCP79412 a lot. The DS1307 seems to be the standard go-to workhorse, and it's a fine chip, but the MCP79412 has several advantages. I've only had them running for less than a week, but they're looking more accurate than has been my experience with DS1307s, and there is the ability to trim them even closer. I also like the fact that they will run on less than 5V.
| Feature | MCP79412 | DS1307 |
|---|---|---|
| On-Chip Calibration | ±127 ppm | N/A |
| Alarms | Dual alarms (single output) | N/A |
| Power Fail/Restore Timestamps | Yes | N/A |
| Unique ID | 64-bit ID | N/A |
| EEPROM | 128 bytes | N/A |
| Battery-Backed SRAM | 64 bytes | 56 bytes |
| Vcc | 1.8 - 5.5V | 4.5 - 5.5V |
| I2C Interface Clock Frequency | 400 kHz (Vcc ? 2.5V) | 100 kHz |
| Square-Wave Output | 1, 4096, 8192 or 32,768 Hz | 1, 4096, 8192 or 32,768 Hz |
