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Knowing how many amps a water heater uses is important if you want to be energy efficient. This guide focuses on electric water heaters as they are the most commonly used in homes, though the formulas can also apply to other types and designs.
A 4000 watt water heater running at 120V draws about 33.3 amps. If the water heater runs an average of 3 hours a day, it will draw 49.8 amps.
Water Heater Amp Size Guide
If you do not want to do any calculations, here is a table with the most common water heater watt capacities and how many amps they draw. As you can see, the same water heater will pull different amps depending on the voltage. The amps pulled will depend on the heater watts, voltage and running time.
These power consumption rates are for reference only. The usage by your water heater may vary due to differences in manufacturing design. Check the owner’s manual that come with your water heater for more information on how much power it uses.
|Watts||Amps 240V||Amps 120V||Amps 24V||Amps 12V|
Water heaters are also measured in gallons. The following table shows the suggested sizes depending on how many will use it. Most of these water heaters are at least 4000 watts, with the larger ones up to 5000 watts or more.
As you can see, a 4000 watt water heater is going to need 416 amps an hour to run off a 12V battery. A couple of Mighty Max 250ah batteries is going to do the job. If you need to run the heater for more than an hour, a bigger battery bank is needed.
These are for reference only. The capacity of a water heater will vary depending on the design. Please refer to the owner’s manual that came with your device for more details.
|Number of Individuals||Size (in Gallons)|
|5 or more||66-80|
Keep in mind that these size recommendations are for general guidelines only. Water usage varies greatly not just per household but per individual, so factor in your personal usage to determine what gallon is appropriate.
The best way to find out is to keep tabs of how much water each person in your household consumes. You can get an idea of the total usage from your utility bill. Using this information it is easy to find out what water heater size will be needed.
Water Heater Amp Calculation Guide
Water heaters come in all shapes, sizes and types. Usage varies as well from 10 minutes a day to over 5 hours, so you have to account for your specific needs when applying these calculations.
The calculation is as follows:
Water heater watts / voltage = amps drawn
If you have a 4500 watt water heater running on a 240V AC outlet, it will use 18.75 amps an hour.
4500 / 240 = 18.75
So that 4500 watt water heater draws 18.75 amps from its power source. Of course this applies only if you use the heater for an hour. If the device runs for 3 hours, multiply the amps by three.
18.75 x 3 = 56.25 amps
And ifyou use the water heater for less than hour, calculations are straightforward too.
18.75 amps an hour is 9.3 amps for 30 minutes, so 10 minutes would be around 5 amps.
This formula is useful not just for water heaters but also for circular saws, coffee machines, microwaves and other appliances and devices with variable runtimes.
Can a 240V Water Heater Run on 120V?
Yes, most 240V water heaters can run on 120V. Not all of them can though, so check the owner’s manual for information. Even assuming it can though, the amps drawn will be different.
Let us use the same example earlier, a 4500 watt heater. On a 240V system the heater draws 18.75 amps an hour. But if you were to run it on a 120V AC outlet:
4500 / 120 = 37.5
A 4500 watt water heater draws 37.5 amps an hour when running through a 120V system. The higher the voltage power, the lower the amps drawn.
Some of these heaters can also run on 110 volts, but again the amps draw will be higher though the difference from a 120V system will be much less.
What Inverter Size Do I Need For a Water Heater?
You can use an inverter to run a water heater and a lot of other appliances including a TV , refrigerator, kettle and more. However you will need a large battery bank to make this work. In addition. a 4000 watt water heater needs 5000 watts of inverter power available.
Assuming you have a 5000 watt inverter ready for the water heater, the question is: how many batteries will you need?
If a 4500 watt water heater uses 18.75 amps on a 240V and 37.5 at 120V, on a 24V inverter system that would be:
4500 / 24 = 187.5
That is 187.5 amps an hour or 562.5 amps for a 3 hour runtime. To round that off, you need a 200ah battery to run a water heater for an hour or 600ah for 3 hours.
Also keep in mind that running the water heater for an hour will completely drain the 200ah battery. That is not advisable because deep cycle batteries need to be recharged at 50%. So only 100ah of the 200ah capacity should be used.
You have two choices, either reduce the water heater runtime to 30 minutes or double the battery bank to 400ah. If you have to run the battery for three hours you are looking at a 1000ah battery bank solely for the water heater.
The requirements go even higher if you opt for 12V batteries, double what the 24V battery needs. Fortunately you can connect several 12V batteries in a series to increase the voltage.
The point is a water heater requires a large inverter and battery bank to work. With these batteries you can already run a lot of other appliances instead of just dedicating it solely for a water heater.
If you are on a grid tied solar system, the heater can run off electrical power. But if you are off the grid, the best option is a smaller water heater or use another power source like a generator available.
How Many Solar Panels Do I Need to Run a Water Heater?
Yes it is possible to use solar panels with a water heater. However it requires a large solar array and you need a battery bank or another backup power source.
A 4000 watt water heater needs at least 4500 watts solar power, or 12 x 350W solar panels. These will take up approximately 210 square feet of space.
With 5 hours of sunlight there should be enough power available to keep the heater running all day. Assuming ideal weather conditions the array can provide 4000 to 4200 watts an hour. The more sunlight hours the longer you can run the heater.
These calculations also assume the heater will be running for an hour at least. The solar power requirements will be a lot less if the heater does not operate as long.
If you have the roof or ground space it is feasible, but is it practical? A 4000 watt solar array can power a refrigerator, air conditioner and other appliances. With a 4000 watt water heater you will not be able to run any of those.
The other drawback is you will not be able to use the heater if it rains or the sky gets cloudy. To run the water heater in any weather, it will require a battery bank. And as shown in the previous calculations, a several large batteries are needed to power the system.
To put it another way, you will need a 4000 watt solar panel, a 5000 watt inverter and 500ah to 1000ah battery bank just for the water heater. Because of these reasons, an off grid system is better off with a smaller water heater or a solar water heater.
Are Solar Water Heaters Worth It?
A better alternative to running water heaters off solar panels is to use a solar water heater. This should not be confused with regular electric water heaters that run off PV modules or batteries, as these are dedicated to water heating only.
Solar water heaters resemble solar panels and are installed on rooftops. Built in collectors draw sunlight into the heater and these are converted into energy, which is used to heat water. These devices only heat water and cannot supply energy to other devices or appliances.
There are two kinds of solar water heaters:
Active solar water heaters have a mechanism that circulates the heated water throughout a house. In an active direct system, the heated water is transmitted to showers and faucets. In an indirect system, the collectors use a heat transfer fluid to heat the water.
Passive solar water heaters depend on convection to heat water and do not have any pumps built in. In some units, an integral collector storage container holds water which gets heated by sunlight. In a thermosyphon system, water gets heated by metallic plates and are then transmitted from the valves to faucets.
A passive solar water heater costs less than active ones because there are no complex mechanisms built in, though active systems are more effective. Either way, both are more practical to use than a regular water heater and use less power.
The good thing about water heaters is they are durable, and you probably need to buy just one and should last for years. Once you know the numbers, you can run the water heater from AC power, batteries or solar panel without issues. Or as shown, you can opt for a solar water heater instead.