How often does a system need topping up?
A well designed system with a good match between supply and demand will only need topping up when it has not rained for some time. Severn Trent monitored a domestic system and found that the only occasionally needed to be topped-up. Most of the time the tank was around 50% full (i.e. an ideal balance between having plenty of water to use, and plenty of space to accommodate the next rainfall).
