I've had this bicycle for many years and have configured it with a couple different electric drive systems. The first was a ZAP DX unit in 1997.
The current drive is a Wilderness Energy 36 volt brushed hub motor with 35 amp controller. It's not real speedy but it gets the job done. There's a thumb throttle on the handlebars.
The XtraCycle thingamajob is an extension to the frame that adds tremendous cargo capacity. It weighs less than five pounds, it bolts into place where the rear wheel was meant to be and provides a place to put the rear wheel. It comes with a pair of things that act as panniers and hang from the frame. It's able to hold four bags of groceries at least.
The batteries are a pair of the DeWalt battery packs which are built from the A123systems cells. I opened the packs and soldered in some wiring so that for discharge I have direct access to the batteries, however I left the BMS in place for use with the charger. The two packs are wired together and carried in this bag which ZAP sells in the DX bicycle kit. The DX kit came with this plastic platform that's held in place with hose clamps. The bag fits into the platform and contains some velcro to hold itself in place.
Electrically the pack is about 33 volt (nominal) and 5 amp-hour. I get about 4 miles with no pedaling (perhaps further) The pack is amazingly light for the amount of power it holds.
I currently mean for this bicycle to be my around-town shopping vehicle, a.k.a. "grocery getter".