Immersion Heater Running Cost UK Per Hour, Daily & Monthly 2026
A standard 3kW immersion heater costs £1.02 per hour to run at the UK rate of 34p/kWh. Heating a full tank (120 litres) takes about 1.5-2 hours, costing £1.53-£2.04 per tank. Most households need 1-1.5 tanks per day, making the daily cost £1.50-£3.00 or £45-£90 per month. Using an Economy 7 tariff (15p/kWh overnight) cuts this to £0.45/hour or £20-£40/month.
Immersion Heater Cost Calculation
The formula is simple: Cost per hour = Wattage × Rate. For a 3kW immersion heater at 34p/kWh: 3 × £0.34 = £1.02/hour. Time to heat a 120-litre tank from cold to 60°C: approximately 1.5-2 hours (depending on inlet water temperature). So one full heat cycle costs £1.53-£2.04. Most households heat water 1-2 times per day. On Economy 7 (15p/kWh): 3 × £0.15 = £0.45/hour. Full tank: £0.68-£0.90. Monthly on E7: £20-£40 — a significant saving.
Immersion vs Gas Boiler vs Heat Pump
Immersion heater (standard rate): £45-90/month for hot water. Immersion on Economy 7: £20-40/month. Gas combi boiler: £15-25/month for hot water (gas is much cheaper per kWh at 6.76p). Heat pump (hot water cylinder): £10-20/month (COP 3+ means 3x efficiency). Solar thermal: £5-10/month (free heat in summer, immersion backup in winter). For homes with gas, a combi boiler is cheapest for hot water. For all-electric homes, a heat pump hot water cylinder or immersion on Economy 7 are the best options.
Timer Settings — Save 30-50%
Never leave an immersion heater on continuously — use a timer. Optimal settings: Option 1 (Economy 7): Set timer to heat during the 7 off-peak hours only. The insulated cylinder keeps water hot for 12-16 hours. Cost: £20-40/month. Option 2 (Standard tariff): Set timer for 1-1.5 hours before your main usage time (morning shower, evening). Heating twice daily for 1.5 hours each: 3 hours × £1.02 × 30 = £91.80/month. Option 3 (Boost only): Only heat when needed using the boost switch. Most economical but requires planning.
Thermostat Setting
Set your immersion thermostat to 60°C. This is hot enough to kill Legionella bacteria (which multiply at 20-45°C) while not wasting energy on excessive heat. Every 10°C above 60°C increases running costs by approximately 10-15%. Never set below 50°C due to Legionella risk. If you have a solar thermal system or heat pump, the immersion serves as a backup boost — the primary system heats to 45-50°C and the immersion tops up to 60°C periodically for Legionella protection.
| Method | Rate | Monthly Cost | Annual Cost |
|---|---|---|---|
| Immersion (standard 34p) | £1.02/hr | £45-90 | £540-1,080 |
| Immersion (Economy 7 15p) | £0.45/hr | £20-40 | £240-480 |
| Gas Combi Boiler | 6.76p/kWh | £15-25 | £180-300 |
| Heat Pump Cylinder | ~11p/kWh eff. | £10-20 | £120-240 |
| Solar Thermal + Immersion | Mix | £5-15 | £60-180 |
Upgrading from Immersion to Heat Pump
A dedicated hot water heat pump (like the Ariston Nuos or Samsung ClimateHub) costs £2,000-4,000 installed and uses 1/3 the electricity of an immersion for the same hot water. Annual hot water cost drops from £540-1,080 (immersion) to £180-360 (heat pump). Payback: 3-5 years. Alternatively, a full ASHP system (£8,000-14,000 minus £7,500 BUS grant) heats both your home and water. For all-electric homes, upgrading from immersion to any heat pump technology is one of the best investments available.
Disclaimer: For educational reference only. Consult a licensed professional.