I posted a bit about this in the December show & tell thread, but figured it would be worth having an ongoing thread to talk about the project and document the progress so others can ask questions and learn.
I’ve wanted to have some degree of energy independence for a while now, mostly to help keep some of the basics running during short outages. Originally my thoughts were along the traditional lines of solar panels and a battery (e.g. Tesla Powerwall). Unfortunately solar panel installation labor is pretty expensive, even with the price of panels continuing to drop significantly. Plus why pay for a clean and prebuilt system when I can cobble together a bunch of components myself and spend way too much time tweaking and optimizing it. 
I like to start all my projects with a set of design goals, though these can change as I learn more about the subject. In this case I needed to know what circuits I have in my house, what their average usage is so that once I identified which ones were critical I could size out a system. My previous experience with sizing battery based solutions led me to immediately ruling out trying to run any major loads off the battery bank, as it would require a lot of space, cost more and I know from experience (e.g. snowpocalypse) that I can do without things like a clothes dryer but not internet. 
A while back I installed an IotaWatt so that I could track my power usage per circuit and it proved invaluable for helping define the design goals. Initially I was only monitoring a handful of circuits, mostly the large ones (HVAC, dryer, EVSE, etc), but for this project I mapped out all of the circuits in my house and added more circuits to the IotaWatt monitoring. This gave me an amazingly detailed view of what devices were on what circuits and how much power they used.
This is what the real-time view on the IotaWatt looks like, for historical data it has a powerful graphing feature but I tend to just use the graphing built-in to HomeAssistant (my smart home system).
Here you can see the installed IotaWatt and the clamps it uses to monitor the power usage of the circuits. I cleaned the exterior cables for it after I put the panel cover back on.
In the end I decided that these were my critical load circuits:
- GarageOutlets
- 120V 20A, GFCI protected
- Idle draw: 20 watts
- These outlets power my fios optical network terminal (ONT), my garage door and the iotawatt itself and the garage door opener.
- AirHandler
- 120V 15A
- Idle draw: 10 watts
- Under load: 550 watts
- This is actually a single outlet in the attic that the air handler is plugged into, I guess that’s just how they did it in the 80s. We have gas heat, so as long as this outlet is powered we can run the fan and heat, we just can’t run the AC compressor (it’s a different, larger circuit).
- Network
- 120V 20A AFCI protected
- Idle draw: 220-250 watts
- This is a dedicated circuit that I had installed during our remodel. It powers a single outlet in the office for the Ubiquiti UDM-Pro router, PoE switch, Intel NUC that runs HomeAssistant and my fileserver. This circuit is the “core” of my home automation, it also provides power to the wifi access points and security cameras.
- Fridge
- 120V 20A
- Idle draw: 5 watts
- Under load: 125 watts
- Food goes in here, cold drinks are also nice to have.
- WaterHeater
- 120V 15A
- Idle draw: 5 watts
- Under load: 200 watts
- Tankless gas water heater, but also a few other random things like lights because it’s pretty common for outlets and lights to be on the same circuit.
- Office
- 120V 15A
- Idle draw: 200 watts
- Under load: 900+ watts
- Outlets and lights for my office and our bedroom. Not exactly critical, but I can easily shed the load if needed by turning off lights/computers.
Adding all that up we get about 485W or idle load and up to 2,045W if everything is on all at the same time. For that reason I decided to go with a 5,000W inverter, as it’ll have more than enough capacity to run everything and will allow me to add a bit more load if my needs change in the future.
There are a few options on the market now for inverter/chargers, the device that will generate AC from the DC battery and also charge the battery when grid power is available. I’ve used Victron Energy gear for some other projects so I decided to use their MultiPlus-II. They have versions that can work with a 12V, 24V or 48V battery, but I chose 48V because higher voltage allows for thinner wires and slightly better efficiency. Additionally I choose to use a 230/240V inverter so that during normal operation, when the inverter is in passthrough mode, I won’t be drawing all my power off of just one of my house’s phases. It also allows me to add 240V circuits later if the need arises.
So with all that in mind I now had a good set of design criteria:
- Able to power my 6 critical circuits
- Ideally with 24h runtime at idle load
- 48V battery
- Ability to expand the system later as needed
- Observability of the system and battery, so I can get runtime estimates while on battery power or identify issues before they cause an outage. Ideally tied into my HomeAssistant system.
- High degree of safety without the need for constant maintenance.
I choose LiFePO4 battery cells as they are highly cost effective, very safe/stable, and have a number of battery management systems available to ensure the battery is only charged/discharged when conditions are safe. Since I knew I wanted to have a 48V battery pack, that means I needed at least 16 LiFePO4 cells (known as a 16s pack).
I was able to find a good deal on Alibaba for 280Ah cells from the Chinese manufacturer EVE, which is one of the newer and decently well rated suppliers of battery cells. At 3.2v nominal cell voltage that is 896 watt-hours per cell or ~14.3kWh for the entire pack. That’s a lot of capacity! It’s actually slightly larger than the latest Tesla Powerwall (13.5kWh) which cost about $10k each.
Initially I had looked for slightly larger cells but the particular supplier I used (Shenzhen Qishou Technology Co., Ltd.) had some in stock at a US warehouse and were able to ship them to me at a reduced rate. The only catch was that they had 22 cells on hand, and while I had initially wanted to buy 20 cells (16 for this pack and 4 for a 12V portable pack) I agreed to buy all 22 for a reduced price, as they didn’t want to be left with just 2 cells on hand. After shipping the cost per cell was $130/ea which works out to $2,080 for the pack! Maybe I’ll make a really big ~5V battery with the two leftover cells. 
There was a lot more back and forth I did in my research, but in the end this is the setup I decided to go with:
The inverter/charger outputs 230Vac, but all of my circuits and devices need normal 120Vac, which is where the autotransformer comes in. It takes 230/240Vac and gives me two 120Vac lines that are split-phase, just like the power I got from the grid. I really liked this style of setup because it lets me run 120Vac and 240Vac loads, automatically balances the load to the phases! If needed the A/T can provide the entire output to just one phase. That helps keep planning a lot simpler and gives more flexibility on what loads I can place on the battery backup.
I don’t have a full breakdown of parts and costs, there’s lots of tools and things I had on hand already (such as wire) but here are the major parts and their cost:
- 22x EVE LiFePO4 cells (280Ah) = $2,860
- Victron Energy MultiPlus-II, 48v/5000W/230Vac/70A inverter/charger = $1,670
- Victron Energy AutoTransformer 120/240Vac/32A = $550
- Overkill Solar 16s/48V BMS = $172
- SquareD QO 125A panel with lugs (no main breaker) = $64
- AFCI and dual function (AFCI/GFCI) breakers = about $240
- 125A DC circuit breaker = $24
- Electrical permit from the city = $76
- What? You think I’m crazy enough to do this kind of work and NOT cover my butt with proper permits and inspection?
- What? You think I’m crazy enough to do this kind of work and NOT cover my butt with proper permits and inspection?
That’s a total cost of about $5700, though I forgot how much I paid for the wood and a few other bits and pieces. Still, that’s well under the cost of just one Tesla PowerWall and I was able to get it now instead of being on a super long waiting list.
If you planned to get one, I would grab one now before they’re out again.