Robbob2112 Answer thread (2024)

#1 - how to decide battery voltage and also figure how many panels you need.

From this thread

Question was how do I decide battery voltage - I also answered on how to determine solar panels needed.

The main decision on battery voltage comes down to how many amps are drawn from the battery - Most of us are of the opinion that 3000w is the highest practical wattage for a 12v system. 5000w for a 24v system. Then 48v for anything beyond.

Watts drawn by your tools/etc multiplied by the number of hours in a day you want to use them. This give you watt hours. Batteries are rated in amp hours and can be crossed into watt hours by mulitplying by voltage.

i.e. 12.8v * 100ah = 1280wh
With LFP batteries you don't want to draw below 10% ever and 20% is used as the floor by many.
1280wh * 80% = 1024 usable wh

If your tool draws 330 watts to run (110v * 3 amps) and you run divide your usable battery watt hours by that to find out how long the battery will run it.

1024wh / 330w = 3.1 hours of constant run time.

So if you assume you want 10 amps at 110v for 8 hours a day of continuous usage you are looking at 8800wh needed.

a 51.2v battery rated at 100ah can do
51.2 * 100 = 5120wh * 0.8 = 4096wh usable

4096wh * 2 = 8192wh

So you would need two of those larger batteries to run for 8 hours roughly. Or you would need 8 of the 12.8v batteries.

For solar panels you need to lookup your address and see how many hours of sunlight you get a day - i.e. Washington state will be less than Florida.

Now - for panel size - if you have full sun on a 300w panel for 5 hours you will get 1500wh out of it.

8192wh / 1500wh = 5.4 panels

Now, you won't get 100% efficent on the panels and the SCC (solar charge controller) and the sun won't shine for your house location as much as the average lists everyday. So you can figure you will need at least 20% more than you think just to break even.

5.4 * 1.2 = 6.5 panels - so call it 7 panels.

300 watt panels are fairly large (65" x 30") and they make up to 550w panels which are a bit larger than a 4' x 8' sheet of plywood. So where you are going to mount things comes into play.

Panel Rules -
Each string of panels - all panels in a string MUST face the same direction - So if you don't have room to install panels into the same plane you need to have multiple strings. i.e. some face east, some face west.

Correction - see below
You can have multiple strings on a single MPPT - but all the panels in a string need to face in the same direction so they have similar power out

Output current will be limited by the charge controller, and thus some people over-panel (despite the max possible panel current exceeding the MPPT output rating). That will just result in some clipping at solar noon on perfect days.

Strings can be in series and parallel or a combo - Series voltage adds and current stays the same. Parallel current adds and voltage stays the same. It is pretty common to have series/parallel combos to get optimal current/voltage

You may NEVER exceed the voltage of your MPPT - even a few volts over will damage it. Panel voltage is measured in a lab at 25c. As heat increases voltage decreases and as it gets colder the voltage increases. So when figuring the max voltage leave at least a 10% buffer or more if you get really cold winters.

If you have more than 2 strings you need a combiner box with breakers that disconnect both poles of the panels.

Grounding - repeat this - 'there can be only one' - Your service entrance from the utility will have a ground rod - ALL other points in the electrical system MUST be tied to that one rod. If you have a independant system in your shed it will need an independant ground rod and the panel frames and the other pieces of equipment will be tied to it. If you later decide to connect to the house power you MUST disconnect the local ground rod..... When removing this rod you must remove the neutral/ground screw 'there can be only one'

The panels should have their frames connected to a grounding system. The shell of the inverter, the power panel, any other equipment with a ground screw will be connected.

Go here to get the estimate for your hours per day of usage

plug in 1k for the panel amount and it will tell you what watts to expect

One thing on energy audit - most tools don't run constantly... I would be willing to guess unless it is a sander it runs only 20% or less of the time depending on the tool

You can use that as a scaling factor to figure out the required number of panels. - here is my result - basically I can expect 4hrs in winter an 6 hours in summer of sun....