How Much Electricity Does a Geyser Use? Units and Cost • The Science 360

In winter the geyser is one of the biggest loads in many Indian homes. It draws 2 or 3 kW while it heats, and people notice the bill. But wattage alone does not tell you how many units it uses. That depends on how much water you heat, how far you heat it, and how long the tank sits hot between baths.

This post works out each part with a 25 litre, 2 kW storage geyser and a family of four, then compares an instant geyser, a heat pump and a solar water heater. Every example opens in the water heater cost calculator, where you can type your own litres, temperatures, geyser size and tariff.

Quick answerA 25 litre, 2 kW geyser heating four 20 litre bucket baths a day from 15 °C to 40 °C uses about 2.8 units a day, 86 units a month, or about ₹604 at ₹7 a unit. Most of that is heat in the water itself; standing loss is about 16 percent.

How much electricity does one bath take?

Warming water takes a fixed amount of heat per kilogram per degree, its specific heat capacity. The US Department of Energy's thermodynamics handbook takes it as 1 Btu per pound per °F for water, which is about 4.18 kJ per kg per °C; the calculator uses 4.184. A litre of water is close to 1 kg, and one unit of electricity (1 kWh) is 3,600 kJ.

Worked example: one 20 litre bucket bath

Cold water comes in at 15 °C, an example winter figure (type your own), and the bath is at 40 °C.

\begin{aligned} Q &= m\,c\,\Delta T \\ &= 20 \times 4.184 \times (40-15) \\ &= 2{,}092\ \text{kJ} \end{aligned}

\dfrac{2{,}092}{3{,}600} = 0.581\ \text{kWh}

So each bucket bath holds about 0.58 units of heat. Four baths a day hold 2.32 units, and a geyser that passes 98 percent of its electricity into the water draws 2.32 ÷ 0.98 = 2.37 units for them.

Notice what is missing from that sum: the geyser's setting. Whether the tank is at 50 °C or 70 °C, you mix its water with cold water until the bucket is at 40 °C. The total heat in the bucket is fixed by its litres and its temperature. A hotter tank only means you use fewer litres of tank water and more cold water.

A tiled bathroom with a 25 litre, 2 kW storage geyser set to 60 °C: a shower fills a bucket with 40 °C water while the tank dial falls to 52 °C as cold water flows in, since each 20 litre bath uses 11.1 litres of the tank. A card shows a month of four such baths: ₹604 and 86 units on the storage geyser, ₹505 on an instant geyser, ₹238 on a heat pump and ₹181 on solar with backup — Figure 1. Each 20 litre bath at 40 °C uses 11.1 litres of 60 °C tank water. Four a day cost ₹604 a month on a storage geyser at ₹7 a unit. Open this in the calculator.

How much does a geyser use in a month?

A storage geyser adds one more term: standing loss, the heat that leaks through the insulation while the tank waits hot. This is what the BEE star label measures. The current table, notified by the Ministry of Power in December 2025 and valid from 1 July 2026 to 31 December 2027, gives limits in kWh per 24 hours with the tank 45 °C above the room. For a 25 litre geyser, 4 stars means a loss above 0.423 and up to 0.465 kWh a day; 5 stars means 0.423 or less.

Worked example: a month of bucket baths

A 4-star 25 litre geyser with the highest loss its band allows, 0.465 kWh a day. Set to 60 °C with 15 °C water, the difference is exactly the label's 45 °C, so the loss is 0.465 kWh if the geyser is left on all day.

\begin{aligned} \text{a day} &= 2.37 + 0.465 \\ &= 2.84\ \text{kWh} \\ \text{a month} &= 2.84 \times \dfrac{365}{12} \\ &= 86.3\ \text{units} \end{aligned}

At ₹7 a unit that is ₹604 a month, or about ₹4.96 a bath. The standing loss is 0.465 of the 2.84 units, about 16 percent.

That 16 percent answers a common worry. Leaving a well insulated geyser on all day does not double the bill. A whole day of standing loss is less heat than one bucket bath.

How long does a geyser take to heat?

This is where the wattage matters. The heater puts heat in at its rated power, so the time is the heat the tank needs divided by the kilowatts.

Worked example: heating a full 25 litre tank

\begin{aligned} Q &= \dfrac{25 \times 4.184 \times (60-15)}{3{,}600} \\ &= 1.31\ \text{kWh} \end{aligned}

\begin{aligned} t &= \dfrac{1.31\ \text{kWh}}{2\ \text{kW} \times 0.98} \\ &= 0.667\ \text{h} = 40\ \text{minutes} \end{aligned}

A 3 kW heater would do it in about 27 minutes. It would not use more units: it puts the same 1.31 kWh in faster.

