Will a 5L instant geyser give you a longer hot shower than a 3L one?

A real same-power comparison

A.O. Smith's InstaPod 3KW product page, checked on 9 October 2026, lists both 3L and 5L capacities at 3,000W. Its official leaflet also lists a separate 3L, 4,500W version.

That gives three distinct combinations:

• 3L tank with a 3kW element
• 5L tank with a 3kW element
• 3L tank with a 4.5kW element

Check both figures on the exact variant before ordering. A comparison of a 3L, 4.5kW model with a 5L, 3kW model changes capacity and heating power. A comparison between the two 3kW versions changes tank size without increasing rated power.

These are specification examples, not tested product recommendations.

What the extra two litres actually change

If both tanks start fully heated to the same temperature, the 5L tank contains more stored heat. That reserve can help maintain useful outlet temperature for longer while water is being drawn. It also takes more energy to heat the larger volume from the same cold starting point.

As hot water leaves, incoming cold water must be heated. Once the initial stored heat no longer covers a shortfall, a larger tank cannot make a 3kW element continuously deliver more than its heating capability. Exact outlet behaviour also depends on mixing inside the tank and its controls, so a five-to-three capacity ratio is not a reliable shower-duration multiplier.

This is an energy-balance inference, rather than a manufacturer runtime claim. PNNL's water-heater sizing guidance identifies heating input, water flow and temperature rise as the key quantities for continuous output.

Why winter changes the answer

Water entering at a lower temperature needs more heat to reach the same outlet temperature. At a heater's maximum output, increasing water flow can lower the delivered temperature. Stiebel Eltron India's explanation describes this limit and the need to reduce flow in demanding conditions, including very cold inlet water.

Here is an illustrative calculation, not a prediction for either InstaPod model. Using the water-heating equation in Stiebel Eltron's engineering guide, 3kW transferred entirely into flowing water would support approximately:

• 2.1 litres per minute for a 20°C rise, such as 20°C to 40°C
• 1.4 litres per minute for a 30°C rise, such as 10°C to 40°C

The calculation is flow ≈ 14.3 × power in kW ÷ temperature rise in °C. It assumes steady operation, full rated heating and no heat losses. It excludes the temporary contribution from stored hot water, thermostat cycling and installation effects. The temperatures are examples, not recommended settings.

The practical lesson is that an appliance which seems adequate with warmer inlet water may require a lower flow during colder weather. Buying two extra litres does not remove that seasonal constraint.

Read the conditions behind continuous hot water

The InstaPod user manual describes its preferred continuous-flow operation with the hot tap adjusted to minimum flow. It does not establish that the appliance can maintain your preferred temperature through a fully opened shower.

Before buying, ask the manufacturer or installer: “At my winter inlet temperature, what flow can this exact model maintain at the outlet temperature I need?” Request a performance chart or a clear model-specific answer. A statement that hot water flows continuously is incomplete without those conditions.

Choose around the way you bathe

For a shower: establish the required flow and cold-season conditions before narrowing the choice to 3L or 5L. If a compact model cannot meet them, compare a suitably sized storage heater or another professionally assessed solution. A.O. Smith's general capacity FAQ points to 15L and 25L capacities for a normal shower, but also says household usage and climate affect sizing. Treat that as a starting point, not a guarantee for your bathroom.

For bucket bathing: decide whether gradual filling is acceptable and how much mixed water you want per bath. The useful question is the filling time under your conditions, rather than whether the nominal tank holds an entire bucket.

For several people in succession: tell the installer how closely baths follow one another. A family headcount alone leaves out the amount of water used and the time available between uses.

Take this checklist to the seller

Write down:

1. Shower or bucket, and the flow or bucket volume you want to use
2. The coldest likely incoming-water temperature, asking the installer for help if unknown
3. The exact model, tank capacity and heating power
4. Whether other hot-water outlets will run at the same time
5. Whether waiting for heating or accepting a reduced flow would suit your routine
6. The installation requirements and total installed cost

Have an authorised installer or qualified electrician check electrical suitability before choosing a higher-power variant. The InstaPod manual requires suitable electrical provision and reliable earthing, prohibits adapters, and distinguishes the connections for its power variants. Leave wiring and protection decisions to the professional.

Bottom line: choose 5L for a larger initial reserve when that benefit suits your use. For sustained hot showers, buy against verified flow and temperature requirements. A 5L badge alone cannot establish that a 3kW heater will keep up.