The unit for power is the watt (symbol: W). Apparent power is often expressed in volt-amperes (VA) since it is the product of rms voltage and rms current. The unit for reactive power is var, which stands for volt-ampere reactive. Since reactive power transfers no net energy to the load, it is sometimes called "wattless" power. It does, however, serve an important function in electrical grids and its lack has been cited as a significant factor in the Northeast Blackout of 2003. Understanding the relationship among these three quantities lies at the heart of understanding power engineering. The mathematical relationship among them can be represented by vectors or expressed using complex numbers, "S" = "P" + "j Q" (where "j" is the imaginary unit).