So the wattage on the box is a speed, not a cost. Two geysers of different wattage heating the same water to the same temperature use the same units. The kettle post makes the same point at a smaller scale: in a bigger kettle is not the problem, heating extra water is, it is the litres that set the energy.

Does turning the thermostat down save money?

A little. Lowering the setting from 60 °C to 50 °C does not change the heat in the bath, for the reason above. Each bath now uses 20 × 25 ÷ 35 = 14.3 litres of tank water instead of 11.1. What falls is the standing loss, which scales with how much hotter the tank is than its surroundings: 0.465 × 35 ÷ 45 = 0.36 kWh a day instead of 0.465.

That saves about 0.1 units a day, roughly 3 units or ₹22 a month. A lower setting does mean fewer baths per tankful, so with several baths back to back you may run short of hot water.

Should you switch off the geyser?

Switching on the geyser only before baths cuts the hours the tank is kept hot. If it is hot for 2 hours a day instead of 24, the calculator scales the standing loss to 2 ÷ 24 of the day's figure.

That brings the month from 86.3 to 73.3 units, about ₹91 a month less at ₹7. The real saving is a little smaller, because a tank that is switched off still loses some heat as it cools, which the next heat-up replaces. The calculator does not count that.

What changes the bill the most?

The litres. A shower uses much more water than a bucket. If each of the four baths is a 40 litre shower, twice the bucket, the heat in the water doubles and the month rises to about 158 units, ₹1,109. Each shower uses 22.2 litres of tank water, so a 25 litre tank barely covers one before it must reheat.

The same geyser with four 40 litre showers a day: each shower uses 22.2 litres of the 25 litre tank, and the month costs ₹1,109 for 158 units, against ₹1,010 on an instant geyser, ₹436 on a heat pump and ₹333 on solar with backup — Figure 2. Four 40 litre showers instead of buckets: ₹1,109 a month. Open this in the calculator.

The season matters next. With cold water at 25 °C instead of 15 °C, each 40 °C bath needs a 15 °C rise instead of 25 °C, and the same four buckets cost about ₹380 a month instead of ₹604. That is why geyser bills fall so much in spring even when habits do not change.

Is an instant geyser cheaper?

An instant geyser has a tiny tank, often 3 litres, and a bigger heater, often 3 kW (V-Guard's 3 litre instant models are 3,000 W). It heats while you draw water, so there is almost no standing loss. For the same four buckets it uses about 72 units a month, ₹505, about ₹99 less than the storage geyser.

The catch is flow. 3 kW can warm only so much water per minute:

\begin{aligned} t &= \dfrac{0.581\ \text{kWh}}{3\ \text{kW} \times 0.98} \\ &= 0.198\ \text{h} = 11.9\ \text{minutes} \end{aligned}

That is about 1.7 litres a minute at 40 °C. Fine for filling a bucket, slow for a shower.

A 3 litre, 3 kW instant geyser with its outlet dial at 60 °C and the meter reading 3.00 kW while the shower fills a bucket: a 20 litre bath at 40 °C takes 12 minutes, and four a day cost ₹505 a month for 72 units with no tank to keep hot — Figure 3. A 3 kW instant geyser fills a 20 litre bucket at 40 °C in about 12 minutes. Open this in the calculator.

What about heat pump and solar water heaters?

Both cut the electricity much more, because most of their heat does not come from the wire.

A heat pump water heater works like an AC in reverse: it moves heat from the surrounding air into the water. The US Department of Energy says these heaters can be two to three times as efficient as conventional electric resistance water heaters. The calculator uses a coefficient of performance (COP) of 2.5, the middle of that range, meaning 2.5 units of heat in the water for each unit of electricity. The same four baths and standing loss then take about 34 units, ₹238 a month.

A heat pump water heater with a fan unit on top of its 25 litre tank, heating from 15 to 60 °C on 0.5 kW, its dial at 45 °C: with a COP of 2.5 the same four baths cost ₹238 a month for 34 units, against ₹604 on a storage geyser — Figure 4. A heat pump at COP 2.5 takes ₹238 a month for the same baths. Open this in the calculator.

A solar water heater gets its heat from a collector on the roof, with an electric element as backup for cloudy days. How much of the year's heat the sun supplies depends on where you live and when you bathe. The calculator takes this as an input, with 70 percent as an example; at that share, the backup uses about 26 units, ₹181 a month.

A bathroom window showing a sunny sky and an evacuated tube solar collector, with the tank dial at 45 °C: if the sun gives 70% of the heat, the electric backup gives the rest and four baths cost ₹181 a month for 26 units — Figure 5. A solar water heater where the sun gives 70 percent of the heat: ₹181 a month for the backup. Open this in the calculator.

