Starting small

[Bach On]

I want to convert our well pump to solar. It turns out that it consumes right much power and adds to our monthly power bill.

Initial Thoughts:

It is an above ground 120 volt pump. I'm not yet sure how many amps/watts it consumes per hour. It is housed in a small pump house. (The roof of the pump house is unsuitable for solar panels.)

The pump feeds a 12 gallon pressure tank (I intend to increase the size of this tank to 20 or 40 gallons so the pump won't run quite as often).

I anticipate that initially I'll use six 120 watt panels to charge two Trojan 105 batteries. These are six volts. So to get 12 volts I'll have roughly 225 reserve amps of current. Going by the formula of using only 20% of their reserve, I'm figuring on roughly 45 amps being available. It is doubtful that the solar panels can fully provide this much power on a daily basis. The number of panels may need to be 8 or 10 - especially for winter months.

So, we have to have the capability to switch the pump back to the grid if the power used exceeds that 45 amps.

I have a south facing roof that is perfect for the panels. It gets lots of sun Summer and Winter. I do not plan to use solar trackers, so I won't get full power all day long.

One thing I'll need to plan is the electronics needed to control the source of the voltage going to the pump. Ideally I'd like to have the pump run from the batteries through an inverter. Then when the drain on the batteries reaches the 45 amp point, switch the receptacle in the pump house over to the grid.

The complicating factor is how to keep the battery/inverter power from backfeeding into the grid in the event of power failure.

I'd guess that some here have done something like this. I'd appreciate any tips and advice you might offer.

My initial thoughts on a budget is about $2 to $3 thousand US dollars. Does that sound anywhere close to realistic? (I wasn't planning on buying a new pump.)

Bach On

 

[dougy83]

The complicating factor is how to keep the battery/inverter power from backfeeding into the grid in the event of power failure.

You can use a changeover relay to switch the pump between the two supplies. You should get an electrician to wire it for you of course.

initial thoughts on a budget is about $2 to $3 thousand US dollars. Does that sound anywhere close to realistic? (I wasn't planning on buying a new pump.)

Yes, that sounds realistic to me... not including installation costs. 5x 120W Panels < $575, batteries < $400, inverter <$300, solar charger <$450; subtotal $1725... + extras

Personally I wouldn't use batteries, nor the inverter and would just run using whatever sun energy is available at the time (directly from the solar panels) while the sun's out to fill a water tank. Household water is then provided using a standard mains powered water pump. 5x 120W Panels < $575, tanks (2x 264 gal) $200, house water pump $149, bore DC 48V pump (2500L/hr) + MPPT controller $790, backup power supply $100; subtotal $1814... + extras. If the pump pumps anywhere near its max rate, the tank will be full in around an hour... this would mean you could downsize the system substantially. Then again, if it only takes 500W to run this pump then that's a bit more than 1kWHr to fill 536 gal tank (which should surey be enough for a day)... is that really that expensive to run? How much water do you use?

 

[tcmtech]

For that level of investment I would just buy a DC powered pump or motor and run that directly off the batteries.

Personally without far more info I can't see the justification of spending $2000 - $3000 on a conversion of one 120 Volt pump that likely costs less than $200 new to save money. The return on investment will likely be far longer than the life of they system for what little money it will save you on electricity.

 

[Mr RB]

For that level of investment I would just buy a DC powered pump or motor and run that directly off the batteries.
...

Agreed! (But running the DC pump directly from the solar panel, no batteries needed).

It would be very much cheaper, and simpler to setup and maintain.

Assuming your mains powered pump turns on with a float switch, it will only be activated when the solar pump is not keeping up with the water demand. So you get an efficient system that will be 100% solar for low water usage and the expensive mains pump is basically for rare emergencies.

 

[BeerBelly]

Solar water pumps are an ideal solution. The more sunshine a household receives-The more water is needed.

 

[bryan1]

For 2-3K one would be better buying a 5,000gal tank and storing rain water and when needed just use the existing pump to top up the tank when needed, connect the pressure pump inlet to the tank outlet at the base of the tank and depending on usage fill up one a month or even longer.

