I just bought my solar panels. Need help sizing my battery bank.

[basicrig]

Greetings.

I am a van dweller living in a cargo van. I have limited roof space, so solar panels are my limiting factor. I decided to max out my roof space with solar panels. I purchased two 360W modules, so that makes 720W total for solar panels. Here's a link to the specs for my panels.

Now, with 720W total, I'm trying to decide what sized battery bank I will need.

My location will be Phoenix, AZ during the winter. That's the worst case for which I must plan. According to the PVWatts Calculator, the PSH for Phoenix, AZ during December is 3.05.

Do the following calculations look correct?

PSH = 3.05 (Phoenix, AZ during December)
Total Panel Power = 720 W
720 W * 3.05 PSH = 2196 Wh produced from the panels
2196 Wh * 2 = 4392 Wh battery bank (for Lead Acid)

Since I can only charge 2196 Wh and since lead acid should only be discharged 50% I'm thinking that my battery bank should be double that. So, 4392 Wh total for lead acid batteries.

 

[DThames]

Do you have your heart set on lead acid for a reason? (Size and weight issues)

 

[basicrig]

Do you have your heart set on lead acid for a reason? (Size and weight issues)

My van has a 4,250 lbs max payload. 160" x 60" cargo space. Basically the largest one they make in both regards.

My primary reason for choosing lead acid: I am wary of the potential fire hazards of lithium ion. I know a BMS can greatly reduce the risk, but it still makes me a tad uneasy. I live in my van 24/7.

 

[DThames]

My van has a 4,250 lbs max payload. 160" x 60" cargo space. Basically the largest one they make in both regards.

My primary reason for choosing lead acid: I am wary of the potential fire hazards of lithium ion. I know a BMS can greatly reduce the risk, but it still makes me a tad uneasy. I live in my van 24/7.

LiFePo4 doesn't catch fire. You might look at them. Smaller, lighter, longer service(typically), more expensive. on the front end..

 

[MichaelK]

How will the panels be oriented? If laying flat on the roof, you can't expect the panels to produce anywhere near their rated output. Assuming they are pointed directly perpendicular to the sun, I'd suggest you'd get 720W X .85 fudge factor = ~610W. Laying flat, maybe 50%. So, 720W X 0.5= 360. So, your solar resources will be much less than what you plan.

If this is going to be a 12V system, then 360W/12V= 30amps, which would be good for a 250-300Ah battery bank. So, maybe a string of two big golf-cart batteries.

 

[DThames]

How will the panels be oriented? If laying flat on the roof, you can't expect the panels to produce anywhere near their rated output. Assuming they are pointed directly perpendicular to the sun, I'd suggest you'd get 720W X .85 fudge factor = ~610W. Laying flat, maybe 50%. So, 720W X 0.5= 360. So, your solar resources will be much less than what you plan.

If this is going to be a 12V system, then 360W/12V= 30amps, which would be good for a 250-300Ah battery bank. So, maybe a string of two big golf-cart batteries.

I think his "PSH = 3.05" suggests they are flat. That is a pretty low number.

 

[MichaelK]

I think his "PSH = 3.05" suggests they are flat. That is a pretty low number.

I thought that PSH meant the average # of sunhours per day? But with the sun low in the sky, and panels completely flat, that's the worst possible combination.

 

[basicrig]

LiFePo4 doesn't catch fire. You might look at them. Smaller, lighter, longer service(typically), more expensive. on the front end..

Thank you. Yes, I heard that LFPs were the most stable against thermal runaway, out of all the lithium-based batteries, because they contain no cobalt. I will reconsider them, but as beginner DIYer with limited up-front budget, I'm leaning more towards AGM for simplicity.

How will the panels be oriented? If laying flat on the roof, you can't expect the panels to produce anywhere near their rated output. Assuming they are pointed directly perpendicular to the sun, I'd suggest you'd get 720W X .85 fudge factor = ~610W. Laying flat, maybe 50%. So, 720W X 0.5= 360. So, your solar resources will be much less than what you plan.

If this is going to be a 12V system, then 360W/12V= 30amps, which would be good for a 250-300Ah battery bank. So, maybe a string of two big golf-cart batteries.

Unfortunately, they will be oriented flat (for now). I may construct an adjustable system later.

I believe PSH stands for Peak Sun Hours, which is measured in kWh / m^2 / day. The 3.05 number is low, because I'm planning for the month of December which is 3.05 in my location. From what I understand, winter is the worst case scenario and we must plan for the worst case. The PSH only goes up from there, especially in summer.

Maybe this is a good case for developing an adjustable tilt-based racking system.

Supposing I angle them, would my calculations above be correct?

 

[MichaelK]

Maybe this is a good case for developing an adjustable tilt-based racking system.

Supposing I angle them, would my calculations above be correct?

Here is what I've determined empiricly, from measuring output throughout the day. The angle from morning till noon, can also approximate the panel's position relative to the sun when horizontal. These are my rough, eyeball measurements.

8am, almost 90 degrees of angle, 10% output
9am, 65 degrees of angle, 35% output
10am, 45 degrees of angle, 50% output
11am, 30 degrees of angle, 70% output
12noon, perpendicular, 85% output.

So, let's say it's 4pm and the sun is low in the sky. With flat panels, I wouldn't expect to get more than 10% output, or maybe 70-75W. Keep in mind though that your panel's rating was determined in a test chamber with artificial sunlight and controlled temperatures. In the real-world, with perfectly positioned panels, I've seen between 85% and 94% output; with Renogy being the single poorest performer, and Astronergy the best.

