For any type of accuracy, depending on the power grid is not a dependable option! There can be noise, interference, and power dips. On the short term, the power grid will be accurate, but over the long term, it can be out by a fair amount.
I have an electric clock that is dependent on the power grid for its timing. This clock can be a few minutes or a few seconds out at times.This is not from the power company being off frequency. It is most likely caused by power dips, or noise in the power delivery.
The accuracy of the power companies timing is very accuracy. It has to be in order to keep synchronized to other systems that they are working with. They have many generating facilities that are working together, therefore all their systems have to be synchronized to within fractions of a degree on the line frequency. From what I am told, here in North America, their margin of error is within about
1 second per month.The problem when connecting a device at home to the power line, such as a clock, the clock can jump off time very easily. It is susceptible to interference, and power disturbance through their distribution. I was also told this by an engineer from our local power company.
Normally, during peak conditions, they allow the frequency to drop very slightly. It may be as much as 0.5 to 1 Hertz or so. During peak conditions, they will give it back. This means that all the power generation systems working together must also drop, and increase by the same amount.
The clocks that I have that are referenced to the AC line, are not dependable for accurate time. Over a few weeks, I found the quartz clocks to be more accurate. I can check them with the NIST broadcast.
Here in North America many stores are selling quartz clocks with built in time receivers. These are getting the time reference from the NIST. The user only has to put in the approximate time to within about 30 minutes. After 12 hours, the clock will be exactly on time, as long as it can receive the NIST signal. The clock automatically checks in with the NIST about every 12 hours. If the NIST signal is unavailable, the clock will keep time to the spec of any consumer type quartz clock. This is about 5 to 10 seconds per month.
Another thought would be to get a low cost GPS that can accept an external antenna and external power supply. This will give you the most accurate possible time for home use.
You can also go to the NIST site with your computer, and get very accurate time. It should be within about 100 to 200 ms. Going from memory, I believe it will tell you its error to your computer. By hitting the reset key, you can sometimes get it down to within a few milliseconds.