Solar panel for powering pool pump - calculation

[onno]

Hi all,

A friend of mine has a pool and he told me he was wondering if the pump could be powered by solar. So I told him I'd look into it.
But I think i'm going wrong with the calculations.

The electrical details of the pump are (see also attached image):
230VAC
P1: 1.1kW / 5A

Say I build a panel from 36 cells, these are 5W 0.5V cells. So total output would be 18VDC / 5W. Right?
Or is the output (when taking in account W=A*V): 5W@0.5V = ~ 10A, so panel output would be: 10A*18V=180W ?

Then I come at ~ 6 panels to power this single pump. Which seems quite a lot. Therefor I believe I'm making a huge mistake somewhere.
Also, there will be a charge controller and battery(s) involved, because the pump needs to run for 12hours, and there are not many places with 12hours of sun ;o)

So I try to figure out what's needed to get this work. I hope someone can post a few hints on the panel/battery calculations. :$ At least thanks!

KR,
Onno.

[SunEagle]

Well first you have to size the battery system to run that pump on days that have no sunshine otherwise the pool can stagnate and grow alga.

So if your pump is 1100 watts and you need to run it 12 hours a day that comes to 13.2kWh (1100w x 12hr = 13200wh). Which is very big and very expensive requiring a panel system over 3000 watts so you can probably forget building your own panels out of 5w cells.

Some people have changed out their pool pumps to ones that are variable speed which should use less wattage to run. But to run that 1.1kw pump for 12hours using solar and batteries would be extremely expensive and IMO wasteful.

[J.P.M.]

If the goal is to have a viable pumping system and reduce pumping costs, and not simply solar powered water pumping for its own sake, get a variable speed pump. Forget single purpose PV and then learn why it probably doesn't pay. Once so informed, if you then choose to proceed with the PV pumping, in spite of the hassles and the economics, you'll be better informed to tackle the project.

[littleharbor]

5 amps @ 230 volts is roughly = to 100 amps @ 12 volts. Considering all the losses involved in battery charging and inverting to 230Vac.and less than peak charging in morning/afternoon hours you would need considerably more than 100 amps of PV @ 12 volt PV to run your pump.

[SunEagle]

5 amps @ 230 volts is roughly = to 100 amps @ 12 volts. Considering all the losses involved in battery charging and inverting to 230Vac.and less than peak charging in morning/afternoon hours you would need considerably more than 100 amps of PV @ 12 volt PV to run your pump.

With that 1.1kw load I would expect a 48V battery system instead of a 12v one. But with a 13.2kWh daily usage the battery would need to be about 275Ah x 4 to keep it within a 25% DOD. That calculates out to 1100Ah at 48V and a 5000watt panel system. Way too costly to just run a pool pump.

[onno]

Hi all,

First of all thank you very much for all the quick replies. Didn't expect so many replies in such a short time :$ So I guess some of my calculations were correct, as in that he better can get a more efficient pump or one running on wind power ;o) Since we have much more wind than sun.

At least the conclusion is all the same: It will be way to costly to run this pump on solar. He'd better use it to run his extra fridge of it ;o)

Thanks again to all for the insights and the calculations mentioned. Really helpful!

KR,
Onno.

[SunEagle]

Hi all,

First of all thank you very much for all the quick replies. Didn't expect so many replies in such a short time :$ So I guess some of my calculations were correct, as in that he better can get a more efficient pump or one running on wind power ;o) Since we have much more wind than sun.

At least the conclusion is all the same: It will be way to costly to run this pump on solar. He'd better use it to run his extra fridge of it ;o)

Thanks again to all for the insights and the calculations mentioned. Really helpful!

KR,
Onno.

Glad we could help.

Based on most feedback a solar / battery system will cost about $2000 to $3000 per kWh it can deliver for small systems and a little less than $2000 per kWh for larger ones..

That price can be trimmed back if you go with a smaller battery system but usually they have a much shorter life due to deep discharging them so you just have to replace the batteries more often.

[J.P.M.]

Hi all,

First of all thank you very much for all the quick replies. Didn't expect so many replies in such a short time :$ So I guess some of my calculations were correct, as in that he better can get a more efficient pump or one running on wind power ;o) Since we have much more wind than sun.

At least the conclusion is all the same: It will be way to costly to run this pump on solar. He'd better use it to run his extra fridge of it ;o)

Thanks again to all for the insights and the calculations mentioned. Really helpful!

KR,
Onno.

It'll probably be as costly or more so to use WECS generated electricity to power the pump than PV generated power. Nice science project, poor use of resources in either case. Much more costly and less reliable than grid power.

[sdold]

For the cost of a battery system that would run that pump, he might be better served with a small grid-tied system, say 5 or 10 panels, if it's allowed there. I bet what he really wants is just to be able to run the pump without paying for the electricity.

[onno]

Hi,

One more 'question'. I'm trying to get hold of the calculation part. So below another 'real live' example. Yes, I know it's way to expensive for PV but as I said, it's just for the calculation part.
I have a device which uses 2240W for 3h20m (lets use 3.25 hours for simplicity). It's running of 230Vac. Rounded up that would be about 10A per hour. It's running at night so I need to store the energy at day.

So that's 2240W @ 230Vac. = 9,74 A (an hour)
At 12V that would translate into 2240W / 12V = 186,67A
So for the batteries, 4x 200Ah batteries (12V) would be needed. Right? (as that would be able to provide 200A for 4 hours?!)
Based on the panel I mentioned before (5W@0.5V, 36 cells), what's the best way to calculate how many I need? I know we also need to take into account the number of sun hours. But do we take the least for year around? (I'm in the Netherlands, so the sun hours won't be much ;o) )
But say the worst is 2 hours.

