I predict these won't ever be smaller, lighter and cheaper than traditional design with an rotating engine and separate generator.<p>In general, the generator weighs and costs more than the engine for the same power level. In the linear design, the generator is used inefficiently because the magnets are moving in a sinusoid instead of a constant velocity past the windings, and the maximum speed of a piston is lower than the optimum speed of a generator. So you'll need more magnet & wire for the same power level, increasing total weight and cost.