People overestimate this dramatically, then buy more machine than they need. The drinking and cooking figure is small; it is everything else that makes household water use look enormous.

The numbers

The gap that confuses everyone

A South African household typically uses somewhere between 150 and 250 litres per person per day in total. Set that against 3 litres of drinking water and the difference looks absurd — until you break down where it actually goes.

Only the last row needs water you would put in a glass. Everything above it needs water that is merely wet.

This distinction is the single most useful idea in planning household water, and it is why a machine producing 20 litres a day is not the absurd proposition it first appears. It is not trying to supply your household. It is supplying the small, critical portion that has to be clean.

Sizing against realistic output, not the rating

The mistake is matching your need to the rated figure. A 20L unit rated at 20 litres per day hits that at 30°C and 80% humidity — not in a Johannesburg August.

Size against your worst realistic month instead. For a family of four needing 10 to 14 litres a day, a 20L unit is comfortable through the humid months and tighter in winter — which is exactly when the purification mode takes over and the machine runs from the tap. That combination covers the year properly.

When to size bigger

• More than six people in the household.
• You are in a drier region, where winter output drops further.
• You want buffer for outages — a bigger unit fills your storage faster when you have supply, and keeps producing when you do not.
• You are also supplying a business, a home office with staff, or a granny flat.
• You have livestock or several large dogs. Animal consumption adds up faster than people expect.

When not to size bigger

If two of you drink 6 litres a day between you, buying an 80-litre machine to feel safe is spending money on capacity you will never use, in a larger footprint, drawing more power per hour. Buy the machine that fits your actual need and put the difference into stored water, which costs almost nothing and covers the scenarios a bigger machine would not have helped with anyway.

How to check your own number

Rather than trusting any table, measure for a week. Fill a marked jug each morning and use it for all drinking and cooking. At the end of the week, divide by seven.

Most households are surprised by how low it is. That number, not a guess, is what you should size against — and it is the number to give any supplier who is quoting you.

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