Bath vs Shower: Water Usage, Costs and Impact on Your Drainage

The bath vs shower debate is one of the most persistent in home energy advice. The conventional wisdom — that showers are always more efficient — is usually correct but not universally so. The actual water and energy use depends on the type of shower, its flow rate, and how long you stand under it.

Typical water usage: the numbers

Standard bath: A full bath uses approximately 80–150 litres of water. The range depends on the bath size (standard UK bath holds 175–250 litres when full; most people fill it to 80–100 litres in practice) and the depth preference.

Power shower: 15–20 litres per minute. A 10-minute shower uses 150–200 litres — more than a full bath. Power showers are driven by a pump that draws from the hot water cylinder, producing high flow at the cost of high water and energy usage.

Standard mixer shower / electric shower: 6–10 litres per minute. A 10-minute shower uses 60–100 litres. This is the middle ground — less than a bath for a typical shower duration.

Low-flow shower head / aerated shower head: 4–8 litres per minute. A 10-minute shower uses 40–80 litres. Significantly more efficient than a bath for all but the most water-conservative bathers.

Electric instantaneous shower: Heats only the water used. Typically 8–10 litres per minute. Very efficient in water terms, but the heating element creates a heavy electrical load.

Energy cost comparison

Hot water energy costs depend on:

• Volume of water heated
• Starting temperature (cold water temperature — varies by season and region)
• Target temperature
• Boiler efficiency or immersion heater efficiency

Very rough guide for comparison:

• A full bath (90 litres from 15°C to 40°C) costs approximately 16–20p in gas (assuming 85% efficient combi boiler)
• A 10-minute low-flow shower (70 litres) costs approximately 12–16p in gas
• A 10-minute power shower (180 litres) costs approximately 32–40p in gas
• A 10-minute electric shower (8kW, 10 minutes) costs approximately 22p in electricity at 27p/kWh

The picture for electric showers changed significantly after energy price increases — at current electricity rates, an 8kW electric shower is often comparable in cost to a comparable gas-heated shower.

Impact on drainage

The volume of water entering your drainage system directly affects blockage rates in kitchen and bathroom drains. This seems obvious but has practical implications:

Power showers and drainage: A power shower discharging at 18 litres per minute produces a flow that most 40mm bathroom waste pipes handle comfortably — but if the waste pipe has a partial blockage from hair and soap accumulation, the high flow rate causes the shower tray to fill and overflow.

This is why power showers reveal drainage problems that conventional showers don’t. The shower isn’t causing the blockage — it’s exposing an existing restriction that a lower-flow shower would drain past.

High water temperature and pipe materials: Power showers draw from the hot water cylinder at the stored temperature (typically 60–65°C). This high-temperature water enters the waste pipe. For plastic waste pipes (UPVC), sustained high-temperature waste water can soften push-fit joints over years. It’s not an immediate failure mechanism, but if you have a power shower connected to plastic waste, use a slightly lower temperature setting rather than the maximum.

Drainage sizing for new shower rooms

When fitting a new shower room or wet room, the waste pipe sizing must match the shower flow rate:

• Standard mixer/electric shower (6–10 l/min): 40mm waste pipe, minimum 1:40 gradient — standard specification, presents no difficulties
• Power shower (15–20 l/min): A 40mm pipe at 1:40 gradient can carry this flow, but the shower tray must have adequate sump depth (the tray’s ability to hold water briefly if drainage can’t keep up with flow)
• Large format rainfall heads (20+ l/min): Consider 50mm waste pipe, particularly if the waste run is long

For wet rooms, the floor drain must be sized for the anticipated flow rate. Standard wet room drains (50mm bodies) handle most shower types; very high-flow shower heads may require a larger drain body and a 50mm waste pipe.

Water-saving measures

Fitting a flow regulator on the shower head: Restricts flow to 6–8 l/min without reducing the perceived quality significantly. A simple, cheap (£5–10) fitting that can halve water usage.

Timed shower: A 4-minute shower at 8 l/min uses 32 litres — genuinely less than any bath. A simple shower timer keeps usage in this range.

Aerated shower heads: Mix air with water to maintain the perceived flow sensation while reducing actual water volume by 30–50%.

Shower head descaling: Hard water areas see progressive flow reduction as scale builds up in the shower head nozzles. Regular descaling (monthly in very hard water areas) maintains design flow rates and prevents the temptation to increase the flow pressure setting to compensate.