I use FWE (Evans own brand) rack bag with zip down panniers, which can hold quite a good bit of gear for a days riding. My one can take minimum of 30ah battery/batteries and in the zip panniers tools/spares/tubes on one side and extra drink,food, pac-a -mac & camera etc,etc on the other side.
Rear main bag has side stiffeners so it doesn't quish flat, side panniers give a useful good load space for all those extra bits. Fixing on my one is a pair of under Velcro straps ann also one at each corner. Place a thing board and some foam inside on the base for added protection against bumps from uneven terrain.
Despite the weight I find handling is unaffected and fit the rack as low and as far forward as possible and use it for off road riding as well on the North/South Downs.