Detached Garage Solar Expansion Planning

[DenverGuy]

I am new at this, so forgive my lack of knowledge. My garage is about 35 feet from my back door. Last summer I wanted to make the garage powered by an off-grid system. It works great.

I have one 100W (monocrystalline) PV on the roof, a 30A PWM charge controller, and two 100Ah AGM batteries balance-connected. That gives me 1200 watt-hours when I figure in the drawdown to 50%. In the summer, I pirate extra power from the system to run my PC, a rice cooker, shop vac, etc. I check the controller to make sure that I don't use too much.

I have been looking into a solar generator for power outages, but instead I am thinking about beefing up the garage system and running an extension cord to power
the refrigerator, a light, my PC, and a microwave oven (for a few minutes).

It's all Windy Nation stuff and their support has been great. They suggested that I add 3 solar panels. But is that enough? Should I switch to an MPPT charge controller and add storage? How much should I have? Are my AGM batteries good enough?
They wrote this, which isn't very clear to me.
"We do recommend 50 watts of solar for every 100Ah of charge, so with
200Ah of charge already, you are good for 400 watts of solar."

Confusing wording. "50 watts of solar?" A 100W PV doesn't produce even 50 watts when you consider the loss. I think their "of charge" means storage. (The other math doesn't work, either, as I only have 1 panel). Any thoughts? Thanks.

 

[jbird526]

I have been looking into a solar generator for power outages, but instead I am thinking about beefing up the garage system and running an extension cord to power
the refrigerator, a light, my PC, and a microwave oven (for a few minutes).

Before buying any additional equipment I suggest an energy audit of all those items for say 24 hours? You will need to know amount of power needed during a period of time before you either waste money or don't build large enough. Purchase a Kill A Watt if you don't already have one to measure.

 

[Johncfii]

It's all Windy Nation stuff and their support has been great. They suggested that I add 3 solar panels. But is that enough? Should I switch to an MPPT charge controller and add storage? How much should I have? Are my AGM batteries good enough?
They wrote this, which isn't very clear to me.
"We do recommend 50 watts of solar for every 100Ah of charge, so with
200Ah of charge already, you are good for 400 watts of solar."

Confusing wording. "50 watts of solar?" A 100W PV doesn't produce even 50 watts when you consider the loss. I think their "of charge" means storage. (The other math doesn't work, either, as I only have 1 panel). Any thoughts? Thanks.

i can understand your confusion. Whoever, from Windy Nation, wrote that note is not the best advisor. To reference “100Ah of charge” is poor practice. The meaning of 100Ah is only useful in the context of the system voltage. Far better would be to reference watt-hours, or kilowatt-hours, of energy that your batteries will store. That way, solar panel capacity makes sense, no matter what the system voltage.

Perhaps the writer meant that “100Ah of charge”, with a 12-volt system assumed, but that is not clear.

Anyway, at best the writer seems to be quoting some rough “rule of thumb“ that was likely given to only slightly-educated sales people. To make the best decisions, the actual optimum total panel wattage, and battery storage capacity, should be evaluated much more carefully. There can be a lot of factors to. consider. These might include: Your latitude; clear sun days per year vs cloudy/partly-cloudy days; panel tilt; panel aspect reference due south; available space; the cost of not being able to charge your batteries for several days in a long overcast period; the size of your budget; possibly building in extra capacity for future expansion; and of course your system load.

It will be hard for anyone to give meaningful advice from a distance without a LOT more information. This is perhaps why the phone sales person reaches for a rough rule-of-thumb.

But, in concept, “beefing up” the garage system to provide essential power to the house, in event of a power outage, is a reasonable idea. The more value, and potential value, that you can extract from your solar system, the easier it is to justify the equipment cost.

When the power is out in the entire neighborhood, being able to run some little LED lamps, to boil hot water for coffee or Mac-N-Cheese, to play soothing/fun music, and listen to news, will go a LONG way to making the days without power less traumatic. Not to mention saving the food in the refer or freezer. At 35 feet, or even 75-feet, a 10-gauge extension cord is a cost-effective power distribution system.

Go as big as you can afford, relative to your expected load. I doubt you would regret it.

 

[DenverGuy]

Thanks. I should have mentioned that I am in Denver, CO. Lots of sun here. This system would be for when I lose power for an hour or two. I think I'll add three panels and one more AGM 12V battery (total of 3). That should be easy enough. That will give me 1800 watt hours of storage. If I use a little of it at a time when I lose power I'll be able to get by for a while.
I appreciate it.

 

[svetz]

Congratulations on your first foray! Hopefully, you'll post a photo or two of your setup, always love seeing what members are doing!

"We do recommend 50 watts of solar for every 100Ah of charge, so with 200Ah of charge already, you are good for 400 watts of solar."
Confusing wording.

Denver has an average summertime insolation of 6.7, so a 100W panel might generate 100x6.7=670Wh on your average day ignoring all losses.
To restore the 1200 watt-hours in your battery you've probably seen it takes about two average summer days to charge with your single 100W panel. If you're only drawing down 500 watt-hours or so a day, it's plenty to keep up in summer.

You're correct they mean the battery by "of charge". Let's use the 400W of solar panels they recommend: 6.7x400=2680Wh, so on an average day four 100W panels should be able to charge both batteries fully from 0% state of charge with a bit left over.

