12 Volts dc supply with 45000 uF 35 Volts buffer capacitor to ensure a reliable operation of the Gents GPO master clock.
My GPO Hipp Toggle pulses every 14 to 17 swings of the pendulum ( 28 to 34 seconds )
Often small 6, 12 or 18 Volt plug packs are used and have not got the capacity to furnish a 250 mA slave clock pulse or the rapid Hipp Toggle pulse to keep the pendulum in motion.
A big capacitor will assist in the current draw taken by the clock and minimise voltage drop.
I charge a 12V 100Ah accu in my basement for back up in case of a power cut.
It will also reduce wear and tear in Hipp Toggle contacts making unnecessary extra pulses to swing a heavy pendulum every 5 or 6 swings.