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The generators we are now using are much more compact & portable, basically being modified commercial turbo trainer stands, the weighted roller being removed and swapped for an energy take-off system:

Power output depends on the bike, rider and load, but we reasonably expect 50W per unit when powered by the average person; you can get a lot more peak load from a fit and trained cyclist.

construction info

Turbo trainers all from the same supplier on eBay (you'll work out which one if you do a search).

hinged base board approx 12" sq 18mm ply (existing, recycled from previous projects) - painted for neatness

hinges - 2off lengths of 16mm box section steel (sold in B&Q) - hinged on M8 bolts (Screwfix) held on with nylock nuts (ditto) - means they will stay on without being done up tightly (hence along hinge to move). Box steel then bolted to base board with four M6 screws (with large washers & more nylock nuts on the back).

The connection box is a diecast aluminium box from CPC, lid removed and bolted through the base board using M3.5 screws. All that really needs to be connected to each individual motor is a diode (so the current doesn't flow back in to the motor and turn the wheel), but on this one I've added a basic ammeter, which is useful. The connectors are my standard 4 pole XLR, both male and female so if necessary I can loop generators together. I've also allowed for a car cigarette lighter socket, which will be switch in along with a basic regulator - this would allow the unit to be used standalone driving equipment directly. I haven't thought about regulators yet - something about 14v would be useful for battery charging and equipent designed for car use (i.e. big amps), but a more precise 12v would be avisable for less robust equipment, such as radios or televisions that come with a 12v mains adapter, but aren't necessarily desisgned for coping with much more than that.

I've also added screw in eyes to neatly anchor the bungee cord that holds the bike wheel against the skateboard wheel; the board folds up neatly against the turbo trainer almsot flat, the motor & connector box sitting neatly between the turbo trainer legs.

The ammeter has proved useful, and shows around 5A under load and an adult pedalling - more or less the estimated 50W output we'd expect.