what percentage of electricity is lost during transmission
Electric power transmission Transmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending on the specific conductors, the current flowing, and the length of the transmission line. For example, a 100 mi (160 km) span at 765 kV carrying 1000 MW of power can have losses of 1.1% to 0.5%. A 345 kV line carrying the same load across the same distance has losses of 4.2%.[20] For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the   I 2  R  {\displaystyle I^{2}R}  losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size (cross-sectional area) is reduced ten-fold to match the lower current, the   I 2  R  {\displaystyle I^{2}R}  losses are still reduced ten-fold. Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. At extremely high voltages, more than 2,000 kV exists between conductor and ground, corona discharge losses are so large that they can offset the lower resistive losses in the line conductors. Measures to reduce corona losses include conductors having larger diameters; often hollow to save weight,[21] or bundles of two or more conductors.
Mains electricity Following voltage harmonisation, electricity supplies within the European Union are now nominally 230 V ±10% at 50 Hz.[6] For a transition period (1995–2008), countries that had previously used 220 V changed to a narrower asymmetric tolerance range of 230 V +6%/−10% and those (like the UK) that had previously used 240 V changed to 230 V +10%/−6%.[7] No change in voltage is required by either system as both 220 V and 240 V fall within the lower 230 V tolerance bands (230 V ±10%). Some areas of the UK still have 250 volts for legacy reasons, but these also fall within the 10% tolerance band of 230 volts. In practice, this allows countries to continue to supply the same voltage (220 or 240 V), at least until existing supply transformers are replaced. Equipment (with the exception of filament bulbs) used in these countries is designed to accept any voltage within the specified range. In the United States[8] and Canada,[9] national standards specify that the nominal voltage at the source should be 120 V and allow a range of 114 V to 126 V (RMS) (−5% to +5%). Historically 110 V, 115 V and 117 V have been used at different times and places in North America. Mains power is sometimes spoken of as 110 V; however, 120 V is the nominal voltage.