How much fuel does APU use?

Could you elaborate on the fuel consumption of APU systems? I'm particularly interested in understanding the factors that influence the amount of fuel an APU utilizes, as well as any variations in consumption across different types of aircraft or APU models. Additionally, I'm wondering if there are any strategies or techniques that can be employed to optimize APU fuel efficiency, and if so, how effective they are in practice. Ultimately, I'm seeking a comprehensive understanding of APU fuel consumption and its implications for aircraft operations and maintenance.