Check out Part 1 in this series for a Solar Power Introduction & our Tesla Solar Roof Tile experience.
I had actually been planning a post talking more about the Tesla Powerwall, but with current events I wanted to make it broader and put in some information about backup power. As we mentioned in Part 1, we opted not to get a Tesla Powerwall for our home mostly due to economics with the current offerings, but I wanted to talk more about one of the benefits which is backup power. This isn’t meant to be a technical report, this post could have easily been a lot longer. This is more of an introduction to different options.
Our Tesla Solar Roof Tiles will not provide backup power in the event of a power outage, if we had purchased Powerwalls we would be able to use the home batteries as backup power. With net metering your solar system will shut down if the utility power goes out, this is because they don’t want your solar system sending electricity to the grid if there is a potential issue with the grid creating a dangerous situation. If you want to use your Tesla solar system during a power outage, you need to have a home battery. This concept is referred to as “islanding” meaning your home is an island from the grid. Your solar will charge your home battery with any excess power during the day and you can use it at night or you can set it to store electricity for backup purposes. Tesla doesn’t offer true islanding or off grid installation, meaning you are completely independent from the power grid. Some other solar installers may offer true off grid installs, but Tesla doesn’t offer this at the time of writing.
A single Tesla Powerwall (Version 2) can store 12.2 kWh (see part 1 for more background on kWh) and output a maximum of 21 amps for output of 5 kW on a continuous basis and 7 kW of peak power. I’ll dive deeper into Powerwall here, but for now we are talking about it as a backup power option. Peak power is for certain appliances that may use more power to start up and then drop down to a lower usage level once they are running or just general flutuations in your power usage, but know that it cannot sustain that output long term.
An Automatic Standby Generator is another option, these are usually connected to your natural gas line (can also be used with the home propane tanks), here is an example of such a system. In commercial or institutional settings these automatic backup generators may be powered by large diesel tanks, which require professional maintenance and upkeep. Think of places like the fire department, hospitals, server farms, cell phone towers, traffic lights, etc.
These are great because you can set them to turn on after a certain amount of time without power and they can produce a lot of power. For example, it can be set to automatically turn on 30 seconds after a power loss. Depending on your generator size it can produce enough to run an AC unit during a hot summer brownout. The downside to these is that they are expensive, large, loud, require pretty involved installation and if you lose your natural gas service (for example, in a large earthquake) then your system will stop working. In theory though as long as you have natural gas this can power your home for days on end.
During install of a backup power system you may need to identify your critical loads, the circuits in your home that will need backup power. What we have done is setup a critical load panel as a subpanel in our home, so the entire panel can be setup for a future Powerwall or setup with a Generator Transfer Switch which will allow us to connect a portable gas generator to a plug on the side of the house to provide power to that panel. Make sure you get the proper permits and install with a generator transfer switch and solar as they may not be compatible, if your solar thinks your grid power is on and starts sending power downstream you could damange equipment or create a hazard.
Here are some suggestions for a critical load panel:
- Medical Devices
- Broadband Modem/Router & Security System
- Garage/Gate Openers
- Water Heater (Tankless gas won’t work without power)
- Select Kitchen Outlets
- Select Lighting
- Select Outlets in bedrooms
- If you have a large enough generator you can also have your AC unit on backup power, this may also be needed if you have people in your home who are medically temperature sensitive.
- Depending on the size of your generator it may be necessary to ration your power, for my purposes I may only be able to turn on one appliance at a time in addition to the refrigerator.
For some of these items I also recommend using an Uninterruptible Power Supply (UPS) like this to help power these devices for short power outages. I have my modem and router connected to one and it can keep it on for about 30-45 minutes for quick power outages.
What I have is a portable gas generator. These come in many shapes and sizes, one thing to note is that if you are using this for electrics you should look for an inverter model that is suitable for sensitive electronics. These are usually more expensive and produce less wattage than the traditional portable gas generators. The simplest way to use this is with power cords, but make sure you do not operate the generator in a confined space. This may be challenging if you are using this during extreme weather as the generator needs to be protected against the elements, but also need to be able to exhaust as it uses an internal combustion engine. You can also connect this to a generator transfer switch as I mentioned above.
