The capacity or size of a domestic water heater is usually specified in which unit or units?

Introduction / Context:
Water heaters used in homes and buildings have a certain storage capacity that tells you how much hot water they can hold and supply. This capacity is a measure of volume, not length or mass. Different countries and manufacturers use different units to express this volume. This question asks which units are commonly used to specify the capacity of a water heater.

Given Data / Assumptions:

• We are dealing with storage-type water heaters (geysers) used in households.
• The main quantity of interest is how much water the heater can store at one time.
• The capacity is a volume measure.
• Different unit systems (metric and imperial) may be used in different regions.

Concept / Approach:
Volume can be measured in many units, but for household water heaters, the most common are gallons in countries using the imperial or US customary system, and litres in countries using the metric system. A heater might be advertised as a "40-gallon" unit in one region and as a "100-litre" or "200-litre" unit in another. Metres, on the other hand, measure length, not volume, so they are not used to state water heater capacity. Therefore, the best answer includes both gallons and litres as acceptable capacity units.

Step-by-Step Solution:
Step 1: Identify that the capacity of a water heater refers to the quantity of water it can hold, which is a volume. Step 2: Recognise that in the metric system, volumes of liquids are commonly measured in litres. Step 3: Recognise that in the imperial or US customary system, volumes of water for such applications are often measured in gallons. Step 4: Note that both units serve the same purpose: they quantify how much water the heater stores. Step 5: Conclude that capacity can be expressed in either gallons or litres depending on the country and manufacturer, so both are correct units for this context.

Verification / Alternative check:
If you look at product descriptions for water heaters from different regions, you will see capacities like 25 litres, 50 litres or 100 litres in metric-using countries, whereas in North America you frequently see capacities like 30 gallons, 40 gallons or 50 gallons. These labels clearly indicate that the capacity is a volume. No legitimate product advertises its capacity in metres alone, because metres are not a volume unit; at most, physical dimensions of the tank (height, diameter) might be given in metres or centimetres, but not the capacity itself.

Why Other Options Are Wrong:
Gallons only: This ignores the widespread use of litres in metric countries and is therefore incomplete.

Litres only: This ignores the use of gallons in imperial or US customary systems and is also incomplete.
Metres: Metres measure length, not volume, so they cannot directly express the storage capacity of a water heater.

Common Pitfalls:
A frequent mistake is to think that only the unit used in one's own country is correct. In general knowledge and science, it is important to recognise multiple systems of units. Another confusion is between volume and length or area; always check what physical quantity is being described. For capacity of tanks and heaters, think in terms of litres, gallons or cubic units, not metres alone.

Final Answer:
The capacity of a water heater is commonly measured in both gallons and litres, depending on the unit system used.