15 litres or 25 litres? Work it out in two minutes
The most common mistake in this category is buying too small, then blaming the geyser. Here is the arithmetic.
18 July 2026 · 5 minute read
Buying too small is the most common mistake in this category. It shows up every single morning, and people usually blame the geyser rather than the choice.
Start with how you bathe, not how many you are
Family size matters, but the bigger variable is bucket versus shower. A bucket bath uses roughly 15 to 20 litres of blended water, of which perhaps 8 to 10 litres is hot. A shower runs 6 to 8 litres a minute, so even a short one uses two to three times as much.
| Household | Mostly buckets | Mostly showers |
|---|---|---|
| 1–2 people | 15 L | 25 L |
| 3 people | 15 L, just | 25 L |
| 4–5 people | 25 L | 25 L, staggered |
Why "just buy the biggest" is not the answer
A larger tank costs more to buy and slightly more to keep hot, because there is more water losing heat overnight. Oversizing by one step is sensible insurance. Oversizing by two is money spent on standing loss you will never get back.
Undersizing, though, is the mistake you notice daily — and no amount of waiting makes a 15 litre tank serve five people at seven in the morning.
The recovery time nobody mentions
A tank does not refill with hot water instantly. Once emptied, a geyser takes roughly 30 minutes in normal winter and 40 to 45 minutes in peak winter to heat a full tank from cold.
So if four people need hot water in the same hour, tank size is only half the question. The other half is whether your morning can absorb a 30 minute gap. If it cannot, buy the larger tank even where the litres-per-person sum says otherwise.
A useful rule: count how many people need hot water inside the same 30 minute window, not how many live in the house. Two people who bathe at seven and at nine are a much smaller demand than two who both bathe at seven.
Where it goes on the wall matters too
If ceiling clearance is tight, or the unit has to sit above a false ceiling, a horizontal model gives you the same capacity in a lower space. It is worth measuring before deciding, because discovering the clearance problem on installation day usually means accepting a smaller tank than you wanted.
The two minute version
- Two or three people using buckets: 15 litres.
- Four or five people: 25 litres.
- Anyone showering regularly: 25 litres, whatever the headcount.
- Tight ceiling: look at the horizontal variants.
Want it worked out for you?
Our size guide asks four questions and gives a straight answer, including which model suits your floor and water pressure.