It really depends on the power consumption of the whole system. For example, a system using 2 watts on average (such as Raspberry Pi 3 with Wi-Fi connected) will need a 48Wh battery to last for 24 hours (24h x 2W = 48Wh) without any additional charging. A 48W solar panel would need just over an hour of full sun to recharge the battery completely. So I used a 80Wh battery and a 100W solar panel which has been sufficient to keep it running.