Serviceability rates and reserves are a good indication of the efficiency of any air force and the sort of campaign for which it has set itself up.
At around the time of the BoB the RAF had a higher percentage of serviceable 'first line' aircraft than the Luftwaffe. Not by much but small margins win battles.
The Luftwaffe, designed for short, sharp campaigns maintained a small reserve, about 20% of front line aircraft. In June 1940 the RAF had more aircraft in reserve than at the 'front' and was far better position to fight a long campaign of attrition through the summer and autumn of 1940. It's why having enough aircraft was not a problem for the RAF in 1940.
Cheers
Steve
The RAF also developed a very good system for cannibalising damaged aircraft and getting some of them back in service not really possible for the LW