 

[Bach On]

Well, I did ask. And the points expressed are all good ones.

1. Yes. This is a lot to spend on one 120 volt pump. I haven't yet measured how many watts the pump consumes over a week or two to get an average daily use figure on the watts needed. I may discover I don't need 4, 5, or 6 panels. But I don't see that the life of the pump has anything to do with the payback period for the system. The only real consumables I see are the batteries. And that is built-in to MOST (though not all) renewable energy situations. But running directly off the panels is an idea I may need to explore. What I like about the battery situation is it helps keep the voltage to the inverter a little more constant.

2. I don't want a tank. I understand the gravity feed thing. And that could be a very definite plus. But it would be difficult to place a tank around our house where it would be aesthetically pleasing. Second, there is a hygiene issue to consider. I worked at a school that had a huge one. I've been up in there. Keeping a raised water tank clean and safe is harder (and more expensive) to do than you might think. And freezing can be an issue.

3. I don't favor changing the pump to a 12 or 24 volt one. It would involve digging a trench and running VERY heavy duty #1 or #2 wire almost 50 or 60 feet (probably even further when you add the run from the solar panels). By using the inverter to power the existing pump I believe we'll experience lower voltage loss from the inverter out to the pump house. And yes - I'll lose some power in the conversion at the inverter. But that is on the 12 volt side - not the pump side.

4. I see this as a stepping stone towards a larger system.

We have higher power rates in our city than in most parts of the country. Our city utility made a bad deal and locked into high rates back when Enron was still in business. Now we're stuck with those rates. It was a stupid move by the manager (who got fired for it). But we're still stuck with high rates. And we can't switch to another power company. Thus, I've been on a quest to lower our electric bill.

While watering some new grass seeds over a two month period, I discovered that the pump was driving our monthly electric bill up. I'm also going to find out if a more efficient pump is an option.

We switched to a heat pump/hybrid water heater (the old electric one had died). This has dropped our use by about $15 a month. Not sure what the life of the unit will be. So don't know if it will ever pay for itself.

We've upgraded our insulation. That is almost always a wise plan.

We upgraded our HVAC unit to a 16 SEER hybrid unit. That saved us between $75 and $!00 a month during the Summer. Our Winter electric use is slightly higher than before, but we use less than a third of the propane our older gas-pack was consuming. That brought our equal pay plan from $465 a month to $345 for electricity. I'm aiming to get it under $300. I may not make it. But that's my goal.

I don't have the money now to convert totally to solar. But I'm looking ways to do it gradually. And the cost of panels is coming down - albeit slowly.

Again, thanks for the good input. I knew I'd get honest feedback. I do recognize that this does look like a lot of money for what must seem like a small gain.

Bach On

 

[tcmtech]

Given its 120 VAC powered the odds are its less than 3/4 Hp (more likely 1/3 or 1/2 hp) so it would have a rough power draw of around 600 watts at worse. From that getting 600 watts of power to a 12 volt pump 60 feet away would give you a amp load of around 50 amps or 25 amps at 24 volts.

At 60 feet you would be able to get by easily 6 ga aluminum (or doubled up 12 gauge copper) for the 50 amp load or 10 gauge copper for the 24 volt which can be bought at any local home building supply center for less than $1 a foot

New larger capacity cable $60 - $120. New shovel $25. 60 feet of digging = a few hours of good old fashioned cardio exercise at worse.

Still not seeing where the $2 - $3K seems like a better solution.

As far as payback on the pump goes it's everything related to this proposed concept.
Say your pump is costing you $2 a day in electricity. given that a investment of $2000 means that your proposed system has to run perfectly with zero maintenance for ~3 years before it reaches theoretical break even. At a $3000 investment its ~4.5 years.

Still think you can get 100% perfect service life for 3 - 4 years from a solar and battery based system with zero maintenance and zero investment of any sorts while still getting 100% service from your water system?

Now to be honest if your daily power consumption for the pump is below $1 a day you would be looking at 6 - 8 years break even points and at only 50 cents a day 12 - 17 years! So yes knowing exactly what your pump uses power wise over a reasonable time frame is very relative to the overall problem here.