How do I power some of my breaker switches with solar-charged battery bank?

[libertyresources]

My name is Ryan and I live in the Tucson, Arizona area. This is my first post. I have bachelor's and master's degrees in electrical engineering so I am comfortable with circuits and math involving electricity but I recognize I need additional help to design/install a solar system to accomplish what I would like. I have done research on and off for a few years, but I keep getting stuck with questions and I want to have a clear answer to all my questions before I start buying stuff and moving forward. I also have two sons who are interested in helping with the project and we are all trying to learn together. So thanks in advance if you can answer any of our questions or point us in the right direction.

MOTIVATION

On the south side of my house, I have a patio that has a sturdy flat roof over part of it with nothing on top of it. I would like to install solar panels on the patio roof and use them to charge a battery bank. I would like to be able to power some of my breaker switches to run off the battery bank instead of the grid power. I would still like to have the ability to manually switch everything back to grid power if the battery bank is not available or not sufficiently charged for some reason.

Currently, the patio roof measures about 13 feet east-to-west and 10 feet north-to-south. In the future, we would like to expand the patio roof to cover more of the patio so it would measure 24 feet by 10 feet.

My main initial goal is to be able to power our chest freezer and refrigerator continuously from the solar-charged battery bank even when there is no grid power. I would like to design the system so it could be scaled to higher capacity to also power other circuits if we added more panels in the future.

INITIAL CALCULATIONS

I found Sunking's post, https://www.solarpaneltalk.com/forum...battery-design, to be helpful as a starting point. In my case, I have measured my chest freezer energy usage to be about 2.1 kWh per day. Its peak power usage is about 100 W and average power usage is 87 W. My refrigerator energy usage is about 3.2 kWh per day. Its peak power usage is about 600 W and average power usage is 130 W. If I use a 1.5 fudge factor to account for system losses, I arrive at 1.5*(2.1 + 3.2) = 8.0 kWh/day as my adjusted daily energy requirement.

Using the worst case month of December for Tucson, I expect to have a solar insolation of about 5.6 kWh/m2/day.

I calculate the needed size of the solar panel array to be 8.0/5.6 = 1.4 kW = 1400 W.

Based on panel array being between 601 to 2000 watts, I assume my battery voltage should be 24 V or higher.

For total battery capacity, the recommendation is to use 5 days. My initial thought is that 5 days is overkill, but I'll use that number for now and revisit later depending on cost/feasibility of the design. So for 5 days, I would have 5*(2.1 + 3.2) = 26.5 kWh. If I use 48 V battery system, I would need a capacity of 26.5/48 = 552 Amp hours which I would round up to 600 Amp hours.

INITIAL QUESTIONS

1. Do you see any flaws in my initial calculations?

2. To calculate how many panels would be needed, do I just make sure they sum to greater than 1400 W? Would four QCell 400 W panels (total of 1600W) be sufficient for my design? Or is there a fudge factor that I should include here too?

3. What would be a possible charge controller that would work for design? If I use 48 V batteries, then I think a 30 A controller would be sufficient. And if I understand correctly, an MPPT controller is usually worth the higher cost as it will result in better efficiency.

4. Any recommendations for types of batteries?

5. What cabling is needed between the solar panels and the charge controller?

6. What cabling is needed between the charge controller and the batteries?

7. Any recommended approaches for connecting the battery bank to the actual breaker switches? What are the options here and pros/cons of each option?

Thanks again for any tips or help!

[SunEagle]

Hello libertyresources and welcome to Soar Panel Talk

I guess a couple of questions I have is what is the cost of a kWh from the grid and why would you want to purchase all that equipment to power a couple of loads?

In most cases the grid is usually cheaper then a solar/battery system (especially the custom one that you have described) so any ROI is very long if not forever and has no payback. So please think about what you really want and determine that before you spend your money.

I would research different types of batteries to determine what is the most cost effective. While FLA type are cheaper they also don't last as long as any of the Li chemistry type. But again you have to do the math to determine what is the most cost effective for you.

