The main considerations affecting the choice of the sizes and groupings of power station boilers and turbines are security of supply and economics.Whilst the capital and operating costs per kilowatt of boilers and turbo-alternators decrease with increasing size of units, the amount of spare plant that must be held in reserve and the required strength of electrical interconnectors increases for a given degree of security of supplies, so that there is an economic size of unit for which the total costs are a minimum. On an interconnected system, this economic size depends, amongst other factors, on the total load in a load centre and on the distance between stations. The evidence suggests that the economic plant sizes for the British Grid during the next few years are, in most cases, 100 MW or above.Six alternative groupings of boilers and turbo-alternators have been examined (Fig. 1). For each of these groupings, system plant capacities have been so chosen that the degree of security of supply is the same when the stations are working on an interconnected network. An economic comparison of these groupings favours the arrangement in which the capacity of each boiler is equal to that of each turboalternator, the boilers and turbo-alternators are all connected to a common steam range and there is no spare boiler unit.