Vascular plant bio-photovoltaics (VP-BPV) is a recently developed technology that uses higher plants to harvest solar energy and the metabolic activity of heterotrophic microorganisms in the plant rhizosphere to generate electrical power. In the present study, electrical output and maximum power output variations were investigated in a novel VP-BPV configuration using the crop plant rice (Oryza sativa L.) or an associated weed, Echinochloa glabrescens (Munro ex Hook. f.). In order to compare directly the physiological performances of these two species in VP-BPV systems, plants were grown in the same soil and glasshouse conditions, while the bio-electrochemical systems were operated in the absence of additional energy inputs (e.g. bias potential, injection of organic substrate and/or bacterial pre-inoculum). Diurnal oscillations were clearly observed in the electrical outputs of VP-BPV systems containing the two species over an 8-day growth period. During this 8-day period, O. sativa generated charge similar to 6 times faster than E. glabrescens. This greater electrogenic activity generated a total charge accumulation of 6.75 +/- 0.87 Coulombs for O. sativa compared to 1.12 +/- 0.16 for E. glabrescens. The average power output observed over a period of about 30 days for O. sativa was significantly higher (0.980 +/- 0.059 GJ ha(-1) year(-1)) than for E. glabrescens (0.088 +/- 0.008 GJ ha(-1) year(-1)). This work indicates that electrical power can be generated in both VP-BPV systems (O. sativa and E. glabrescens) when bacterial populations are self-forming. Possible reasons for the differences in power outputs between the two plant species are discussed.