Integral collector storage (ICS) solar hot water systems are remarkably simple and cheap when compared to the other types. They don't use pumps, they don't need controllers, they use only one tank and they require very few extra components beyond a big water tank, some pipes and a pile of insulation materials.

Before you opt for an ICS, make sure you have considered the two important limitations:

• The tank is the collector, so your roof must be capable of supporting a lot of weight.
• ICS systems must not be installed where freezes occur.

An ICS solar hot water system is an open, passive system: the water in the tank is fed directly to hot water outlets, rather than using a separate solar loop (and solar fluid) to transfer the sun's heat. The tank is also the collector – it sits in the sun and heats the water supply directly.

The ICS system is usually plumbed straight into the cold water feed to a standard grid-tied water heater. When someone opens a hot water faucet (or otherwise requests hot water), cold water enters the ICS tank (which is also the collector) and pushes the sun-heated, warmer water out.

This warmer water goes to the standard water heater. If it's hot enough to be supplied direct to the outlet, it goes there; if not, the grid-tied heater does the work of warming it further.

This means that when the sun is shining and the ICS tank/collector is working, hot water uses only as much grid-tied energy as is needed to "top up" the heat level. If the weather is nice and hot, there's no grid-tied cost.