So, under perfect conditions, don't expect to ever see more than 650W from your panels. If you ever do, mark your calendar. It's a day for celebration!

 

[basicrig]

I think his "PSH = 3.05" suggests they are flat. That is a pretty low number.

That's correct. The calculator I was using asked for a tilt, and I put 0 degrees:


Here are PSH results that I got:


So perhaps my original calculations are correct, and flat is fine (for now)? Just going by the government website. I know this is a bit of a tricky art... thanks for the feedback thus far.

 

[MichaelK]

So perhaps my original calculations are correct, and flat is fine (for now)? Just going by the government website. I know this is a bit of a tricky art... thanks for the feedback thus far.

No, your calculations are NOT correct. You will NEVER get 720W out of 720W of panels laying flat! In the summer, at high noon, you might get close, but for the rest of year at any time of day, no.

 

[basicrig]

Here is what I've determined empiricly, from measuring output throughout the day. The angle from morning till noon, can also approximate the panel's position relative to the sun when horizontal. These are my rough, eyeball measurements.

8am, almost 90 degrees of angle, 10% output
9am, 65 degrees of angle, 35% output
10am, 45 degrees of angle, 50% output
11am, 30 degrees of angle, 70% output
12noon, perpendicular, 85% output.

So, let's say it's 4pm and the sun is low in the sky. With flat panels, I wouldn't expect to get more than 10% output, or maybe 70-75W. Keep in mind though that your panel's rating was determined in a test chamber with artificial sunlight and controlled temperatures. In the real-world, with perfectly positioned panels, I've seen between 85% and 94% output; with Renogy being the single poorest performer, and Astronergy the best.

So, under perfect conditions, don't expect to ever see more than 650W from your panels. If you ever do, mark your calendar. It's a day for celebration!

Thanks for the explanation. So it looks like I may be overestimating the battery bank by a fair margin. Which begs the question: what is the downside of that, for lead acid? The worst thing that I see happening is I never reach 50% discharge. Maybe only 75% discharge (wild guess). That will surely extend the life of the batteries, no?

Aside from wasted money.

 

[basicrig]

No, your calculations are NOT correct. You will NEVER get 720W out of 720W of panels laying flat! In the summer, at high noon, you might get close, but for the rest of year at any time of day, no.

Of course... So to account for that, isn't that why we multiply by a PSH? That's why I'm multiplying 720W by a PSH factor, which the government website has determined to be 3.05 for a flat 0 degree tilt.

Do you know what the correct parameters for the PVWatts website might be, then, considering my screenshots above? Maybe I'm missing parameter(s)?

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

 

[basicrig]

No, your calculations are NOT correct. You will NEVER get 720W out of 720W of panels laying flat! In the summer, at high noon, you might get close, but for the rest of year at any time of day, no.

Have you ever tried using PVWatts? It asks for tilt and even accounts for system losses %. Perhaps there is where my issue lies? I just left it to the default system losses (14%) as that's over my head. What would be the correct parameters there, then?

 

[DThames]

Have you ever tried using PVWatts? It asks for tilt and even accounts for system losses %. Perhaps there is where my issue lies? I just left it to the default system losses (14%) as that's over my head. What would be the correct parameters there, then?

PVWatts was pretty close for me, in the total KWHR per year produced.

 

[MichaelK]

Have you ever tried using PVWatts? It asks for tilt and even accounts for system losses %. Perhaps there is where my issue lies? I just left it to the default system losses (14%) as that's over my head. What would be the correct parameters there, then?

I've never used the modeling software much. Most of my information comes from the observations of systems running in real-time, when actually producing power. In my application, I'm mostly concerned with running my 240V well pump, so I've always carefully monitored solar output to know when my system is producing more than 2000W.

 

[Pnw_steve]

Thanks for the explanation. So it looks like I may be overestimating the battery bank by a fair margin. Which begs the question: what is the downside of that, for lead acid? The worst thing that I see happening is I never reach 50% discharge. Maybe only 75% discharge (wild guess). That will surely extend the life of the batteries, no?

Aside from wasted money.

Flooded Lead Acid (FLA) have a number of disadvantages. The one that comes to mind first is the fact that you need to charge them at at least 8%-10% of C in order to "stir" the electrolyte and prevent sulfating. You need to size your battery and charge source to "fit". With limited solarthat can be a problem. Oh, and they emit explosive gases when you charge them. They also take up too much space and are too heavy.

In their favor: They are fairly cheap.

I was a FLA fan boy until I learned about buying LFP's at a competitive price. The more I learned the more enthused I am about LFP. They are safe. They are compact and light. They have no minimum charge rate and their max charge rate is MUCH higher than FLA. They are also more efficient. With an FLA you are able to use about 70-80% of the energy that you put into the battery when you charged it. Put 1kwh in and have 700 watts available to use. LFH are 95%+ efficient. 1hwh in gets 950watts out.

The biggest downside to LFP has been the cost. What does a Battleborn 12v 100A/H cost? $1000? I have a good quality 12v 200A/H FLA battery in my trailer and it cost me about $250. Now that folks who are a little handy can put together batteries that will run circles around FLA for just a few dollars more price is not the hurdle that it was. My new house battery in my RV is 24v 280A/H and the package with cells, busbars & cables, BMS and shipping is costing me less than $1500.