Inexpensive and Easy Hybrid Solar Grid Tie Backup With Goal Zero Yeti




Hi there. While a 'solar battery generator' and portable solar panels are a nice thing to have on hand for emergency backup or recreational use, letting them sit idle the rest of the time is like tossing dollars down the drain. So in this instructable, I'm going to detail a simple and easy way to put them to use that can save you just a little bit on your power bill every single day. Obviously, the "inexpensive" part of this instructable assumes you already own a Yeti and some panels. There are less expensive ways to backup your power if you don't!

While I am going to refer to the Goal Zero product exclusively, this could be adapted to use with any other solar generator that meets your design specifications.

Goal Zero recommends that you keep your Yeti plugged in anytime it's not in use, so utilizing it as a source of supplemental and backup power from your panels in case of an outage is perfect way to take up the slack and gain a little peace of mind in the process. Unlike installing a switchable critical loads panel which only provides a backup source of power, this is something you can easily do yourself without monkeying with your household wiring and to take better advantage of your generator and panels when backup power is not needed.

In short, the system I describe here is designed to make full use of solar power when it is available, then switch to grid power to keep the Yeti nearly fully charged and ready to go at all times when the solar power is not. Keeping important equipment powered up in case of an outage is a bonus!

What you will need:

  • Goal Zero Yeti (or other similar solar generator with simultaneous AC and solar charge capability)
  • Solar Panels of 100 Watts or less each (to comply with the 18-22V input limit of the Yeti)
  • MC4 to Anderson Power Pole Adapter (Not needed if you your panels are Goal Zero brand or you have other adapters to make them fit the Yeti)
  • Y-Adapters to parallel however many panels you have to connect (No more than 400W total per input with the Yeti, but up to 800W overall!)
  • A Reliable Timer or Smart Outlet

Step 1: Getting Started

The first thing you need to figure out is what items you want to back up. I have mine connected to a 12 cu. ft. freezer in my garage.

I believe that a garage or outdoor freezer or refrigerator is as close to perfect a designed load as one can find, as it is generally about the right amount of load all year round. During the winter when solar power is greatly reduced, the demand from the freezer is also typically reduced. Likewise, in full summer sun, the panels and freezer are working their hardest.

Next, you need to assess how much power the connected devices are realistically going to draw over the course of a day. I know from using a KIll-a-Watt meter on my freezer that it uses around 1.2 kilo-Watt-hours (kWh) per day, or an average of 50 Watt-hours every hour (1200 kWh / 24 = 50 Wh). You will need to figure out the same average Watt-hour load with your devices. If you don't have a meter to check it, most things have some reasonable average consumption data available online if you search for them. A best guess is probably fine if you keep an eye on things and you try to fine tune how it all operates. My freezer is really close to what I found online, but your mileage may vary. Overestimate by 20% or so if you are unsure and can't afford to lose power. In the case of the Yeti products, the display will show you how much power the devices are really using at any given moment.

After you have figured out what to connect and how much power it will need, then you will have to determine how long your device can be powered by your solar generator alone. This is your failsafe. This is what you get when the grid goes down and solar power is not available or is well below expected production. You also want to figure out what depth of discharge you are willing to accept for your solar generator. For daily use, I have chosen to limit mine to no more than 20% to assure plenty of backup energy in reserve if needed, as well as to prolong the life of the battery. (The Yeit 1000 battery is rated for 500 discharge cycles to 0%. Using only 20% depth of discharge theoretically raises the number of cycles to more like 2500, or close to 7 years, which is a reasonable service life for many lithium based batteries.)

So for my freezer, using 50 Watt-hours on average, one could expect a 1000 Watt hour Yeti (1000 divided by 50) to provide up to 20 hours of emergency backup power to the freezer. Because I want to limit the depth of discharge on my Yeti to no more than 20%, that means I can comfortably take my freezer off of grid power for at least 4 hours every day even without the solar panels assisting in providing power. (But keep in mind the AC wall charger for the Yeti only provides about 60 Watts when it is connected - or just enough to balance out the load from my freezer over the other 20 hours of a day.) Thus 4 hours is our equilibrium point for going off-grid. Any longer, and it is likely that the depth of discharge would increase beyond the ability for the AC charger to make up the difference, and any shorter just creates wasted opportunity.

Step 2: Logistics

After figuring out what you want to power and how much backup power you will have, attention to location of each device deserves some attention.

