Step 1: Work out how much water your household uses
Don't guess — use your municipal water bill. It shows consumption in kilolitres (1 kilolitre = 1,000 litres) for the billing period. Divide the litres by the number of days in the period to get your daily use.
Example: a bill showing 18 kL over 30 days is 18,000 ÷ 30 = 600 litres a day.
If you only want the tank for essentials during an outage — toilets, cooking, washing — your daily need is lower than your normal use. Garden watering and filling a pool are usually the first things to cut.
Step 2: Decide how many days of cover you want
Multiply your daily use by the number of days you want to get through without municipal water.
Example: 600 litres a day × 3 days = 1,800 litres. Using only essentials at 400 litres a day, the same tank would last 4½ days.
Round up to the next common tank size. Tanks are widely sold in sizes such as 2,500, 5,000 and 10,000 litres, with slimline shapes for narrow side passages.
Step 3: Check how much rain your roof can collect
If the tank is fed from your gutters, your roof decides how quickly it refills. The rule of thumb: every 1mm of rain falling on 1m² of roof produces about 1 litre of water. Allow for some loss to splash, evaporation and first-flush diversion by multiplying by about 0.8.
Litres collected ≈ roof area (m²) × rainfall (mm) × 0.8.
Example: a 150m² roof in a 20mm shower collects about 150 × 20 × 0.8 = 2,400 litres — enough to refill a 2,500-litre tank from nearly empty in one good rainfall.
Only the roof area draining into the gutters connected to the tank counts. If just one side of the roof feeds the tank, use that side's area.
Step 4: Plan the base, pump and filtration
Water weighs about 1kg per litre, so a full 5,000-litre tank puts roughly 5 tonnes on the ground beneath it. The base must be level, solid and at least as large as the tank's footprint, following the manufacturer's installation instructions. An uneven base can distort a tank over time.
Gravity alone gives very little pressure. To run showers and taps from the tank you need a pressure pump, ideally under a weatherproof cover.
To use harvested rainwater inside the house, add filtration. A typical 3-stage system — sediment, carbon block and a fine micron filter — improves clarity, taste and odour. Note that standard filtration does not disinfect water.