Need assistance sizing a off-grid solar power system.

OffgridAquaponics · April 2017

Basic problem: I need to size a battery bank and solar array to run a 120 volt air pump 24 hours a day, indefinitely. It is for an off grid aquaponics greenhouse. I clocked the pump for 24 hours using a Kill-A-Watt meter and I think it pulls 46.5 watt hours (not sure I did it correctly). Some online calculators ask for lowest expected temperatures for the batts (I presume 20-30 F) I have three 15 watt panels (45 watt total), 12 volt/35 amp batteries (2), and an 800 amp inverter. I also calculate for 50% usage of battery and 3-5 days operation without sun. I may purchase a second 45 watt solar kit soon.

I know it is under powered. I just have no clue to calculate the appropriate size. I am also at a loss. Should I do the batteries in series or parallel? I am an intelligent person but I have no clue about electrical engineering and the online calculators are more confusing than helpful.

Solar Kit
Battery
Inverter
EOCP268 951 GPH Pump

Any assistance is appreciated.

Estragon · April 2017

To sanity check the 46.5 wh/day you could watch the draw live for a bit to see if it makes sense.

One problem though is this is measuring AC current after the inverter. I suspect the 800w inverter will use much more than that just being on. If you have a multimeter and can check dc amps from battery to inverter that may give you a better sense of actual load.

Photowhit · April 2017

Please DO NOT buy another set of Harbor freight 3 panel sets, They will certainly be a waste of money for this application!

Photowhit · April 2017

Is the 45.6 watts at 12 volts? or off an inverter?

Basics are (I'm going to use 50 watts per hour, 50 x 24hours = 1200 watt hours, as a 12 volt load this would be 100 amphours. For lead acid battery use 16 hours a day (winter) you will also need 20% more current to charge the battery, (you will recoup some of this during day time hours when you will run pretty much directly off the solar array)

so 100 amps x 3 days autonomy = 300 stored amps, if you don't want to discharge the battery bank more than 50% that would be a 600 amp hour battery bank or 6 - 6 volt golf cart batteries wired in series and parallel for a 12 volt @620 amps.

To charge this you would want a 60 amp charging current so you would need about a 1000 watt array...

Just quick numbers I've got to get out for league tonight.

Estragon · April 2017

@Photowhit
I'm pretty sure his estimate was 46wh over the 24hr period, not a continual 46w draw. A 2w draw seems kinda low, but a bubbler pump might be something like that?

The thing is, even if that's right, the inverter losses could easily be 20w or so. It would help to check the DC amp draw to the inverter.

Also, could the pump be low voltage DC with an AC transformer? If so, maybe it could be fed DC to avoid inverting?

OffgridAquaponics · April 2017

Estragon said:

@Photowhit
I'm pretty sure his estimate was 46wh over the 24hr period, not a continual 46w draw. A 2w draw seems kinda low, but a bubbler pump might be something like that?

The thing is, even if that's right, the inverter losses could easily be 20w or so. It would help to check the DC amp draw to the inverter.

Also, could the pump be low voltage DC with an AC transformer? If so, maybe it could be fed DC to avoid inverting?

I added the user manuals for the products in the system. Would they help clarify? I am not sure where else to get the information you need.

Horsefly · April 2017

So the EOCP268 951 GPH you pointed to in the linked spec sheet clearly says the power draw is 41 watts. In a 24 hr period that works out to be 24 x 41 = 941 watt-hours per day at 115VAC. With an 85% efficient inverter, that means you need 941/0.85 = ~1100WH. That's quite a bit more than you were thinking.

Steve

EDIT: I see @Photowhit already got to about the same point.

OffgridAquaponics · April 2017

Horsefly said:

So the EOCP268 951 GPH you pointed to in the linked spec sheet clearly says the power draw is 41 watts. In a 24 hr period that works out to be 24 x 41 = 941 watt-hours per day at 115VAC. With an 85% efficient inverter, that means you need 941/0.85 = ~1100WH. That's quite a bit more than you were thinking.

Steve

EDIT: I see @Photowhit already got to about the same point.

Thank you. It makes a little more sense to me now. So what would be the ideal set up (brand/model of batts, inverter, and size/number of panels) for the task to run the pump 24/7?

OffgridAquaponics · April 2017

Photowhit said:

Is the 45.6 watts at 12 volts? or off an inverter?

Basics are (I'm going to use 50 watts per hour, 50 x 24hours = 1200 watt hours, as a 12 volt load this would be 100 amphours. For lead acid battery use 16 hours a day (winter) you will also need 20% more current to charge the battery, (you will recoup some of this during day time hours when you will run pretty much directly off the solar array)

so 100 amps x 3 days autonomy = 300 stored amps, if you don't want to discharge the battery bank more than 50% that would be a 600 amp hour battery bank or 6 - 6 volt golf cart batteries wired in series and parallel for a 12 volt @620 amps.

To charge this you would want a 60 amp charging current so you would need about a 1000 watt array...

Just quick numbers I've got to get out for league tonight.

Great information. Thank you @Photowhit

Estragon · April 2017

Photowhit's numbers are likely in the ballpark, but I still think you should check DC current from batteries to inverter.

The inverter is big for the load, and a square wave inverter feeding an inductive load may be heating the motor and using a lot more than ~4adc. A small Samlex or Morningstar sine wave inverter may be better.

Choice of controllers and panels depends on how far the array will be from the batteries. If they're more than about 40' apart you might want to use 4x250w panels wired into 2 strings to a MPPT controller like a Midnite classic or Outback. For short distance, depending on local cost of panels, you could do 6 or 7 150w 12v panels to something like a Morningstar 60a PWM controller.

For batteries 6x6v deep cycle golf cart size wired as 3 strings suggested by Photowhit would probably be cheapest option. Depending on local pricing, 4x6v L16 size in 2 strings for ~700ah might not be much more and would have fewer cells and interconnections (easier maintenance).

This is a rough guide. You may need to adjust based on your location (hours of winter sun), and availability of alternate charging to avoid battery damage during long periods of bad solar condtions.

Photowhit · April 2017

As Estragon said check your numbers, you might be able to find lower energy aerators or reduce the number of fish, I know aerators for hydroponics are lower wattage but likely don't produce enough oxygen... You will likely also be running a circulating pump? unless your growing on top of you fish pool. I think fish like flowing water as well.

OffgridAquaponics · April 2017

Photowhit said:

As Estragon said check your numbers, you might be able to find lower energy aerators or reduce the number of fish, I know aerators for hydroponics are lower wattage but likely don't produce enough oxygen... You will likely also be running a circulating pump? unless your growing on top of you fish pool. I think fish like flowing water as well.

The air pump powers an airlift pump to move water from the sump to the fish tank. It simultaneously oxygenates the water. The rest of flow is gravity fed from the fish to swirl filter, mineralization tank, Depp Water Culture grow beds, then back to the sump tank and airlift pump. I wanted as low of energy inputs as possible.

I have thought of a pond aerator wind mill powering the airlift pump and a float switch if the wind dies and the sump starts to fill too much.

Thanks for the help everyone. Great information. I love this collaborative knowledge base.

Need assistance sizing a off-grid solar power system.

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