The power is likely more important than the voltage. The rating of the transformer is Voltage and Volt/Ampere which is technically the same as Wattage. For this puppy, 10VA is not going to make it. It seems like it requires 15VA alone so with a chime and the power kit, you can count an additional 5VA. You also have to account for the power loss due to the wiring (distance and gauge) I am running mine on a 40VA transformer with a mechnical chime. Voltage, as long as it is above 12V it seems to be ok though higher voltage actually helps lower the wire resistance loss and improve power efficiency by driving a lower current. If you take a voltmeter to it, you will actually notice that without load, it is always a little higher than the rating so I chose 16V as 24V seemed to be on the high side.