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I have a 120/240 volt generator with an output of 3250 watts peaking at 3750. I want to be able to power my well water pump but I'm getting conflicting answers on whether this generator packs enough power. I'm also unsure how large my well pump is but everything else in the house is pretty standard so I'm going to say between 3/4 and 1 hp.

Consumer reports says a well pump runs about 700 watts. Multiply that by 3 to get a rough estimate of the starting wattage for the pump and you run about 2100 which still leaves plenty of wiggle room in terms of wattage.
Consumer Reports Wattage Report

Doing some other research online there are a lot of folks saying you need at least a 5k watt generator to power a well water pump. Obviously that's a far cry from what consumer reports is listing and well below the output of my generator.

I don't want to just plug it in and ruin my water pump or generator so I'm trying to find a solid answer or direction before doing so.

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2 Answers

up vote 1 down vote accepted

If you already own the generator I say go for it and try it out. If the generator is anywhere decent then it will regulate the voltage more than enough for a motor so you won't over power the motor with voltage. If the generator can't handle it you will not harm the motor unless you are able to stall it for an extended period of time; the generator shouldn't be harmed either it would be like trying to use a dead battery if it can't push out enough juice it just won't.

The reason I've notice that people suggest higher wattage ones is because they are typically built a little better and since they are capable of more wattage they respond to a peak significantly better, I'm going into peak discussion. Generators supply electricity on demand (even the power stations'), a household generator will produce approximately a few hundred watts at idle, as more items are plugged in/turned on demand rises and the generator senses it and runs harder. Since there isn't a preemptive notice about this, the generator will typically spike/peak higher than the demand and recovery back around what it needs to put out. A motor turning on with capacitors will typically be able to handle that lag, the capacitors will just charge up slower (almost not noticeable in most cases) and the capacitors work as rectifiers handling another other voltage changes during operation. Motors without capacitors also typically handle it but since they don't get a beginning zap from the capacitors you run a small chance of burning out your coils, imagine plugging a 220v motor under load to 110v source it will energize the coils but remain stalled.

If this is a more permanent installation (off grid scenario as opposed to hurricane prep, etc.) I recommend a separate motor starter kit - bad with naming here - before the motor or a heavy duty UPS/Battery system but for just when the power fails I think you'll be fine.

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I had the very same question when I sized my generator to run my well-pump during power outages. I purchased a clamp meter for under $100 and tested the pump on household current. I simply attached the meter to the hot line and then turned on the pump. My pump is 220 volt with two hot wires so I had to multiply the results by 2. Unfortunately, I have moved-on from that home and don't remember how much power mine needed, but it was well under the 5500 watt stated capacity of my genset.

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Oh, also, It is important to note or set the "hold" feature on the clamp meter so you know the starting requirements and the running requirements. –  Jerry Jun 26 '13 at 14:28
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