The size of the investment for energy-saving retrofits of air compressors depends on multiple factors, including the type, scale, and usage of the existing air compressors, as well as the energy-saving retrofit technologies and solutions chosen. Let's roughly go through these factors:

Condition of existing air compressors: If your air compressors are very old or less efficient, energy-saving retrofits may require a larger investment. Newer and higher-efficiency compressors may require less retrofit cost.

Energy-saving retrofit technology: Different energy-saving retrofit technologies have different costs. Some common energy-saving technologies include replacing with high-efficiency compressors, improving the insulation of piping and gas transmission systems, and installing variable frequency drives. The specific technical solution will affect the investment amount.

Expected energy savings: The anticipated reduction in energy consumption after the retrofit also affects the size of the investment. The more significant the energy savings, the shorter the payback period, and the higher the potential investment.

Energy costs: The level of energy costs influences whether it is worthwhile to invest in energy-saving retrofits. In regions with higher energy costs, there may be more motivation to implement such upgrades.

Policy support: Some regions may have policies and subsidies related to energy-saving retrofits, which can reduce the investment burden.

Overall, the size of the investment for air compressor energy-saving retrofits varies by region. It is best to conduct some detailed research and consultations first to understand your specific investment budget and potential energy-saving benefits. Energy-saving retrofits are generally a worthwhile investment for reducing long-term energy consumption and operating costs.