The Nvidia CMP 40HX payback period depends on its efficiency (hashrate), selling price, and energy consumption. The estimate features Nvidia recommended retail price - $699, but you can change it to the price you actually paid for your GPU. The energy expenses are not taken into account in the calculation. The GPU hashrate corresponds to optimal overclocking parameters. Make sure to read: GPU Overclocking on Different Algorithms.
To maximize hash rate you are probably going to need to configure additional settings. Choose an optimal GPU overclocking value to maximize hash rate and keep energy consumption at an acceptable level. Algorithms may require core overclocking, memory overclocking, or both. The main overclocking principle is stable work and maximum hash rate at an acceptable energy consumption level. It’s important to remember that each GPU is unique. If for one it is possible to overclock the core to 2000 MHz, for a similar one you may be able to overclock only to 1900 MHz. Make sure to adjust overclocking settings for each card individually. Find out more: GPU Overclocking on Different Algorithms.
