What Inverter Size for Well Pump to Use?

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If you have an off grid solar system set up, it is crucial that the well pump is properly configured. If you decide to go with an AC well pump, the inverter must be the appropriate size to run the motor. But how many watts do you really need?

A 4000 watt inverter is the ideal size for most 1 to 1.5 HP AC well pumps. These pumps typically consume 1500 watts, but the surge wattage is double that, which is why a 4000 watt modified or pure sine inverter is the best choice.

Solar Inverter Well Pump Requirements

An AC well pump requires a lot of power to start up and run. It is comparable to running an air condition on solar, so make sure your inverter can handle its requirements before proceeding.

To calculate what inverter your well pump needs: total surge watts + 25% = inverter size

The 25% reserve power is the minimum amount. You can increase that to 50%, or any percentage you like. Here is a guide for the minimum inverter and well pump combination.

HPMin. Inverter Size (Modified Sine) KwhMin. Inverter Size (Pure Sine) kwhRunning Watts
0.5 2.01.51000W
0.75 2.42.01300W
1.02.82.51600W
1.5 4.54.02150W
2.0 5.04.52500

In the figures above you will notice that the inverter size is not always double that of the running watts. The rule of thumb is the surge watts is double the running watts.

However the inverter specs listed above can run these well pumps though you might see an overload indicator flash for a few seconds. If you only use the pump for a few times the inverter should hold up.

The figures above assume there is no other load on the inverter. Adding extra load will require a larger capacity. If you have to use other devices, add the total wattage plus 25% to get the suitable inverter.

There are other well pump sizes available and they may have larger requirements if equipped with extra features. You have to decide if these are worth the additional watts.

Inverters are not 100% efficient. Some energy will be lost ranging from 5% to 15% for good models. Pure sine wave inverters lose less power than modified sine wave, which is why you can run well pumps with less power on pure sine.

Calculate Well Pump Power Usage Example

Majority of AC well pumps used in homes are 1 to 1.5 HP, so that is what we will use here.

If you have a 1.5 HP pump, its running watts is going to be around 1500 watts, more or less. However, pump motors need a surge of power to start up. In a 1.5 AC pump this is about 3000 watts.

Add 25% to 3000 watts and you get 3750 watts. Round that off to 4000, and that is the inverter you need.

For other types of well pumps you can repeat the same formula. Simply replace the numbers from the chart with your pump’s specs. Some manufacturers already list the surge watts, saving you time and effort.

Should you get the minimum inverter specs given? If a 4000W is too much you may settle for something slightly under it. This will not be a problem if you run the pump only occasionally. But for frequent usage, a 4000W inverter is the best and most practical option.

Can you go higher than a 4000 watt inverter for a 1.5 HP well pump? It will not damage the pump but it will not improve performance. What you are aiming for with the 25% reserve power is for any sudden power surges or spikes.

Without any reserve power, a sudden voltage surge could damage the pump, so extra capacity helps. But doubling the inverter capacity is not going to help much as 25% is just about enough. The only time you should increase the inverter size more is if you plan to use a bigger pump or extra load.

One more thing needs to be pointed out. Most AC well pumps are not designed to be used for hours on end, so make that adjustment in your calculations. This is explained further in the following.

How Long Can an Inverter Run My Well Pump?

A 4000W inverter can run a 1.5 AC well pump for 2 to 3 hours. The runtime assumes the pump runs for 20 minutes an hour and is rated 1 to 1.5 HP.

Most well pumps are designed to run for 20 to 30 minutes at a time. Some pumps recommend a 4 or 5 minute cycle. So you have to make adjustments to the power consumption calculations depending on how long you use it.

If you are running a solar powered shed for instance, it is easy. Just multiply watts per hour x runtime and you know what inverter to get. With a pump, it is different. If you have a 1500 watt well pump that runs for 30 minutes an hour, it only consumes 750 watts.

A 1.2 HP pump may have a limited number of cycles to run per day, say 300. Divided over 24 hours that is 4.8 minutes per cycle. Some newer AC well pumps can run for long stretches so these variables have to considered.

If you use the pump for less than 15 minutes an hour, power consumption could be 500 watts or lower. But if you run the pump for 15 minutes twice an hour, power use goes up to 1000 watts.

You do not have to worry too much about the calculations though. As long as the inverter can handle the surge watts, you should be able to run the pump without trouble. Of course the rest of your solar system must be of sufficient size too.

Does this mean you should get a 500W inverter? No, because the well pump still needs 3000 watts approximately during startup. This only takes a second or two, but the inverter must still be able to supply it. A 1.5 HP pump will not run if the inverter cannot supply at least 3000 watts.

So even if you only use the well pump for a few minutes per hour, the inverter capacity LNK must still be higher than the surge watt requirement. This applies not just to well pumps but any appliance or device that has surge watts.

AC vs. DC Well Pumps For Solar

AC well pumps are more affordable than DC pumps and easier to maintain. DC pumps are more efficient, but for home use, an AC system works well enough.

Both well pumps can be used with a solar PV system. A DC pump can be connected directly to the solar system. You do not need an inverter or battery to run it. However the pump will only run as long as there is sunlight. Once the sun goes down, the pump will shut down along with the solar panel.

The solution is to connect the pump to a battery bank, which is what you will do with an AC pump too. If you are going to install a battery bank, might as well get an inverter. Even if you add the inverter cost, an AC system will still cost less.

The biggest disadvantage of a DC well pump is the maintenance. It requires more upkeep than an AC motor and repair is difficult. If something goes wrong you need to call a specialist and that is going to cost you.

There are benefits to using a DC pump. As we mentioned it is more efficient and second it does not produce as much noise as an AC pump. The lifespan is usually longer for a DC well pump, but some AC units have improved their design and can compete in this aspect.

If you have an AC well pump you can use not only solar panels but also a generator or any AC power source. So whether you are on or off the grid you can use this pump.

Conclusion

A well pump is going to be one of the biggest power draws in any solar system. That is why you need to configure the system so it is as efficient as possible. This starts with having the right inverter, so hopefully this guide was able to help you.