But government's impact on technology can't always be measured in the same way as the return on investment of a venture-capital fund. If it could, then it would in fact be optimal to leave investing to the private sector. Government is needed, though, when there are spillovers from investment that exceed the private return. In the case of solar and battery power, these could come from economies of scale, and from networks of manufacturers and suppliers. Also, government can afford to be more patient than private investors — after all, A123 Systems recovered from its bankruptcy, and is now growing again.