Ideally, the shorter your DC leads are from your solar panels to the charger, the less power loss you will end up with in the system - even if you have to make longer runs with your AC power. So if you can, try to put your solar generator in a protected area as close to your panels as possible, then run standard household extension cords as needed. Things should work fine if you have to do the opposite of this, but just know that you may be sacrificing some of your solar power to do it, although at anything 50 feet or less with 10 AWG wire, it probably isn't going to end up being much loss with the lower power panels used for this project. It may be splitting hairs to worry about this very much.

Step 3: Putting the Basics Together

After you have decided where everything is going to go and what your available power and needs are, this is the easy part. No fancy engineering at all, but just plugging it all together.

Plug your solar generator charger in to AC charge power at the chosen spot and connect your backed up devices into the 120V inverter source.

BE SURE TO PRESS THE BUTTON TO TURN THE INVERTER ON! You aren't powering anything plugged into it until you do that. ;)

You should be able to see some load on the output side of your generator if the device is running (or open the door to turn on your fridge or freezer light to create a small load if it is not). If you don't see any output power even when you know your device should be running or your device is not running, stop here because something is wrong! In the very likely event however that there are no problems at this stage, take a look at your input side of the generator display. Unless your battery is at or near 100%, you should also be seeing power going into your generator from the grid.

You should not expect your input and output power to be the same. One or the other may drop to zero, but the rate of charge in does not equate to the rate of draw. Either one can be higher than the other.

Congratulations, you now have a reliable backup battery connected! Too easy, right? But clearly, we are not stopping there!

Now because devices like the Goal Zero Yeti provide for multiple charging sources, this is where you get to take advantage of the solar panels. Connect as many as 400 Watts worth of 100W panels in parallel using Y-adapters for a single Anderson power pole input or use the 8mm Goal Zero connections if you are using their panels. If you have the optional MPPT charge converter, you can plug in another 400 Watts of panels to the additional Anderson power pole connection.

According to Goal Zero, you can use as many input ports for charging as are available for a maximum combined input of up to 800 Watts, with or without the MPPT adapter, although real usable input is limited to 650 Watts. So doubling up your AC charger with the solar panels is not a problem. It is an intended feature. If you have 800 Watts worth of 100 Watt panels at your disposal, use them!

At this point, we have now connected a battery backup to a device as well as integrated solar panels to provide at least some of the power the device normally consumes. But there is a caveat here: The Yetis will default to whatever charge source is available and take the combined input of those sources to meet demand. There is no built in way to decide which charge ports are active. It all just adds together. So if you are able to generate more power from your solar panels than needed to make up for your device load, it makes sense to automate disconnecting the AC charge circuit, doesn't it?

The panels will provide the maximum power possible to whatever charger you have, whether it is the basic charge input or the optional MPPT, but only if your battery actually needs a charge. If there is nothing to charge, all they are doing is providing backup if grid power fails and you are wasting energy from those panels!

You could probably stop here and be a happy camper knowing that your backed up device has both battery and solar panels at the ready in the event that they are needed. But why do that with money still on the table?

Step 4: Doubling Down on Those Panels

Remember the math I had you doing at the start? It's important because now you need to take another look at the Watt-hours your devices are expected to use.

Where I live, solar power really could be very nearly zero on some days. So you will have to decide how deeply you are willing to risk discharging your solar generator if that is potentially the case.

I am choosing to err on the side of caution and long life with a 20% depth of discharge maximum. Most days I hope to see 10% or less if I have done my own math correctly.

At most temperate latitudes, you can expect an average of 5 hours worth of solar power production per day over the course of a year at your solar panel combined production ratings (rain or shine, winter or summer).

So a 100 Watt panel can typically be expected to produce 500 Watt-hours of power on an average day.

Obviously it will be higher when the weather and season is right, and lower or nothing at all when both are not.

Knowing that, if you have a device that draws only 500 Watt-hours of energy in a day, you could theoretically power that device exclusively with a single 100 Watt panel on the average day. Overnight, the battery provides the extra power that your panel cannot, and then is recharged by the panel when the sun shines again, but that is a very general and optimistic way to look at it.

In reality, your panel may not get adequate sunshine to recharge your battery for days or even weeks depending upon your weather, shading, and all of the other wonderful factors that play into how much power solar panels produce. The equilibrium point discussed earlier comes into play here. For my freezer, I know that I can disconnect power 4 hours every single day, regardless of the weather and be safe in not significantly discharging my Yeti over the course of several days or weeks.

Many off-grid folks like to plan for as many as 5 days of reduced or no solar production. But for something plugged into the grid such as our solar generator, I don't think we need to be so pessimistic. We just want to use as much of our solar power production capacity as possible while providing emergency backup. With that in mind, a single solar day is probably a more reasonable benchmark.

