Ultra-long silver nanowires (AgNWs) with an aspect ratio of >2000 were prepared by the hydrothermal synthesis method. The influence of reaction time (4-32 h), reaction temperature (150-180 °C), polyvinylpyrrolidone (PVP) molecular weight (10 000-1 300 000 g mol−1), PVP concentration (50-125 mM), glucose concentration (5.6-22.4 mM) and CuCl2 concentration (2-20 μM) on the AgNW length was investigated systematically. The optimum conditions provided nanowires with an average diameter of 207 nm, an average length of 234 μm and a maximum length of 397 μm. Finally, a AgNW electrode was prepared on a glass substrate and used in transparent heater application. The transparent heater enabled outstanding heat-generating properties, reaching >200 °C within 70 s with an applied voltage of 5 V. Our results demonstrate how increasing the aspect ratio of ultra-long AgNWs is beneficial for both optical and electronic applications in terms of increased transmission and a more efficient Joule effect in the heater application. In addition, our results show that AgNWs with different lengths can be simply obtained by tuning synthesis parameters.