How much hot water does a cylinder deliver in the first hour?

1. Why first hour rating matters more than size

People shop for a hot water cylinderby its nominal size — a “50-gallon” or “190-litre” cylinder — and then run cold halfway through the second shower. The reason is that the number on the front is only the water the cylinder holds; it says nothing about how fast the hot water cylinder makes more. Two cylinders of the very same size can behave completely differently in a morning rush, depending on how powerful their burner or element is. First hour rating fixes this by answering the question you actually care about: starting full, how much hot water can I get out in one hour?

In the United States and Canada the figure is required on the yellow EnergyGuide label, where it is called the First Hour Rating (FHR). Elsewhere the same physics applies even though the label does not always print one number — a cylinder with a strong burner delivers more in the first hour than a weak one of identical volume. That is why sizing by first hour rating beats sizing by gallons or litres alone.

2. The formula: usable storage plus recovery

First hour rating is the sum of two parts. The first is usable storage: the hot water already in the cylinder that you can draw before incoming cold dilutes the outlet below a useful temperature. You can never use the full nominal volume, because cold water enters as hot leaves; a common planning figure is that about 70% of the cylinder is usable, though the real draw-down depends on the cylinder's internal design.

The second part is recovery: the extra hot water the hot water cylinder can heat during that first hour. It comes straight from the heat balance — the energy to warm a volume of water is the volume times its specific heat times the temperature rise. Rearranged for volume per hour, the recovery is the heat input multiplied by the efficiency, divided by the energy needed to lift that volume of water through your temperature rise. In imperial units that is the familiar formula gallons/hr = BTU/hr × efficiency ÷ (8.33 × ΔT °F); in metric it is litres/hr = watts × efficiency × 3600 ÷ (4186 × ΔT °C), and the two agree exactly.

Put them together: first hour rating = usable storage + one hour of recovery. Take a 40-gallon (150 L) gas cylinder with a 40,000 BTU/hr burner at 80% efficiency, heating from a 50 °F inlet to a 120 °F setting (a 70 °F rise). Usable storage is 70% of 40 = 28 gallons. Recovery is 40,000 × 0.80 ÷ (8.33 × 70) ≈ 55 gallons in the hour. So the first hour rating is about 28 + 55 = 83 gallons (around 314 litres). The calculator runs exactly this arithmetic, in either unit system.

3. First hour rating by cylinder size, at a glance

These figures are estimated from the formula above— not manufacturer label values — using a 70% usable fraction, a 70 °F (39 °C) temperature rise, a 40,000 BTU/hr gas burner at 80%, and a 4.5 kW electric element at 98%. Your unit's real, tested figure will differ, so always read its label; this chart shows the pattern, especially the gap between gas and electric:

Notice that the gas column runs roughly 25–30 gallons (about 100 litres) ahead of electric at every size. That whole gap is recovery: the gas burner simply reheats more water within the hour. It is also why a 40-gallon gas cylinder can deliver more in the first hour than a 50-gallon electric one — the headline size can mislead if you ignore recovery.

4. What first hour rating do you need?

Size to your busiest hour, not your daily total. Picture the heaviest 60 minutes your home ever asks of hot water — typically a weekday morning — and add up what runs in it. A back-to-back shower uses roughly 20 gallons (75 litres) of hot water; a bath far more; a dishwasher or washing machine adds a few gallons each. Three showers in a row plus a quick kitchen load might come to 65–70 gallons (around 250 litres) in that hour. Choose a hot water cylinder whose first hour rating meets or beats that number and you will not run cold when it matters.

Working from the demand side rather than the cylinder size is the key habit. If you already know roughly how many people draw hot water at once, the hot water cylinder size calculator turns that into a recommended size and a busy-hour figure to match, and the first hour rating calculator then checks whether a specific cylinder clears it. Used together they answer both halves of the question: how much you need, and whether a given unit supplies it.

5. Gas, electric, heat pump and tankless

The first-hour idea is sharpest for storage cylinders, and the fuel changes the answer through recovery. Gas burners deliver heat fast, so gas storage units have high first hour ratings for their size. Standard electric elements are slower, so an electric cylinder leans more on stored water and benefits from being a size larger. Heat-pump (hybrid) cylinders recover slowly in their efficient mode, which is why they are usually sold in larger sizes and quote a first hour rating that assumes a back-up element kicks in.

Tankless (on-demand) hot water cylinders are the exception: they store almost no water, so first hour rating is not the right measure for them at all. They are sized by continuous flow rate and temperature rise instead — how many gallons or litres a minute they can heat without ever running out. If you are weighing a tankless unit, size it by flow with the flow-rate calculator rather than by first hour rating.

6. Getting an accurate number for your cylinder

Three inputs move the answer most. The temperature rise is your hot setting minus the incoming cold; colder winter mains or a hotter setting both increase the rise, which slows recovery and lowers the first hour rating, so a cylinder that copes in summer can struggle in winter. The recovery efficiency reflects how much of the fuel actually ends up in the water — read it from the spec label rather than guessing. And the usable fraction depends on the cylinder's design; 70% is a fair planning figure but better-stratified cylinders do a little better.

Enter those three, plus the cylinder size and the heat input, into the first hour rating calculator and you get a like-for-like estimate you can compare across models before you buy. Treat it as a planning figure: the printed FHR comes from a standardized test under set conditions, so use the calculator to compare options and to understand the trade-offs, then confirm the final choice against the unit's own label and, for anything you are installing, your local code and a licensed professional.

Common questions

What is first-hour hot water delivery?

It is the amount of hot water a full hot water cylinder can deliver in the first hour of heavy use. In the United States and Canada it is printed on the unit's EnergyGuide label as the First Hour Rating (FHR). It combines the usable water already stored hot in the cylinder with the extra the hot water cylinder reheats during that hour, so it reflects a real morning rush far better than the nominal cylinder size on its own.

How do you calculate first hour rating?

First hour rating = usable storage + one hour of recovery. Usable storage is about 70% of the cylinder's nominal size. Recovery is the heat input × efficiency divided by the energy to raise that much water through your temperature rise. A 40-gallon (150 L) gas heater with a 40,000 BTU/hr burner comes to roughly 83 gallons (about 314 L) in the first hour; a 50-gallon (190 L) electric one to roughly 61 gallons (about 230 L).

What is a good first hour rating?

Size it to your busiest hour, not your daily total. Estimate the hot water used in your heaviest hour — each back-to-back shower is roughly 20 gallons (75 L) — and pick a hot water cylinder whose first hour rating meets or beats it. Many households land in the 60–80 gallon (230–300 L) range; a large home with simultaneous demand may want more.

Why does a gas cylinder out-deliver an electric one?

Because a gas burner adds heat much faster than an electric element, so it recovers far more water within the hour. A typical 40,000 BTU/hr burner recovers about 55 gallons (207 L) an hour, against roughly 26 gallons (98 L) for a 4.5 kW element — which is why a smaller gas cylinder can beat a larger electric one over the first hour.