Capacitor have thin conducting plates (usually made of metal) separated by a layer of dielectric, then stacked or rolled to form a compact device. Many types of capacitors are available commercially, with capacitors ranging from the Pico farad range to more than a farad, and voltage ratings up to many kilowatts. In common, the higher the capacitance and voltage rating, the bigger the physical size of the capacitor and the higher the cost. Tolerances in capacitances value for discrete capacitors are usually specified as a percentage of the nominal value. Tolerances ranging from 50% (electrolytic types) to less than 1% are commonly available. Another figure of advantage for capacitors is stability with respect to time and temperature, at times called drift. Variable capacitors are generally less stable then fixed types.