Since you have EE knowledge just look at the amount of amps that are going through the cables to determine their size and overcurrent protection. Following the NEC is a good practice.

I am sure others will make comments but try to remember we have people here that pay a very high electric costs and some (like me) that pay very low. Others are just trying to be Green and don't care about the cost or any ROI but some do not have the money to burn so an ROI is important.

Good luck and let us know what your plan is.

[J.P.M.]

A couple of comments/respectful suggestions:

1.) An off-grid system for a small load will not be cost effective and take more maint. and attention and babysitting than a grid tie system - just so you know.
If this is for a hobby, enjoy but know that often the bloom on the solar lily fades rather quickly, especially for those with off grid systems.
As for particulars of wiring, I'd hire a reliable outfit that installs PV. I'm a retired M.E., P.E. and that's what I did after I did the system design.
2.) Assuming your flat roof is mostly horizontal, plan on tilting any array off the horizontal by at least 10 or more degrees for at least 2 reasons: Better annual and especially winter performance but mostly to keep the array from turning into a shallow sandbox which will need very frequent cleaning. Even at that relatively low tilt, expect frequent trips to the roof with a blower or other means of cleaning.
3.) Continue more relevant education. Download a free PDF of "Solar Power Your Home for Dummies". It's a bit dated but still relevant for most of the basics of residential PV.
4.) If you don't already have one (or more) get something called a "Kill-a-Watt" meter. The freezer load looks familiar but the fridge load looks a bit high. The meter is cheap and easy to use. Since fridges/freezers are pretty constant loads (say over 24 hours), I'd skip the 1.5 fudge factor.
5.) After you read the dummies book, download something from NREL called PVWatts. it's a user friendly model for residential PV sizing. Before you do any runs, read all the help screens a couple of times and download the user manual. Consider reading that as another part of your education.
6.) As a PVWatts modeled output of long term average annual system production, a 1 STC kW 20 degree tilt, south facing grid tied array will likely produce about 1,800 or so kWh/yr. If winter output is more important look at the output for a higher tilt (say 45 degrees or so) but know it will take more design and calcs for racking and external (mostly wind) loads.
Confirm/play with the model. Scale the system as your load estimates change.
7.) Check with your utility for how or even if net metering is available and if so, any required hoops to jump through. Many POCOs are in the process of making residential PV less favorable to their customers.

Welcome to the neighborhood and the forum of few(er) illusions.

[SolTex]

Hello Ryan,

I live in the Midland, TX area so probably similar "solar" conditions as far as weather, sunshine, ambient temps, etc. We have a ground mounted array that is pretty close to optimum in its orientation. Our panels face true south, 25 degree tilt, and zero shading. I will try to address just one of your questions: how many panels do you need. Since my system uses micro inverters, I know exactly how much AC power is produced by my panels on any given day. You are talking about 400W (DC) panels. I have 380W (DC) panels. The STC rating of a panel doesn't tell you much about how much actual power it will produce. You need to consider how the panels will be oriented, whether or not they will ever be shaded, the capability of your controller, etc. You also need to consider the capability of the inverter you choose (You will need a inverter between the battery bank and the fridge/freezer). It's how much AC power comes out "the end of the pipe" that really matters. You must start there, and size everything else to meet your end objective.

So anyway, my system has 40 panels rated at 380W STC. My inverters can produce a peak output of 295W (AC) at which point they start clipping (which they do every sunny day). On really perfect sunny days we will get 100kWh of AC power from our system, which equates to 2.5kWh per panel. On cloudy, rainy days (few and far between) we might only get 50 kWh (1.25 kWh per panel). At night, we get zero (duh). Assuming you get similar production from your panels, to meet your goal of 8 kWh/day you would need a minimum of 4 panels (if everything was perfect) and perhaps 6 panels to cover the less-than-perfect days and/or conditions.

