A power conditioning unit for delivering power from a dc power source to an ac mains power supply output includes a dc power input to receive dc power from a dc power source, an ac power output for delivering ac power to the ac mains power supply, a dc link coupled between the dc power input and the ac power output, first and second dc-to-dc converters coupled between the dc power input and the dc link, wherein one or both of the first and second dc-to-dc converters comprises a pair of resonant capacitors, a sensing element connected in parallel to one of the pair of the resonant capacitors for sensing a ripple voltage, and a controller operative to control the first and second dc-to-dc converters in response to the sensed ripple voltage.