What size water heater do you actually need? An HDB sizing guide

Most water heater conversations focus on brand — Joven vs Ariston vs Rheem. Important, but actually less impactful than getting the size right. A perfectly-sized budget tank beats a too-small premium tank every morning at 7am.

Here's the sizing logic we use when we recommend storage tanks in HDB and condo flats.

The starting point: hot water per person

A regular shower in Singapore uses roughly 30–40 litres of hot water over 8–10 minutes. Hair wash days, bath fills or extended showers can push that to 50L+.

But — and this is the bit people get wrong — you don't need a litre of tank capacity for every litre of hot water you use. Tanks mix: as you draw hot water out the top, cold water flows in the bottom and the heater keeps working. A 25L tank can deliver roughly 35–40L of hot water in one go before the temperature noticeably drops.

Real-world sizing

15L tank

• Good for: Single person, short showers, one bathroom.
• Reality: Often too small. Even one long shower or a hair wash empties it.
• Our take: Skip unless space is genuinely the only constraint.

25L tank

• Good for: Couple, single bathroom usage, or a family of 3–4 who shower at different times.
• Reality: The most popular HDB size for a reason. Re-heats in 25 minutes.
• Our take: Default pick for 3 and 4-room HDB.

40L tank

• Good for: Family of 4+, multiple bathrooms on the same line, morning shower pile-up.
• Reality: Lets two people shower back-to-back without losing heat.
• Our take: Default for 5-room HDB and condos. Worth the extra $80.

60L or larger

• Good for: Big landed homes, multiple bathrooms drawing simultaneously, or homes with a bathtub people actually use.
• Reality: Overkill for most HDB. Slow to reheat (45+ minutes).

The morning shower test

The easiest way to size your tank: think about a typical weekday morning.

1. How many people shower before 8am?
2. How close together are those showers (15 minutes apart? Back-to-back?)
3. Does anyone wash their hair daily?

One person showering alone → 15L works (but 25L is safer).
Two people, 30 minutes apart → 25L is fine.
Two or more people, back-to-back → 40L minimum.
Three or more people on a school morning → 40L plus consider staggering, or split the heater across two bathrooms.

When instant beats storage entirely

For some setups, an instant heater is the right answer regardless of family size:

• Single bathroom, single user — instant gives endless hot water and saves the standby heat loss
• Service yard too small for a tank
• You take very long showers (15+ minutes) and one tank can't cover that

The trade-off: instant heaters can only serve one outlet (usually just the shower). No hot water at the basin tap from the same unit.

Real install considerations

Sizing isn't just about capacity. Three things to check before ordering:

Where it'll mount

A 25L tank is roughly 50cm tall × 35cm diameter. A 40L tank approaches 65cm tall × 45cm. Measure your service yard or false ceiling space before you buy.

Electrical capacity

Storage tanks under 40L generally use 13A circuits. 40L+ often needs 20A. If you're upgrading from a smaller tank, you may need an electrician to upgrade the breaker as well — budget $80–$150 for that.

Plumbing distance

The further the tank is from the shower, the more hot water sits in the pipes cooling down between uses. If your tank lives in the kitchen yard and your shower is across the flat, the first 10 seconds of every shower is "hot water in transit" — not actually heating.

The simple recommendation

If you're sizing for an HDB flat in Singapore and don't want to overthink it:

Live alone? Instant or 15L.
Couple? 25L storage.
Family of 4 with one bathroom line? 40L storage.
Big family or multiple bathrooms? 40L + a second instant for the secondary bath.

Get the size right and the brand becomes less critical. Get the size wrong and you'll resent even the best brand every morning at 7am.