varies 17,520 times a (non-leap) year. The TNUoS charges for a HH metered customer are based on their demand during three half-hour periods of greatest demand between November and February, known as the Triad. Due to the nature of electricity demand in the UK, the three Triad periods always fall in the early evening, and must be separated by at least ten clear working days. The TNUoS charges for a HH customer are simply their average demand during the triad periods multiplied by the tariff for their zone. Therefore, () a customer in London with a 1 MW average demand during the three triad periods would pay £19,430 in TNUoS charges. TNUoS charges levied on NHH metered customers are much simpler. A supplier is charged for the sum of their total consumption between 16:00 and 19:00 every day over a year, multiplied by the relevant tariff. Constraint payments Constraint payments are payments to generators above a certain size, where the National Grid gives them dispatch instructions that they are unable to take the electricity that the generators would normally provide. This can be due to a lack of transmission capacity, a shortfall in demand, or unexpected excess generation. A constraint payment is recompense for the reduction in generation. Major incidents Power cuts due to faults in the national grid, or lack of generation to supply it with sufficient power, are very rare. The overall performance of the system is published on National Grid's website and includes a simple high-level figure for the transmission system availability. this was 99.999612%. In 2020–21, issues affecting the low voltage distribution networks – for which National Grid is not responsible – caused almost all the 60 minutes or so per year, on average, of unplanned domestic power cuts. Since 1990, there have been a small number of prominent power outages which were linked to National Grid: August 2003 In the early evening of 28 August 2003, a power cut affected 476,000 customers in the south London area, as well as the London Underground and some rail services, for approximately 40 minutes. A total of 724 MW of load was lost. An oil leak had been left untreated, except for top-ups, for many months, pending a proper fix. This caused an alarm which was misinterpreted by the National Grid control room. While switching the presumed faulty equipment out, an incorrectly-sized protection relay installed several years prior