110V and 220V AC. It is also using four 1.5V AAA size batteries as a backup, which will be connected automatically with the circuit in the absence of mains power. The batteries will only be connected automatically with the circuit during power outage due to which they will last longer.

After building the circuit, it requires a simple adjustment for the first time. To adjust the circuit first adjust the output pulses of the 555 timer to one pulse per minute so the calculator add one digit per minute, this can be done by adjusting the 1M variable resistor. Any low cost calculator can be used with the circuit. 

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This simple power outage time estimator will automatically starts calculating the time of power outage in your absence and shows on the screen of the calculator in minutes. The circuit is simple to build and using very few common and low cost components. The circuit will work on both
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Power Outage Time Estimator Circuit Diagram