How to size your batteries? system?

System · June 2007

Ok forgive me because I know this question shows just how much I dont know.

I'm just starting to learn about solar and last summer I changed my pool over to solar heat and salt water.

Now I want to start playing with solar electricity.

My first project would be to make my 1.5 HP pool pump run off solar. The pump runs 8 hours a day in the summer and as low as 6 hours a day during the winter. Sometimes like if it flashes over green I may have to run the pump a full 24 hours but I figure if that happens I could pull from the grid. (It happens about once a year during the summer where I need to run it 24 hours.)

Now my problem is how do I size the batteries and then how do I size the panels to make sure they keep the batteries charged. I know there are simple math formulas for this but I can find them... I searched this forum and see examples but not the formulas.

Also do you think this project is to big to start learning with??

Thanks for you responses.

mike95490 · June 2007

Re: How to size your batteries? system?

That's a pretty large pump, and while you may get it running off solar, you are looking at a fairly large array.

What does "flashes over green I may have to run the pump a full 24 hours" mean ?

I'd start very simple, 1 panel, 1 battery, and a nighttime flood light for the pool.

Next I'd suggest for the most bang for your buck, is just a gird-tie system, no batteries to have to mess with, just spinning the electric meter backwards in the daytime. Use the "grid" as your battery !

BB. · June 2007

Re: How to size your batteries? system?

Answer somewhat depends on where you live.

Your best solution would be to install a Grid Tied Inverter solution. But, you have to 1. have a utility that allows Grid Tie and 2. in some locations, you can get a government/ and/or utility rebate for installing solar PV panels (but only for Grid Tied solutions--battery off-grid systems are not allowed in US rebate programs). Generally, net metering and government rebates are not a good job for a first time do it yourself installation (many states have stringent contractor licensing requirements for their rebates).

Any off grid solution with battery banks would decrease efficiency from ~95% down to ~60% efficiency--requiring you to almost double the number of very expensive solar panels for the same amount of useful power.

Your only other option would be to find a DC compatible pump or pump system that will direct connect to your solar PV panels. to replace your AC pump. It can be done, but the expense and maintenance requirements for these systems usually make it not worthwhile.

So--since solar PV panels charge controllers, batteries, and inverters are expensive, you will 1. want to most efficient pool pump and filter possible, your approximate location for solar irradiation, and 3. know the exact power requirements (not just nameplate ratings--so either a current measurement or watt meter and exact number of hours planned use and seasonal usage).

Next you will need to look up how much sun you get for your area:

http://rredc.nrel.gov/solar/pubs/redbook/

Next calcluate the amount of solar PV you will require... Your numbers will be like this (insert your real numbers if ddiffernt than samples provided here):

Solar PV (STC) using grid tied = Watt of Load * 1/(0.75 PV derate) * 1/(0.95 grid tie inverter) * 1/(hours of sun)

OR

Solar PV (STC) off grid = Watt of Load * 1/(0.75 PV derate) * 1/(0.95 MPPT charger) * 1/(0.8 batt eff) * 1/(0.85 inv eff) * 1/(hours of sun)

Battery Storage Watt*Hours = 3 (days of no-sun) * 1/(0.5 for battery life) * Watt*hour (load per day)

Battery Storage Amp*Hours = BS W*Hr (above number) / (12/24/48 volt battery bank voltage)

We can further fine-tune the numbers but this should give you a good place to start.

I am sure others will have better comments to add too.

-Bill

System2 · June 2007

Re: How to size your batteries? system?

mike90045 wrote:

What does "flashes over green I may have to run the pump a full 24 hours" mean ?

With a salt water pool your chlorine level is always very low. Just low enough to keep the pool from turning green. I'm in Florida and if we get a bad storm in the morning it can dilute the chlorine level. Also in Florida green algae can turn a pool green in about 5 to 10 minuets. It happens so fast we call it flashing over green. I have watched it happen. It's actually neat to watch but stinks because you cant swim for about a day.

Also thank you just reminded me of something extra on that breaker panel. The pool light and the plug outlets by the pool are on that pool pump panel.

I do want my system on the grid but the main reason for batteries is I can be without power for weeks after a storm. So I kinda want both. My real goal would be to have my home off the grid and making extra power to run the meter backwards most of the time. But I could never afford to pay someone to do it so I need to learn it myself. LOL

Bill, THANKS!! That was just what I was looking for.

BB. · June 2007

Re: How to size your batteries? system?

If you want more Florida pool information, Solar Guppy is both a designer of Grid Tied Inverters and has his home/lab in Florida. He can tell you about the ins and outs of the various options based on experience. He is here sometimes and also has is own BB:

http://www.solar-guppy.com/forum/

Just as an FYI--you are not talking about a small system and battery off-grid requirements can easily double the price (or triple because it may not be eligible for government rebates).

In many cases, a grid tied system with generator backup may be more cost efficient (yea, I know, this is a solar power board).

-Bill

crewzer · June 2007

Re: How to size your batteries? system?

My first project would be to make my 1.5 HP pool pump run off solar. The pump runs 8 hours a day in the summer and as low as 6 hours a day during the winter.

Now my problem is how do I size the batteries and then how do I size the panels to make sure they keep the batteries charged. I know there are simple math formulas for this but I can find them... I searched this forum and see examples but not the formulas.

I'm in Florida… I do want my system on the grid but the main reason for batteries is I can be without power for weeks after a storm. So I kinda want both. My real goal would be to have my home off the grid and making extra power to run the meter backwards most of the time.

Rawley,

Here are some back-of-the-envelope estimates and concepts to consider.

Assuming 75% efficiency, your 1.5 HP pump running for 8 hours/day will consume (1.5 HP x 746 W/HP / 75%) x 8 hours/day = ~11.94 kWh/day. Assuming ~67% overall RE system efficiency and an average of 5.2 hours/day of “full” Sun in the summer, (i.e, Tampa, FL), you’ll need a PV array rated at (11.94 kWh/day / 67%) / 5.2 hours/day = ~3,430 W to meet that energy requirement.

One rule-of-thumb for sizing battery banks for off-grid systems is to spec the bank at ~6x daily energy need. This allows for 3 days of autonomy (no Sun) without discharging the batteries by more than 50%. Assuming 90% inverter efficiency, you’d “need” a battery bank rated at 11.96 kWh/day x 6 days / 90% = ~80kWh. Assuming a nominal 48 V battery bank, that would be ~1,667 Ah – a fairly large bank.

A 48 V x 1,667 Ah battery bank would be a bit large for a 3,430 W PV array (48 V x ~1,000 Ah would be better balanced), so this part of the sizing exercise would need some additional work.

There are few systems that blend grid-intertie, grid-back-up, and battery back-up. I think there’s the SMA Sunny Island inverter, and Xantrex might still have a system. There's also the OutBack Power family of GTFX and GVFX inverter/chargers, the OutBack PS1 system, and there are GridPoint products, which I believe use OutBack subsystems.

Energy back-up systems are rarely configured to sustain all of a traditional home’s loads. The usual configuration is to identify critical circuits and power them them via a sub-panel. During normal operation, non-critical loads are powered from the grid, and critical loads are powered by the PV energy system, the batteries and/or the grid. Additionally, any “extra” energy from the PV energy system is fed to the grid. If the grid fails, the batteries will power the critical loads until they discharge.

With grid-intertie and grid-back-up, I would think that you could get by with a battery bank rated at two- or three days of capacity.

HTH,
Jim / crewzer

How to size your batteries? system?

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