My first Off grid solar system

[LawrenceP]

Hey everyone, I'm a newbie when it comes to designing solar systems. However, I am very much interested in learning what is required to have a proper functioning setup. For this I would definitely need some assistance. Basically I'm putting together a system to power a small fridge. I already have 2x 100W panels from HSTQ, Renogy Rover 20A controller, connecting cables from panels to controller. I know I need the power rating of the fridge. I want to assume power draw is under 280KW/year. I need help now in choosing the battery. I can get 6v 232Ah US AGM deep cycle. I'm thinking I need 2 of these in series to produce a 12v system. Can anyone tell me if i'm on the right track and what other information I need to consider. Thanks.

[chrisski]

Really need the draw of the fridge.

This needs to be pretty precise with an energy audit. For my energy needs, I found leaving my inverter on all day at idle draws 288 wh. That's 105 kwh a year, or a bit more than a third of that 280 kwh you want.

When choosing batteries, you need to plan for days without clouds, and short winter days. Here is a very good explanation: Solar Off-Grid Battery Design - Solar Panels - Solar Panels Forum (solarpaneltalk.com). A quick summary is if you live in Tucson, you're better off than when you live in Seattle where the days are a lot shorter, but in each case, a grid powered battery back up system is much more expensive than grid power (7 times the grid price for Tucson or 22 times the grid price for Seattle).

So, you need to be very careful about how you design it or you'll easily have an underpowered system and kill your batteries or Overdesign it, which is hard to overdesign taking into account how expensive stuff is.

[MichaelK!]

Let's do a bit of math to get you started. If this frig uses 280kWh per year, then the daily consumption is likely to be 280kWh/365days= 0.767kW/D, or ~ 770Wh per day. Throw in a smaller sized inverter like Chriski mentions, and your daily consumption is likely to be at least 770Wh + 290Wh = 1060Wh per day. For planning purposes, assume you'll get 5sunhours per day in summer, and 3 sunhours in winter. Understand that the # of sun hours is NOT the same as how many hours of daylight you have. It's an approximation of what you can expect your panels to produce.

So, in the winter, you'd expect your panels to be producing 200W x 3sunhours = 600W total per day. So, you can see that your design as is will be inadequate for winter. Most likely you'll need to at least double your solar input to get you through a winter without destroying your batteries. Look at getting at least two grid-tie panels putting out 250-300W. Right now I've been getting 500W of grid-tie panels for the price of one single 12V renogy panel. You are really wasting your money sticking with 12V components.

Now, the charge controller. Let's assume you want to charge your batteries at least 1/10C or 0.1 X 232Ah = 23.2A. You really should not have bought a controller with a 20A limit. Try to see if you can exchange it for a 40A controller. Your batteries will be happier in the long run.

Now, the batteries. Assume you have a total of 232A X 12V = 2784Wh of electricity. Assuming you will use 1060W per day, then expect the batteries to be depleted 1060Wh/2784Wh = 38% of capacity. Assuming you don't want to deplete your batteries more than 50%, you have just day of autonomy. So, one cloudy day and your batteries get depleted more than 50%.

Basicly, you bought the wrong/inadequate equipment before you had any understanding of what you needed to accomplish your goal. So, here's what I think you need to do.

1) Trade in your 100W panels and get your money back and switch to grid-ties. If you can't, go out and buy at least two or better four more 100W panels. Alternatively, you can wire the two 100W panels in series, and pair that with at least one 250W grid-tie panel wired in parallel.

2) Return your charge controller and get a 40A unit. The 20A unit you have now is rather inadequate.

3) Get four of your batteries instead of two and wire them for a 24V system instead of 12V. With 24V X 232Ah X 50% usage =2784Wh of available power, or 2-3 days of cloudy weather autonomy.

Do it that way, and you'll have a functional system less likely to let you down.

[Mike90250]

... I want to assume power draw is under 280KW/year.......

I want to assume my new car gets 45 mpg. ( it does, going downhill )

The best energy star fridges are about 1kwh daily. Getting lower than that, and you are looking at manual defrost, or very expensive machines.

If you intend to run it thru the winter, you will often need about 1kw worth of panels, to have enough harvest to go for a couple cloudy days. I suggest four 6V 200ah golf cart batteries wired for 24V, they are cheap, and will run a modest 1kw pure sine wave inverter needed to be able to start the motor in the fridge.

[littleharbor]

.

1) Trade in your 100W panels and get your money back and switch to grid-ties. If you can't, go out and buy at least two or better four more 100W panels. Alternatively, you can wire the two 100W panels in series, and pair that with at least one 250W grid-tie panel wired in parallel.

Most 250 watt panels are 60 cell count. Two 100 watt, 12 volt panels would total 72 cells, in most cases. If you are going to series/parallel wire panels you need to watch this as the voltage of a 60 cell panel wont match two 36 cell panels, in series. Most new 72 cell panels are in the 300 + watt range these days and are large (39 x 78)but you should be able to find some used 175 -195 watt, smaller panels (32 x 64) if size is an issue.