If you want a more scientific estimate of what size system (how many panels) you may need, try using the PVWatts calculator. Enter your physical location to start the calculator. Then you can play with the number of panels and how they are oriented (azimuth and tilt) to see what kind of AC production you get. Open the "Advanced Parameters" to enter more details. Read the "Note" (the little "I") for each entry to get a quick solar education. For instance, I entered my address, then started with a 1.6kW system using (4) 400W (DC) panels. The calculator says I would get an average of about 233 kWh of AC power per month, or 7.75 kWh per day. Pretty close to your goal! Give it a try - it's fun to play around with the inputs.

https://pvwatts.nrel.gov

[libertyresources]

Hello libertyresources and welcome to Soar Panel Talk

I guess a couple of questions I have is what is the cost of a kWh from the grid and why would you want to purchase all that equipment to power a couple of loads?

In most cases the grid is usually cheaper then a solar/battery system (especially the custom one that you have described) so any ROI is very long if not forever and has no payback. So please think about what you really want and determine that before you spend your money.

I would research different types of batteries to determine what is the most cost effective. While FLA type are cheaper they also don't last as long as any of the Li chemistry type. But again you have to do the math to determine what is the most cost effective for you.

Since you have EE knowledge just look at the amount of amps that are going through the cables to determine their size and overcurrent protection. Following the NEC is a good practice.

I am sure others will make comments but try to remember we have people here that pay a very high electric costs and some (like me) that pay very low. Others are just trying to be Green and don't care about the cost or any ROI but some do not have the money to burn so an ROI is important.

Good luck and let us know what your plan is.

Thank you for your feedback! By cabling, I was mostly referring to the wiring diagram showing which signals go between components, but your points about following NEC are good. ROI is not as important to us as having some independent solar-powered backup energy in case the grid goes down. I'll give an overall update in a separate post.

[libertyresources]

A couple of comments/respectful suggestions:

1.) An off-grid system for a small load will not be cost effective and take more maint. and attention and babysitting than a grid tie system - just so you know.
If this is for a hobby, enjoy but know that often the bloom on the solar lily fades rather quickly, especially for those with off grid systems.
As for particulars of wiring, I'd hire a reliable outfit that installs PV. I'm a retired M.E., P.E. and that's what I did after I did the system design.
2.) Assuming your flat roof is mostly horizontal, plan on tilting any array off the horizontal by at least 10 or more degrees for at least 2 reasons: Better annual and especially winter performance but mostly to keep the array from turning into a shallow sandbox which will need very frequent cleaning. Even at that relatively low tilt, expect frequent trips to the roof with a blower or other means of cleaning.
3.) Continue more relevant education. Download a free PDF of "Solar Power Your Home for Dummies". It's a bit dated but still relevant for most of the basics of residential PV.
4.) If you don't already have one (or more) get something called a "Kill-a-Watt" meter. The freezer load looks familiar but the fridge load looks a bit high. The meter is cheap and easy to use. Since fridges/freezers are pretty constant loads (say over 24 hours), I'd skip the 1.5 fudge factor.
5.) After you read the dummies book, download something from NREL called PVWatts. it's a user friendly model for residential PV sizing. Before you do any runs, read all the help screens a couple of times and download the user manual. Consider reading that as another part of your education.
6.) As a PVWatts modeled output of long term average annual system production, a 1 STC kW 20 degree tilt, south facing grid tied array will likely produce about 1,800 or so kWh/yr. If winter output is more important look at the output for a higher tilt (say 45 degrees or so) but know it will take more design and calcs for racking and external (mostly wind) loads.
Confirm/play with the model. Scale the system as your load estimates change.
7.) Check with your utility for how or even if net metering is available and if so, any required hoops to jump through. Many POCOs are in the process of making residential PV less favorable to their customers.

Welcome to the neighborhood and the forum of few(er) illusions.