I would guess that, the panel would provide around 8A per hour (~ 80% of the 10A it could produce). So when one panel provides 8Ah for 2 hours (16Ah), I would need 50 panels [(4*200)/16]?!?!
Or is this calc complete BS ;o)

I know this question might be redundant as there are tons of calculators to find. However, that for me a bit the problem, because many work a bit different than the other giving all kinds of results.

At least, thanks again for your time from all of you.

KR,
Onno.

[SunEagle]

Lets get you on track with the proper calculations. You don't determine the amps used. You determine the watt hours used.

So for a 2240w load running for 3.25 hours you get (2240w x 3.25hr = 7280watt hours

Now you divide the watt hours by the battery voltage or 7280wh / 12v = 606Ah. That is the amount the battery will use to run your load.

But to keep a battery life as long as possible most should not be discharged more than 25%. So your battery would need to be 606Ah / 25% = 2427Ah.

That is a big battery. It would be better to use a 48v battery which would change that 606Ah to 152Ah (7280w / 48v = 151,7Ah) And the battery would be 4 times that or 606Ah at 48V.

Now you can build that battery system with 8 x 6Volt 600Ah batteries all wired in series to create a 48V 600Ah system.

With that you will need about 60amps of charging at 48volts which calculates to 60A x 48V = 2880 watts of panels using a 60Amp MPPT Charge controller.

With say 3000watts of panels and 4 hours of useful (or charging) sunlight. You will be able to put 12000wh back into a battery that has used 7300 wh. But if you only get 2 hours of sunlight you will need more than 4000watt of panels (2hr x 4000w = 8000wh).

That is how you get a rough idea of how big the solar / battery system needs to be.

[onno]

Hi SunEagle,

Thank you very much!! That's clarifying things greatly! Based on your information, I did a calculation for a fridge rated at say 60W. So with the info provided by you, I come to the following:
Lets say the fridge is running continuously (If I'm correct, a fridge turns on/off many times during a hour, so I guess my calculation provides more power than needed, right?)

When it runs continuously at 60W, in a day that's:
60W x 24h = 1440Wh

That results in:
1440Wh/12V = 120Ah

Then I found a battery with a DoD ~80%, so then I need a battery of:
120Ah/75% = 160Ah (DoD~80%)

Charge current of a battery is: 10% of Ah
So when I would buy this battery: Ultracell UCG200-12 Deep Cycle Gel Accu 12V 200Ah (20hours)
200Ah x (10/100) = 20A
20A x 12V = 240W

So with two hours of sun, I would at least 720W of panels.

Just wanted to know:
- Is my calculation correct?
- Am I right that the usage of the fridge indeed would be less than 1440Wh?
- Would in this case, this single battery sufficient to run the fridge?

Yet thanks again! Guess/hope this post will help many others with the calculations like it helped me.

KR.
Onno.

[SunEagle]

I see two things that bother me.

The first is that a GEL type battery is really not made for daily discharges and may live a very short life.

The second is that just about any lead acid battery will not perform an 80% daily discharge without killing it.

So going back to your fridge calculations. If you need a 120Ah (@ 12v) usage each day your battery needs to be 4 times that amount so you only discharge it 25% each day. That would make it somewhere around 480Ah which should be matched up with 48amp of charging or about 580watts of panel using an MPPT CC.

Now if it is not critical you can try to go with a smaller Ah rated batter and panel wattage but unless you really know how many watt hours that fridge will use you are making a guess which can result in spoiled food.

[onno]

The first is that a GEL type battery is really not made for daily discharges and may live a very short life.

Ah, you're right. I thought it was an AGM, but that was another one which I also had open in my browser (and overlooked the 'gel' part in the name ;o) ). Well it was purely used for the calculation. But thanks for the hint. (Edit: Yes I know, AGM is also not the way to go as they are very expensive. (just read some other posts on this forum about batteries)

The second is that just about any lead acid battery will not perform an 80% daily discharge without killing it.

Completely true. I've read that.

You're saying "about 580watts of panel" : I thought I needed to divide the total usage (1440) by the amount of sun hours a day to calculate the needed power/panels. But that's wrong?

[SunEagle]

Ah, you're right. I thought it was an AGM, but that was another one which I also had open in my browser (and overlooked the 'gel' part in the name ;o) ). Well it was purely used for the calculation. But thanks for the hint. (Edit: Yes I know, AGM is also not the way to go as they are very expensive. (just read some other posts on this forum about batteries)



Completely true. I've read that.

You're saying "about 580watts of panel" : I thought I needed to divide the total usage (1440) by the amount of sun hours a day to calculate the needed power/panels. But that's wrong?

There are two ways to determine your panel wattage. The first is determining the proper charging amps. A rule of thumb would be to get 1/10th the battery Ah rating. Once you determine that amp amount you multiply it by the battery voltage. That should get you an approximate panel wattage.

The second calculation for panel wattage is knowing you shortest hour of sunlight. Use that number divided into the estimated watt hour usage and you then get a panel wattage.

Sometimes the second panel wattage comes close to the first but you can increase the amount of charging amps for lead acid batteries to 1/8th the Ah rating. That allows you to install more panel wattage if there are not enough sunlight hours to charge the batteries yet will not charge your batteries too fast. AGM type batteries can be charged faster then FLA and depending on the manufacture I have seen as fast as 1/4th the battery Ah rating but to be safe you really should go faster then 1/6th.

Hope that help explain it better.