Water

You need at least one gallon of water per person per day. This takes into account that you will need to drink at least 2 quarts and will use the rest for hygiene and cooking.

Water, water purification chemicals, and/or purifying filter. Enough to provide one gallon per person per day.

Three gallons per person is heavy (24 lbs), so I strongly suggest that you include a water filter and water treatment chemicals.

I suggest pump type back country filters, such as those made by Katadyn or MSR, that are rated to filter out all bacteria and have a carbon core to remove toxic chemicals, bad tastes and odors.

Boiling kills all bacteria and viruses but is not always an option and does nothing to remove toxic chemicals, bad tastes and odors.

Also, supplement your filter(s) with purifying iodine crystals (or other chemicals), such as a “Polar Pure” water purification kit, to kill all viruses that may not be removed by filters.

Pump filters that are rated for virus removal have tiny pore sizes and tend to clog quickly (a clogged filter is worthless).

At Ready.gov/water it is recommended that "To determine your water needs, take the following into account:
One gallon of water per person per day, for drinking and sanitation.
Children, nursing mothers and sick people may need more water.
A medical emergency might require additional water.
If you live in a warm weather climate more water may be necessary. In very hot temperatures, water needs can double.
Keep at least a three-day supply of water per person."

From the Red Cross:
Having an ample supply of clean water is a top priority in an emergency. A normally active person needs to drink at least two quarts (half gallon) of water each day. People in hot environments, children, nursing mothers, and ill people will require even more.
You will also need water for food preparation and hygiene. Store at least one gallon per person, per day. Consider storing at least a two-week supply of water for each member of your family. If you are unable to store this quantity, store as much as you can.
If supplies run low, never ration water. Drink the amount you need today and try to find more for tomorrow. You can minimize the amount of water your body needs by reducing activity and staying cool.
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There is a container called a WaterBob. It can store up to 100 Gallons of water in your bathtub. It takes about 20 minutes and includes a pump so you can fill other containers with it. If you don't store water this might be the way to go, but you need to do it before the electric goes out or the disaster hits.