How the two differ in kind
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
Show
| 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. |
| What limits its capacity | FLOW, at a given temperature rise. Its rating is a flow rate rather than a volume, and it can sustain that flow indefinitely. | Volume, then recovery. It delivers its stored contents at full temperature and then falls back to how fast it can reheat. |
| Winter performance | Falls, and by more than people expect. Incoming mains water is colder in winter, so the required temperature rise is larger and the flow it can deliver at temperature drops in proportion. | Barely changes. A colder inlet lengthens the recovery, but the stored volume is still delivered at temperature. |
| Standby loss | None. It heats only while water is flowing, which is the efficiency argument for it. | Continuous, whether or not anyone uses water — though a modern well-insulated cylinder loses far less than the reputation suggests. |
| Small draws | Needs a minimum flow to fire at all, so a trickle at a basin delivers cold water — and a short draw followed by a pause produces the slug of cold water between two hot ones. | Indifferent. Any draw, however small, comes off the top of the cylinder at temperature. |
| Supply it needs | Substantially more. A gas unit typically needs a larger gas supply and its own flue; an electric one needs a circuit far beyond a normal domestic provision. | Modest and continuous, because the heat is added slowly over a long period rather than instantaneously. |
| Hard water | Scales in a narrow heat exchanger, which reduces output and eventually blocks it. Descaling is a scheduled maintenance item rather than an option. | Tolerates scale for longer, but accumulates sediment in the base that reduces usable volume and insulates the element. |
Which one, and when
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.
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.
- Why does my tankless heater run cold in winter?
- Because its rating is a flow rate at a stated TEMPERATURE RISE, and the rise it has to achieve changes with the season. Incoming mains water that arrives at fifteen degrees in August may arrive at five in February, so a unit asked to deliver water at fifty is doing a thirty-five degree rise in summer and a forty-five degree rise in winter — and its deliverable flow falls in proportion. A unit sized against a summer inlet temperature, or against a manufacturer's headline figure quoted at a small rise, will be adequate for most of the year and disappointing exactly when hot water matters most. Size it on the coldest expected inlet temperature and the flow you actually want simultaneously.
- What is the cold water sandwich?
- A slug of cold water arriving part-way through a draw, and it is characteristic of instantaneous heaters rather than a fault. It happens when a tap is turned off and on again quickly: the water left in the heat exchanger and the pipe is still hot and arrives first, then the heater takes a moment to fire and modulate for the new flow, and the water that passed through during that moment arrives cold behind the hot. Modern units reduce it with faster modulation and small buffer tanks, and a recirculation loop largely removes it. It is most noticeable in showers where a mixer is adjusted, and least where a draw is continuous.
- Will I need to upgrade my gas or electrical supply?
- For gas, very often. An instantaneous unit delivers in minutes the energy a cylinder spreads over an hour, so its input rating is several times a storage heater's — and that can exceed what the existing meter, regulator or pipe size will deliver, particularly if other appliances run at the same time. The pipe run from the meter is frequently the binding constraint and it is not a small job to change. For electric instantaneous units the problem is more severe: the current required for a whole-house unit is beyond a typical domestic supply in most places, which is why electric tankless is usually a single-point solution rather than a house one. Establish this before ordering, because it is where the cost lands.
- Does a tankless heater actually save money?
- The saving is the standby loss, and on a modern well-insulated cylinder that is a smaller number than the marketing suggests — real, continuous, and modest. Set against it are a higher capital cost, often a supply upgrade, and a descaling regime in hard water. The cases where it wins clearly are a household with low and intermittent demand, where a cylinder spends its life keeping water hot for nobody, and installations where the space a cylinder occupies is worth money. The case where it loses is a large household with simultaneous draws, where a single unit cannot supply two showers at winter inlet temperatures and the answer is either two units or a cylinder.