[LawrenceP]

Thanks for everyone of your responses guys. Seems like I can use a smaller fridge with power rating of 120W. I understand the fridge cycles on and off so its not running 24hrs. I did try a calculation before using a daily consumption of 1800Wh per day. I live in the tropics so its sunny most of the time but I still use 1 day autonomy for rainy days. Since I have the panels and controller already, I can try to finish the setup with enough storage to sufficiently run this load. So its just the battery bank and a suitable inverter I need help with to complete this project.

[MichaelK!]

Thanks for everyone of your responses guys. Seems like I can use a smaller fridge with power rating of 120W. I understand the fridge cycles on and off so its not running 24hrs. I did try a calculation before using a daily consumption of 1800Wh per day. I live in the tropics so its sunny most of the time but I still use 1 day autonomy for rainy days. Since I have the panels and controller already, I can try to finish the setup with enough storage to sufficiently run this load. So its just the battery bank and a suitable inverter I need help with to complete this project.

Let's update the math a bit with your new numbers. Since you are tropical and won't have winter lows or summer highs like we do, let's take an average and say you get 4 sunhours per day. So, 200W of panels X 4 sunhours = 800Wh, far, far short of the 1800Wh you are predicting per day. Don't kid yourself that it's going to work well. You will end up with ruined batteries and a frig full of spoiled food.

Though your frig of interest might consume only 120W, it's the duty cycle that's important. Will the frig be running 1/2 of the time, 1/3 of the time, or 1/4 of the time? Even at a very conservative 1/4 cycle, 1/4 of 24 hours is 6 hours, so the watts consumed is 120W X 6h =720Wh, basicly every single watt your little system MIGHT produce. And, that will be in the dark with no entertainment, like TV or stereo. In the tropics, where I assume it's going to be hot, I can't imagine a frig on only 1/4 of the time.

So, get more panels, four batteries to create a 24V system, and a 1000-1500W 24V inverter. If you really insist on keeping the 20A controller, go buy a second one and hook up additional panels to the second one. With two controllers charging the same battery bank, you'll have some flexibility of having different panels with mis-matching voltages.

Do that, and you can start with a workable system that will power the frig, a few lights, and maybe a small window fan.

[cansno]

Most people usually start off too small(as I did) and then start changing up down the road. Better and cheaper in the long run to go all in at the start, Within budget of course. Especially with voltage determination, charge controller and inverter. Go to at least 24v system. You can always add panels and batteries later. You state you just want to run a fridge. I chuckle in respect, because that's exactly what most people(including myself) say in the beginning. Renewable energy is addictive. At first you just want to run a few things, and then when you see the convenience, you just want more. That's why you should start off with more than you think you need.

[GeorgeF]

Cansno, how would you add batteries to an already used battery-bank?

[Gforce686]

Hi I am a Nubia to this sight . Not sure if I am in the correct location. I live in Newfoundland Canada. I have 45 years in the Electrical and Electronic field . I just got interested in solar power . I am going to build my first solar system . I would love to get as much advice as I can get. The very first question I will ask is between the PV and the controller. I have 6 100w panels and I wan’t to use 2/ 40A MPPT EPEVER Tracer An Series charger controllers (1) should I run all 12 AWG wires down to the controllers this would be 6/12 AWG pair of wires to the controllers and use a combiner box near the controllers . Or Should I use the combiner box near the PV and run 2 pair of 2 AWG wire to the controllers

[LawrenceP]

Well thanks for putting everything into perspective for me. You see I was hoping to get into the business of supplying and installing PV systems, but as you all point out, I will have to do a lot more research and proper calculations when designing a system to meet specific goals. What I will have to do now is just setup this system with a 12V battery bank and inverter, test it's capabilities, sell it, gain some experience, then start from scratch with proper calculations and put together a bigger system. I'll make sure and check back with my estimates BEFORE I purchase. Best regards to everyone

[cansno]

Cansno, how would you add batteries to an already used battery-bank?

Just add more banks. No problem as long as your original batteries are in good shape. Four banks is max recommended. That's why using 6v batteries with 24v system, you can have more batteries than a 12v system without maximizing banks. After all, if you get a weak battery or two that won't take a charge, they would have to be replaced with new.

[cansno]

Hi I am a Nubia to this sight . Not sure if I am in the correct location. I live in Newfoundland Canada. I have 45 years in the Electrical and Electronic field . I just got interested in solar power . I am going to build my first solar system . I would love to get as much advice as I can get. The very first question I will ask is between the PV and the controller. I have 6 100w panels and I wan’t to use 2/ 40A MPPT EPEVER Tracer An Series charger controllers (1) should I run all 12 AWG wires down to the controllers this would be 6/12 AWG pair of wires to the controllers and use a combiner box near the controllers . Or Should I use the combiner box near the PV and run 2 pair of 2 AWG wire to the controllers

Hey Gforce. I'm on da rock as well. What's the distance between panels and c controllers? I don't think you need 2 40a controllers for 600w's. I have two of the same controllers with 4 300w panels going to each.

[Gforce686]

Hi Cansno

[cansno]

Hi Cansno

What is your system voltage?