Simple water storage at home

Tap water from the public supply is clean when you fill it. Whether it stays that way depends mainly on two things: whether germs get in during filling and withdrawal, and whether the container is suitable. Months on the shelf are less of a problem than a dirty bottle neck, a finger on the inside of the lid or a can that once held something else.

Light and warmth encourage the growth of algae and germs, especially in translucent containers. Plastic can also absorb odours from its surroundings – water stored next to paint, solvents or petrol will eventually taste of them. That is why a stock belongs in a cool, dark place with no chemicals nearby.

Bought, sealed water is the most reliable because it was filled cleanly in industrial conditions and tightly closed. It carries a best-before date; you use it in daily life before then and buy new. That keeps the stock fresh without anything having to be thrown away.

The amount follows your authority. Germany's BBK gives around two litres per person per day for emergency preparedness, Ready.gov in the US one gallon (about 3.8 litres) per person per day including sanitation. Example calculation: four people times two litres times five days makes 40 litres. Children, sick people, pets and hot weather need extra reserves – and do not forget water for cooking pasta or rice.

For your own containers: food-grade, with a tight screw cap and easy to clean. Milk and juice bottles are poorly suited because residues stay in crevices. Wash the container with hot water and washing-up liquid, rinse thoroughly, wash your hands and do not touch the inside of the lid or opening. Then fill to the brim with tap water, close tightly and note the date.

You replace self-filled water at fixed intervals; Ready.gov gives six months for this. A fixed date helps, such as when the clocks change in spring and autumn. The old water is not wasted: it can be used for watering plants, cleaning or flushing the toilet.

Containers that ever held fuel, chemicals or cleaning products are off-limits for drinking water. Residues cannot be reliably removed, and some substances are harmful even in small amounts. A can once designated for water should never be used for anything else either – ideally label it “Drinking water only”.

If it is unclear whether stored water is still fine – the container is damaged, the water smells or is cloudy, or nobody knows how old it is – do not simply drink it. Either treat it before drinking (the “Water” world explains how) or use it only for flushing and cleaning. Also watch for official advice: if the tap water itself is contaminated, the authorities will say what to do.

Think about weight: one litre of water weighs about one kilogram. A full 20-litre can is therefore heavy and belongs on the floor or a sturdy low shelf, not up high. That way nothing tips over and nobody has to lift heavy containers above head height.

If a disruption is announced – a severe-weather warning or a planned interruption of the water supply – you can also fill clean pots, bottles and jugs shortly beforehand. The bathtub can be filled as a reserve for flushing the toilet. Bathtub water is not drinking water, however, because the tub is not clean enough for that.

In high-rise buildings, electric pumps often push water up to the higher floors. If the power fails, the water can stop there even though the mains still work. For people on upper floors a stock is therefore especially useful – also because water cannot easily be carried upstairs in an emergency.

If you are short on space, you can split the stock: a few packs of bottles under the bed, in a cupboard, in the basement. What matters is that every spot is cool and dark and that you keep track. A small list with place and date on the inside of a cupboard door helps with that.

Authorities recommend a water stock because it is available immediately, independent of power, pumps and pipes – without any treatment. It is the simplest and cheapest form of preparedness there is. The exact quantities can change, so it is worth checking the current version of your authority's guidance.

This entry only covers storing water that is already safe to drink. Water from a rain barrel, stream or well must be treated before drinking; filtering alone is not enough for that. Chemical agents for long-term preservation are deliberately not described here.

A good stock is alive: it is used, replaced and checked. If you drink your bottled water in daily life anyway and always buy a new pack before the last one is opened, you always have a fresh stock without much effort.