Thanks for the feedback and recommendations. I downloaded the Solar Power Your Home for Dummies PDF and read it all. It was very helpful. I do have a handheld power/energy meter that has been helpful to assess average and peak power for the fridge and freezer (and other things). I am leaning toward hiring an installer and finding a kit rather than trying to put together a design component by component.

[libertyresources]

Hello Ryan,

I live in the Midland, TX area so probably similar "solar" conditions as far as weather, sunshine, ambient temps, etc. We have a ground mounted array that is pretty close to optimum in its orientation. Our panels face true south, 25 degree tilt, and zero shading. I will try to address just one of your questions: how many panels do you need. Since my system uses micro inverters, I know exactly how much AC power is produced by my panels on any given day. You are talking about 400W (DC) panels. I have 380W (DC) panels. The STC rating of a panel doesn't tell you much about how much actual power it will produce. You need to consider how the panels will be oriented, whether or not they will ever be shaded, the capability of your controller, etc. You also need to consider the capability of the inverter you choose (You will need a inverter between the battery bank and the fridge/freezer). It's how much AC power comes out "the end of the pipe" that really matters. You must start there, and size everything else to meet your end objective.

So anyway, my system has 40 panels rated at 380W STC. My inverters can produce a peak output of 295W (AC) at which point they start clipping (which they do every sunny day). On really perfect sunny days we will get 100kWh of AC power from our system, which equates to 2.5kWh per panel. On cloudy, rainy days (few and far between) we might only get 50 kWh (1.25 kWh per panel). At night, we get zero (duh). Assuming you get similar production from your panels, to meet your goal of 8 kWh/day you would need a minimum of 4 panels (if everything was perfect) and perhaps 6 panels to cover the less-than-perfect days and/or conditions.

If you want a more scientific estimate of what size system (how many panels) you may need, try using the PVWatts calculator. Enter your physical location to start the calculator. Then you can play with the number of panels and how they are oriented (azimuth and tilt) to see what kind of AC production you get. Open the "Advanced Parameters" to enter more details. Read the "Note" (the little "I") for each entry to get a quick solar education. For instance, I entered my address, then started with a 1.6kW system using (4) 400W (DC) panels. The calculator says I would get an average of about 233 kWh of AC power per month, or 7.75 kWh per day. Pretty close to your goal! Give it a try - it's fun to play around with the inputs.

https://pvwatts.nrel.gov

Thanks for sharing the details of your similar real-world example. The PVWatts calculator is a really useful tool!

[libertyresources]

So we did hire a contractor to extend the patio, so we'll wait until that project is done to move onto solar. After reviewing the suggested resources and other online resources, I am leaning toward purchasing a kit as I expect to have less risk of incompatibilities between components that way as well as support from the seller. The one I have found that is closest to my needs is https://sungoldpower.com/products/of...nels-sgr-8k10e. I would probably hire an installer to help me with the installation/permits/etc.

I would be interested in any recommendations for adjustable tilt angle mounting for a flat roof. I was expecting carbon fiber would be a good fit for this but didn't find any and I'm guessing it would just be too expensive even if it would weigh a lot less.

Also, if you have any feedback on the kit I put the link to, that would be helpful.

[bcroe]

Understand just covering your flat roof with flat panels will
collect as much energy as tilted up panels, but requires more
panels. The flat arrangement avoids the shadowing problem
that occurs at times with high angle panels, and may be easier
to build. But do use at least a minimum tilt as J. P. M. suggests.
Bruce Roe

[J.P.M.]

I would be interested in any recommendations for adjustable tilt angle mounting for a flat roof. I was expecting carbon fiber would be a good fit for this but didn't find any and I'm guessing it would just be too expensive even if it would weigh a lot less.

Also, if you have any feedback on the kit I put the link to, that would be helpful.

The weight of what's commonly aluminum racking of most any suitable type is almost inconsequential for design purposes. Carbon fiber is nice but gross overkill for PV racking applications.

As for kits, do your homework and you'll be better off if for no other reason than you'll have more design flexibility. As you probably found from the Dummies read, residential PV design is not rocket science.