I have this portable gas inverter generator, with a 3700 watt continuous output and 4500 watt peak output. It is compliant for use in California with their stricter emissions requirements. They also make Duel Fuel generators that can be connected to a propane tank as well as use gasoline. One benefit of this is that propane tanks are easier to store than gasoline cans. You can use a meter like this to see how much power your appliances use to see what size generator will work for your needs. Some of these generators offer the ability for parallel operation, which allows you to connect two together for increased output and capacity. With a generator this size, you won’t be able to run an AC unit during a summer brownout.
We talked about how to calculate watts in part 1 of these series and this size generator is enough to power a refrigerator and perhaps and a toaster oven or water kettle. Please real all manuals and follow all safety information. A couple things to note is that gasoline cannot be stored indefinitely, you must use a fuel stabilizer like this in your gas can. I would start your generator once a month to test it to make sure that is starts and once a quarter test it with a load, such as with your electric water kettle or toaster to make sure it can produce enough power. Fuel stabilizer claims it can last up to 2 years under ideal conditions, but I change out my fuel every 6 months.
You can put the gas into your car to use and then refill the gas can with fresh gasoline and fuel stabilizer. In a severe incident you may not be able to refill your gas can, so plan out how much you will need or how you will ration generator usage for a multiple day power outage. Your refrigerator should be able to stay cool for several hours as long as you minimize opening it, so will likely not need to be powered 24 hours a day depending on your climate. May be useful to have multiple gas cans, please follow all storage and safety guidelines for storing gasoline.
They also sell inverters that connect to your car battery than can provide power, but it’s a very inefficient way to produce power by letting your car idle. Unless you’re going to stay with the car it’s probably also an good way to get your car stolen. The 2021 Ford F-150 Hybrid comes with a 7.3 kWh generator built in if you are shopping for a new truck.
I would also recommend keeping a small butane/propane stove like this one with extra butane canisters around just in case all else fails and you need to boil water or prepare food. You can also buy an adapter like this to connect it to your BBQ propane tank for more supply. I don’t want to get too doomsday prepper on you, but also keep ample water, shelf stable food, medication, batteries, flashlights, radios, etc. on hand for these type of events.
It may be useful to use your BBQ (if you have one) to prepare food and boil water. I read somewhere a long time ago that the propane exchange tanks are not that well maintained and that it was better to buy your own tanks and get them refilled locally. That is what I have done for several years now, I have two propane tanks that I purchased brand new and I refill them myself at local distributors rather than use the exchanges. I know they haven’t been dropped or abused and are in good working order. Another thing you may not know is they put less propane in the exchange tanks than they do when you refill on your own, so you are paying more for less. A standard tank is 20 lbs and these exchange tanks are usually only filled with 15 lbs so you are getting 25% less propane with exchanging. Keep an extra around space permitting in case you run out of propane mid-BBQ and also for emergency preparedness.
Powerwall is a cool concept for home battery, here are some ways Powerwall can be used:
- Solar will charge your Powerwall during the day and you can use it to power your home at night or augment during peak power usage, especially useful if you don’t have access to net metering.
- To serve as a battery backup for your home, useful if you have medical devices in your home or cannot have the other power backup options I mentioned.
- I have read about people charging their home batteries at night on Time of Use (TOU) power plans and using them to power their home during peak hours. TOU means that, for example, using power at midnight is cheaper than using power at 6pm. It would take over a decade for this to make financial sense though. This setup is not paired with a solar system.
- If you just think this is really cool technology and want to have it and can afford it.
Downsides to Powerwall
- It’s expensive, and for me, it was mostly for convenience as I already have a portable gas generator. Your situation may be different where this is the only/best option for you to get backup power.
- If you want to use it with an AC unit, you will require 2 or more units.
- This is a giant battery and batteries deteriorate with every cycle, the Tesla Powerwall warranty is that it will hold 70% of its capacity for 10 years. Just know that you are ok with up to 30% loss of capacity over 10 years if you purchase this product.
Check out your rebates:
- California has Self-Generation Incentive Program (SGIP) allocated by installer. Tesla did not have any allotment left so if you went with a third party installed you could save thousands with this rebate.
- Your Powerwall may also qualify for the newly extended 26% federal tax credit if paired with a solar system.
- These rebates often dictate how you use the Powerwall, such as a requirement to use it during peak energy usage times to relieve strain on the grid and not only using it for emergency backup.
I hope this was useful information, I think many people are surprised that if the power goes out during broad daylight their solar systems will not power their home unless they have home batteries installed. If you want more information or are interested in ordering Tesla products, please use my referral link to save $100. This link can also be used to order a Tesla car and you will receive free supercharger credits.