I have already a 5kW grid connected solar system and I want to use energy for mining which is not used in my house and would otherwise be delivered to the grid.
Is it possible to reduce the cpu clock to reduce the power consumption of the miner?
I don't think you'd get anything done with a 5kW PV system even if you could underclock the miners, no offense. I don't think it's possible to do so without opening them up and tinkering yourself, though, which is risky. Anyway, yours is not a large system at all, enough to cover the average home's electricity needs perhaps but definitely not enough to handle a miner's power draw. A single S9 under normal operating conditions will draw 12.7 MWh a year, or 12,702 kWh. If you're in an excellent location for solar, your 5 kW system may produce... what, 7,000 kWh a year? If you used no electricity yourself and it all went to the miner, that still would barely cover it running at half-power. I've done the math on it myself, and in my location (which is average, but not exceptional, for solar PV production) I'd need an 11 kW system (with installation costs of $30k+) to cover one single miner costing $1.5k.