Water purifier (Fallout 4)

A water purifiers are two settlement objects in Fallout 4.

Gameplay
The water purifier and industrial water purifier generate varying amounts of purified water in whichever settlement they are built. Their production rates are much higher than that of a basic pump (10 and 40 respectively, instead of 3), usually making them the superior choice. However, this is counterbalanced by their need for electrical power (2 and 5 units of power respectively, vs 4 for the powered pump, none for the manual pump); the considerably larger amounts of crafting components required to build them; and the fact that only some settlements can support purifiers at all (refer to the list below).

Water production
Similarly to water pumps, should the water production exceed the settlement's demand, a proportion of any excess water (usually about 75% of the surplus) will be placed in the workshop inventory on a regular basis, subject to certain caps (see below). Building multiple purifiers increases surplus production accordingly.

A settlement will stop producing purified water if there are more than 5 drinkable items in the workbench (but see Notes), plus 1 for every 4 settlers. This extremely low cap is the reason water seems to only be produced after the player takes all the purified water from the workbench.

Water consumption
Drinkable items stored in workbenches are consumed at random by settlers, at a rate of one per settler per 24 in-game hours. This consumption occurs before water is deposited in the workshop at some settlements, and afterward at others.

If there are not enough drinkable items in the settlement's workbench, items stored in workbenches in other settlements linked by supply lines will be consumed instead. If there are not enough drinkable items in all workbenches in the settlement's supply network, settlers who had to do without drinking will have their happiness capped at 30 for the next 24 in-game hours, lowering the settlement's happiness as a result, and triggering complaints of lack of water, even if the settlement has enormous production capability.

Detailed analysis
As a result of the extremely low storage cap, settlements of population over 6 will experience occasional water shortages when the stockpile runs out, regardless of actual production, unless linked by a supply line to settlements with populations under 6. The most efficient water storage and supply settlement, maintaining sufficient population to have at least one provisioner, one farmer, and one vendor, is one with a population of exactly 4, which, assuming they are producing at least 6 units of water per day, will always have at least 2 purified waters available to supply to other settlements. Settlements with a population over six can thus sustain themselves in a supply network with a certain number of smaller water farming settlements with population 4, as follows:
 * 7-9 population: 1 pop 4 settlement
 * 10-12 population: 2 pop 4 settlements
 * 13-14 population: 3 pop 4 settlements
 * 15-17 population: 4 pop 4 settlements
 * 18-20 population: 5 pop 4 settlements
 * 21-22 population: 6 pop 4 settlements

If a vendor is not required to maintain a high level of happiness, a settlement with a population of 2 will be able to maintain at least 3 units of purified water, as long as it produces at least 5 units of water every day.

Crafting
Water purifiers must be placed on top of a water source such as a stream or pond. The regular water purifier requires two power to operate, the industrial variant requires five power.

Possible building sites
Not all settlements offer the opportunity to build water purifiers due to a lack of suitable water sources. Below is a list of settlements that can support at least one purifier.
 * County crossing
 * Dalton farm
 * Echo Lake Lumber
 * Egret Tours Marina
 * Finch farm
 * Kingsport Lighthouse
 * Longfellow's cabin
 * Murkwater construction site
 * Nordhagen Beach
 * Nuka-World Red Rocket
 * Sanctuary Hills
 * Spectacle Island (allows for the production of 10,000+ units of water)
 * Starlight Drive In
 * Taffington boathouse
 * The Castle (completing Taking Independence automatically activates one industrial purifier, albeit without power)
 * The Slog
 * Vault 88 (already comes with its own variation of water purifier, the Vault-Tec water pump)
 * Warwick homestead