Which costs less to run?

The honest answer is 'less than the difference people expect'. Tankless saves the cylinder's standing loss, which on a modern insulated tank is modest; heavy hot-water households save proportionally less. Meanwhile a heat-pump CYLINDER can beat both on running cost by moving heat instead of making it. Efficiency lives in the whole system and tariff, not the format — run the sizing calculators, then price your actual usage.

Can a tankless fill a big bath?

Yes — slowly if the unit is small. A bath is a quantity job done at whatever flow the unit sustains at your winter rise; it never goes cold, it just takes the time it takes. If fast deep baths are the household ritual, storage suits the ritual better.

What maintenance does each need?

Tankless: descaling matters in hard water — the heat exchanger's narrow passages scale first, and an isolation-valve flush kit fitted at install turns an annual chore into twenty minutes. Tanks: anode inspection every few years is the cheap insurance almost nobody buys, and it is why 'tanks just die at ten years' is folklore rather than fate.

Honest comparison

Tankless vs Tank Water Heater

A tank runs out of hot water but never out of flow; a tankless runs out of flow but never out of hot water. Which limit your household hits first is the whole decision.

Strip the marketing and the two systems fail differently, and that failure shape is the real comparison. A storage cylinder holds a fixed quantity of hot water and delivers it at whatever flow the plumbing allows — three showers at once, happily — until the tank is spent and everyone waits for recovery. An instantaneous unit heats water as it passes, forever, but only so many litres a minute; open a second shower and both cool or the flow throttles.

The winter number decides more than the brochure. An instantaneous unit's output is flow × temperature rise, and the rise depends on incoming mains temperature — a unit that manages two showers on summer mains manages one and a half in February. Sizing on the January inlet figure is the single thing that separates happy tankless owners from the disappointed.

Infrastructure is the quiet gatekeeper. Whole-house instantaneous heating draws enormous instantaneous power: as gas it frequently needs a larger meter and supply pipe; as electricity it wants current most domestic boards cannot offer. A tank sips the same energy politely, spread across the day — which is also what makes it the natural partner for off-peak tariffs and heat-pump water heating.

The factors that actually differ

	Tankless (instantaneous)	Storage tank
The limit you hit	Flow: simultaneous use splits a fixed litres-per-minute budget.	Quantity: heavy back-to-back use empties it, then recovery time rules.
Winter sensitivity	Output drops as mains temperature falls — size on the coldest inlet or regret it.	Insulated storage barely notices the season.
Supply demands	Huge instantaneous draw: gas pipe/meter upsizing common; whole-house electric rarely feasible.	Modest steady draw; content on ordinary supplies and off-peak tariffs.
Standing losses	None — nothing is kept hot.	Real but small on a modern insulated cylinder; the old uninsulated horror stories are about old uninsulated cylinders.
Space	A briefcase on a wall.	A cylinder plus clearance — floor area a small home feels.
Legionella discipline	No stored water, no stored-water regime.	Storage temperature rules exist for a reason and must be respected.

Choose tankless (instantaneous) when…

Space is precious and the household rarely runs two big draws at once.
Long, sequential showers are the pattern — the never-runs-out property earns its keep.
The gas supply already supports the burner, or can cheaply.

Choose storage tank when…

Simultaneous use is normal life: family bathrooms, filling baths, laundry overlapping showers.
Off-peak electricity or a heat-pump cylinder is in the picture.
The electrics/gas can’t take an instantaneous unit without expensive upsizing.

Now run your own numbers

This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes:

Tankless Water Heater Sizing Calculator →Water Heater Sizing Calculator →

Frequently asked questions

Which costs less to run?: The honest answer is 'less than the difference people expect'. Tankless saves the cylinder's standing loss, which on a modern insulated tank is modest; heavy hot-water households save proportionally less. Meanwhile a heat-pump CYLINDER can beat both on running cost by moving heat instead of making it. Efficiency lives in the whole system and tariff, not the format — run the sizing calculators, then price your actual usage.
Can a tankless fill a big bath?: Yes — slowly if the unit is small. A bath is a quantity job done at whatever flow the unit sustains at your winter rise; it never goes cold, it just takes the time it takes. If fast deep baths are the household ritual, storage suits the ritual better.
What maintenance does each need?: Tankless: descaling matters in hard water — the heat exchanger's narrow passages scale first, and an isolation-valve flush kit fitted at install turns an annual chore into twenty minutes. Tanks: anode inspection every few years is the cheap insurance almost nobody buys, and it is why 'tanks just die at ten years' is folklore rather than fate.