So for my example of wanting to limit my battery draw to only 20%, I can easily time it to run about 4 hours on battery alone before I reach my 20% discharge limit, making this a virtually zero risk scheme for utilizing our panels and battery system. One could easily use a mechanical timer to run the switching during those hours and be done with it, but because this is figured for the worst case we are still leaving money on the table!

Step 5: Making the System Smart

Now that we have a nearly fool-proof backup and timing system in place, we can step beyond the guaranteed equilibrium a little bit. This is where smart outlets are fantastic little devices, as they bring with them an easy way to time our AC charging to the length of every day with sunset and sunrise timers, allowing us to maximize the amount of time our solar panels can run the system.

Because there are so many different devices and systems available to do this, I won't go into detailed setup, but describe what I have figured will work for me.

I have decided for my own system than I can probably tolerate running an additional hour off-grid with minimal potential risk of discharging my battery more than my 20% threshold on the worst winter day. So for December 21, with a sunrise of 7:44 AM and sunset of 4:37 PM at my latitude, we get about 8 hours 53 minutes of the sun being above the horizon. Subtracting my equilibrium time period of 4 hours, as well as the additional 1 hour I estimate I should be able to be off grid, we are left with 3h 53m of daylight hours. Split that in half and we end up with being able to take our solar battery off the grid about 2 hours after sunrise, and to put it back onto the grid about 2 hours before sunset. Or in the case of a typical smart switch timing, we can use a rule of 'Sunrise + 2 hours' off, and 'Sunset - 2 hours' back on.

Using this scheme, as the days grow longer to June 21, the off-grid window will automatically be broadened to as much as 11 hours 30 minutes (15:30 from sunrise to sunset - 2 hours post-sunrise and - 2 hours pre-sunset).

For my freezer, that means it is potentially running almost half the day during the peak solar day of summer on solar power alone. With an average length of a day being about 12 hours locally and an average off-grid time of 8 hours using the window I figured above, I can provide 400 Wh of average daily solar power for my freezer's daily 1.2kWh load. This translates to saving an average of 146 kWh over the course of a year. In real money using my local utility rate of 10 cents per kWh consumed, that's $14.60 saved per year. So a smart outlet at an average of about $30 cost pays for itself in about 2 years.

Step 6: Some Important Tips

These may not be obvious to you at first if you are new to smart home automation:

1) DO NOT TRUST THE CLOUD! Really, it's all caps because it is important if your solar generator is a critical source of power for a device. Many wifi based smart plugs rely on cloud connected applications that often have less than stellar reliability for switching them day after day. If you need an app to run it, you probably don't want the fate of your freezer pizzas depending upon it. It WILL fail eventually, so checking operation daily is an absolutely requirement for something like a freezer or refrigerator with perishable foods. (Perhaps warm beer and soda may be a world ender for you as well, so plan accordingly!)

2) Wifi devices in general are less reliable - even if you have them setup to run on a local hub or server. If your router fails, so does your switching, and you'll never know it unless you check on things regularly or have a way to monitor them.

3) More panels means more potential off-grid time. While a single 100 Watt panel would be enough to power my freezer for 5 hours per day at a minimum, additional panels can increase that window of time even if they are not producing at full capacity. This however has significantly diminished returns if your device load is low like my freezer, as you may be producing more energy than either your device or the solar generator can use. So buying more panels just to run a freezer off-grid for longer isn't necessarily going to drastically improve your run time and efficiency, but if you have the panels anyway, plug them in. They will add to your overall charging - even if the AC charger is also powered on.

4) When using a smart outlet to control your solar generator, it is highly advisable to not allow voice assistants such as Alexa or Google Home to control them if it provides backup power to an important device. It may seem convenient at first - until the first time that somebody says "Alexa, turn everything off." If 'everything' includes your important backed up device that you have forgotten about being connected, you may have a problem that you might only discover by the stench of rotting meat!

Step 7: Wrapping It All Up

Using a solar generator as a source of backup power like this is a limited substitute for a real off-grid or hybrid solar power system. It is however a way to get more out of what you already have available or plan to buy, and provide a way to save a few bucks in the process by taking a device load off the grid every day - even if only for a few hours. It all adds up, and it's always better when it adds up in your favor instead of the power company's.

If you have read this far and learned nothing, you should be writing instructables yourself, but hopefully you have. Either way, thanks for playing along, and please feel free to point out whatever flaws or errors you see. We all learn more when we all participate in the process!



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    Tip 3 months ago

    It's also worth noting that additional AC chargers can be connected to available charge ports up to a maximum of 4, so one could potentially power more devices this way without significantly drawing down the battery when solar power is not available.