A 400 watt inverter should take car of business, ive done some serious powering with one. Why do you think you need an inverter though? Are you worried about the dc-dc converter being to complicated? Or think its just not capable of powering your system? A good basic way of determining the size you need for power is using your factory power supplies spec to figure it out. Mine for instance is only 100w. I dont use powered usb hubs or anything else. That way i know if my factory psu can handle the load, then the m2-atx dc-dc i plan on buying that has more output on every line can too.