It will need to be a tank based water purification system, as opposed to the GE Merlin reverse osmosis system which filters your water without a tank. Most of your taps will be connected but it may be wise to bypass washing machines and toilets as they don't need filtered water.

Size of the RO filtration system

Typical daily consumption is 70 gallons of water per person in the household. It may sound much but it includes all usage - washing, drinking, bathing, showering etc. So for a normal family of 4-5 people, you may need to size up the system for 300-350 gallons a day.

Reverse osmosis tank size

In order to cope with the two peaks at morning and evening, you will need a holding tank. This is because the RO system filters water at a slow rate whilst water usage is often concentrated to the peaks. Your holding tank will need to be somewhere around half the daily usage in order to make sure you are not running out in the morning rush.

Size of the reverse osmosis filter

This is a trade-off. If the RO filter is too small, it will need to run 100% of the time and it will wear out quickly. RO filters are expensive, so you don't want to oversize it (too much). A good trade-off is to have a filter that is running for 1/3 of the time - so 8 hours a day running and 16 hours a day turned off.

Cost of a whole house reverse osmosis system

The cost will depend a lot on what "extras" you need - i.e. prefiltration, UV light sterilization (for removing bacteria, virus etc), extra large holding tank etc.

A fairly basic RO system for a 4-person family home, no prefilters and no UV light unit will typically cost $6000 - $8000. Prefiltration is expensive, if you need it you are looking at another $2000 or so. UV-sterilization is another £1000-£2000.