To address the insufficient reliability margin of orthogonal frequency division multiplexing (OFDM) in high-speed power line communications (HPLC) for photovoltaic monitoring, this paper proposes a chirp spread spectrum (CSS)-based power line communication scheme. The CSS modulation and demodulation principle is first presented. Considering the transmission characteristics of long-distance underground cables in photovoltaic stations, a frequency- selective fading channel and a Middleton Class A noise model are constructed. The communication performances of CSS and OFDM are then compared. Experimental results show that CSS achieves a 35 dB lower demodulation threshold than that of OFDM due to its spreading gain, thereby significantly improving the reliability margin. Meanwhile, its constant envelope property reduces the peak-to-average power ratio, ensuring excellent anti-noise robustness at low transmit power. The proposed CSS scheme effectively overcomes constraints including harsh channels, severe impulsive noise, and power limitations, offering an effective solution for reliable sensing node communication in complex power grid environments.