But, the SCC has losses and the AGM has losses while charging (which you might be able to find on their datasheets). Let's use an SCC efficiency of 94% and battery charging of 88%. So four 100 panels on the average summer day would generate: 4 x 100 x 6.7 x .94 x .88 = 2200 watt-hours. Way more than you need (1200 watt-hours) if you're running your two 100 Ah 12V batteries at 50% DoD.

But what about the average winter day? The Denver insolation is 4.56 in December. Now four 100W panels gives: 4 x 100 x 4.56 x .94 x .88 = 1500 watt-hours. So, still plenty for the average day and some leeway for the below-average day.

... Should I switch to an MPPT charge controller and add storage?

MPPTs are more efficient than PWM SCCs. To know if you should switch, run the math changing out the SCC efficiency.

...But is that enough?... Should I ... add storage? How much should I have?

Only you can answer that. Since you're "pirating" power from the house and managing the SoC it doesn't seem like you need to do anything beyond that. Adding more storage means pirating less power if you have the solar to keep it recharged. Hopefully, the math examples above give you an idea on how to adjust your system to get what you want.

Ultimately, how much is needed to recharge depends of course on how much you use and the AGMs self-discharge rate.
Even though four 100 watt panels provide more than enough even for "average" winter days there will be below-average days. Also, if those AGMs get cold, they're going to have a lot less energy.

The Energy Audit might be able to help you lock down your power usage. The FAQ also has numerous other math examples that can help you out with things like temperature correction for your panels or understanding the Peukert effect of your AGMs.

 

[Johncfii]

Denver has an average summertime insolation of 6.7, so a 100W panel might generate 100x6.7=670Wh on your average day ignoring all losses.

Svetz:
Lots of good information you’ve given. However, one small thing: The arithmetic isn’t quite as simple as 6.7 x 100w. The 6.7 figure for Denver is in units of kWh of solar irradiance, per square meter of surface, per day. The typical 100w panel is a bit less than 0.6 square meters. So, the 100w panel might receive 6.7 x 0.6 = 4.0 kWh of solar irradiance in the average summer day. But the best panels are less than 25% efficient at converting sun energy into electrical energy. So, the yield will be more like 800 to 1000 watt-hours per summer day, per 100 watt panel.

Unless I am not understanding the reference correctly, in which case, please correct me.

Solar Irradiance Calculator

www.solarelectricityhandbook.com

 

[Johncfii]

Comment Deleted

 

[svetz]

However, one small thing: The arithmetic isn’t quite as simple as 6.7 x 100w.

It is correct for "rough" calculations for an average Denver day in June for zero-tilt 100W panels (especially if you toss in the SCC and battery losses). You've confused irradiance (W/m²) with insolation (watt-hour/d/watt). You can use SAM if you want more accuracy though, definitely suggest watching the video tutorials. Hope that helps!

 

[MJB8706]

I am new at this, so forgive my lack of knowledge. My garage is about 35 feet from my back door. Last summer I wanted to make the garage powered by an off-grid system. It works great.

I have one 100W (monocrystalline) PV on the roof, a 30A PWM charge controller, and two 100Ah AGM batteries balance-connected. That gives me 1200 watt-hours when I figure in the drawdown to 50%. In the summer, I pirate extra power from the system to run my PC, a rice cooker, shop vac, etc. I check the controller to make sure that I don't use too much.

I have been looking into a solar generator for power outages, but instead I am thinking about beefing up the garage system and running an extension cord to power
the refrigerator, a light, my PC, and a microwave oven (for a few minutes).

It's all Windy Nation stuff and their support has been great. They suggested that I add 3 solar panels. But is that enough? Should I switch to an MPPT charge controller and add storage? How much should I have? Are my AGM batteries good enough?
They wrote this, which isn't very clear to me.
"We do recommend 50 watts of solar for every 100Ah of charge, so with
200Ah of charge already, you are good for 400 watts of solar."

Confusing wording. "50 watts of solar?" A 100W PV doesn't produce even 50 watts when you consider the loss. I think their "of charge" means storage. (The other math doesn't work, either, as I only have 1 panel). Any thoughts? Thanks.

Hi - you literally described my setup and what i'm trying to do. My 2-car detached garage is 40 feet off back of my house and I'm trying to create an off-grid system for it. I purchased a Bluetti AC300 and 2-B300s which means i have 6kwh storage now and I can expand that. The unit permits up to 2400 watts of solar input. So im contemplating adding 6-10 panels to my garage roof and thus I'm curious how you mounted yours and how did you bring the cables into the garage. also, did you ground the panel? I'm probably going to lean towards a 24v panel because my system will allow up to 150v on each of the 2 solar inputs (the unit has 2 mppt and each allows 1200 watts/150v max). any input is welcome - thanks

 

[DenverGuy]

Sorry - I just saw this. How cool. Good luck with it. I mounted the panels with standard Z connectors from Windy Nation. I connected the panels in parallel (for now), with the branch connectors tucked under the panels to keep them out of the weather. There are two holes in the roof where the wires were feed through. I used branch connectors inside and feed the charge controller as below. Let me know if you have any questions.