The Ministry of New and Renewable Energy's figure gives a useful check. It says a 100 litre a day solar water heater can replace an electric geyser and may save about 1,500 units a year. Heating 100 litres by 35 °C every day takes 100 × 4.184 × 35 ÷ 3,600 = 4.07 kWh, or about 1,485 units a year. So MNRE's figure assumes that much hot water every day, almost all of it from the sun. A family using less, or a north Indian winter with foggy weeks, will save less. Neither the heat pump nor the solar figure includes the purchase price.

How does the calculator work it out?

Heat in each bath: litres × 4.184 × (bath temperature − cold water temperature) ÷ 3,600 kWh.
Standing loss: the star label figure × (setting − cold water temperature) ÷ 45 × hours kept hot ÷ 24. It takes the room as the same temperature as the cold water.
Electricity a day: storage = baths' heat ÷ efficiency + standing loss; instant = baths' heat ÷ efficiency; heat pump = (baths' heat + standing loss) ÷ COP; solar = (1 − solar share) × the storage figure.
A month is a day × 365 ÷ 12, priced at a flat rate or on top of the rest of your house on a slab tariff.
Heating time: the heat a full tank needs ÷ (kW × efficiency), or ÷ (kW × COP) for a heat pump.

Water heater working for four 20 litre baths: Q = 20 × 4.184 × (40 − 15) = 2,092 kJ = 0.5811 kWh; each bath uses 11.11 litres of 60 °C tank water; standing loss 0.465 kWh a day; 4 × 0.5811 ÷ 0.98 + 0.465 = 2.837 kWh a day, 86.29 units a month, ₹604 at ₹7; a full 25 litre tank needs 1.308 kWh and takes 40.03 minutes at 2 kW — Figure 6. The working panel for four bucket baths. Open this in the calculator.

It leaves out heat lost in long pipes, the cooling of a tank while it is switched off, purchase prices, fixed charges and taxes. For how kWh, joules and calories relate, see joules, calories and kWh explained, and for the other big seasonal load, how many units a 1.5 ton AC uses per hour.

To see your own numbers, open the water heater cost calculator, set the litres and temperatures your family uses, and compare the four kinds of heater on your tariff.

Sources

US Department of Energy, Thermodynamics, Heat Transfer and Fluid Flow handbook, volume 1: heat added = mass × specific heat × temperature rise, with the specific heat of water taken as 1.0 Btu per lbm per °F (about 4.19 kJ per kg per °C).
Ministry of Power, notification S.O. 5970(E), 23 December 2025, Gazette of India (copy of the gazette PDF): Table 3.5, star bands for electric storage water heaters by standing loss in kWh per 24 hours at a 45 °C difference, valid 1 July 2026 to 31 December 2027.
V-Guard, Victo 25 litre storage water heater (2,000 W) and Zio 3 litre instant water heater (3,000 W): typical heater ratings.
US Department of Energy, heat pump water heaters: two to three times as efficient as conventional electric resistance water heaters.
Ministry of New and Renewable Energy and REEEP, Solar water heating systems: a review of global experiences: a 100 litre capacity solar water heater may save about 1,500 units of electricity a year under Indian conditions.
IEEFA and JMK Research, Advancing residential rooftop solar adoption in India (October 2025): ₹7 a unit as a typical household tariff assumption.
Delhi Electricity Regulatory Commission, tariff order press note and schedule: domestic slab rates used for the calculator's slab option.

Frequently asked questions

How much electricity does a geyser use per day?

For four 20 litre bucket baths at 40 °C from 15 °C water, a 25 litre, 2 kW geyser left on all day uses about 2.8 units a day: 2.4 for the water and 0.47 of standing loss. That is about 86 units, ₹604 a month at ₹7.

How long does a 25 litre geyser take to heat?

About 40 minutes for a 2 kW geyser heating 25 litres from 15 °C to 60 °C. The water needs 25 × 4.184 × 45 = 4,707 kJ, 1.31 units, and 2 kW delivers that in about two thirds of an hour.

Should I switch off the geyser after use?

It helps a little. A 4-star 25 litre geyser kept at 60 °C all day loses about 0.47 units a day; switching it on only 2 hours a day saves about ₹91 a month at ₹7. Using fewer litres saves more.

Is an instant geyser cheaper than a storage geyser?

It saves the standing loss, about ₹99 a month in our example, because it has no tank to keep hot. But a 3 kW instant geyser warms only about 1.7 litres a minute to 40 °C, so it suits bucket baths more than showers.

How much does a solar water heater save?

MNRE says a 100 litre a day solar water heater may save about 1,500 units of electricity a year. The real saving depends on how much hot water you use and how sunny your winters are.