how to know what to buy

Welcome mrprepared - you'll need to know a few things to determine your panel needs. This sounds like it might be for a hydroponics or aquaponics setup - am I correct? Firstly - I assume you won't only be running your led light when the sun it out and shining, so what are your plans for storing the energy collected? You'll probably need both battery storage and a charge controller. You'll first need to determine the battery you plan on using (based on your loads), then work towards what panel arrangement will allow you to replenish your battery with PV. You'll need to determine the watts that those two bubblers require - and the hours those will run per-day. You'll also need to determine the hours that the LED light will run per-day.

Once you know the total watts (average continuous watts), multiply this by the hours you need to run these loads per day (watt hours). Take that result and divide by the voltage to determine the Amp hours per day. You can't discharge most batteries to 100%, so you need to account for this by making sure that your battery has the available amp hours after being discharged to your depth of discharge. Example... if you take a 100 AH battery and discharge it to 50%, you have 50 available AH to use. But it's not quite that simple. You also need to account for the discharge rate and the fact that capacity of lead acid batteries (assuming you go that route) will decrease as the rate of discharge increases. This is described further in Peukert's Law. The faster you remove power from a battery, the less capacity you'll get. So... that means, you need to size your batteries not only according to the AH you'll need, but also the discharge rate. Lastly - you'll need to plan for days without sun and consider having sufficiently-sized batteries for such. That is often called Days of Autonomy. There are many good articles on this site about C Rates, batteries, etc. Take the time to read through them and you'll spare yourself some headache and cost.

Once you know the battery bank size in AH, you should be able to determine the panel arrangement that will produce the amount of watts sufficient to replace the amp hours that are used. This too requires knowing a few key things. First - a reasonable and safe recharge current for your batteries. You'll need a charge controller that can support such. You can't just go putting 150 AMPS into 100 AH battery or things will go boom and crackle. Second, you need to know the hours of average sun per day in your area, and how many watts x volts it will take to produce the charge current necessary to recharge your batteries during those hours.You'll also need a charge controller for such - which you've not mentioned. It's not as trivial as many would expect. You'll also have to account for panel efficiency, temperature, and so on. There's a bit of analysis and calculating involved in sizing systems.

With all the above said - i'm going to say this before Sunking beats me to it: Why do you want to use this approach? Are you somewhere where grid power is available? If so, grid power is a fraction of the cost of off-grid power. Producing power with PV and batteries is the most expensive way to make power. If you need to then you need to, but if you're trying to save money - you won't do so in this way - not if you require batteries. If your main interest is saving money and don't really need off-grid power, you might achieve greater benefit by adding a small grid-tied system to your home that gains you credit with your POCO, offsetting the cost of running your equipment and also lowering your electric bill.

This. Get a grid tied system.

a 1500 watt LED light? As has already been stated, If grid is avail. you will save nothing by using batteries. I have two ponds and I had converted both over to a solar battery set up. Deep cycling of deep cycle batteries cost big bucks in battery replacement and maintenance. I was spending 4 to 6 times what grid power cost for same water flow and I was using a highly efficient 24 volt battery bank and 12 volt pump and bubbler set up. If your goal is to be clean and green, grid power is about 10 time greener than any solar battery set up. If your itching to have solar get a nice grid tie system and run your pond from an AC outlet, If you need backup power, build yourself a standby battery back up power system.

Nothing green about lead acid

If you are talking about taking it off grid and using batteries there is nothing green about it except all the cash you will loose. Anything you take off-grid is going to cost you 5 to 10 times more than just buying it from the POCO the rest of your life, and a heavy heavy polluter.

You did not say how many hours per day you want to run this imaginary 1500 watt LED that does not exist but assuming 24 hours per day you need:

18,000 watt Solar Panel. $25,000
4 very expensive charge controllers at $600 each
48 volt 3750 AH, $43,000, 11,000 pound battery you get to replace every 5 years at higher cost. That does not include EPA permits, Spill Containment, and year FD inspections.

In addition you will need to build an additional room to your house to hold all the batteries and a 1/2 acre of property for 18,000 watts of panels. In the end you should not pay a penny more $150,000. Never going to happen. Battery cost alone will cost you $10,000/year or just pay that mean ole POCO $1500/year for the same amount of electricity. God help you if you ever have a spill. The Employment Prevention Agency will crucify and flog you, and make you pay for the clean up which will be a 6 digit number.

Never going to happen.

I literally laughed out loud reading this... I might add - your face might turn that shade when you realize the ongoing expenses as well.

You can install Off grid 36V/300W*2pcs off grid solar panel system

And Audreyy goes to the cornfield. Everything is good