Solar Gas Station

Off-grid solar & battery shed for EV Charging and RV Power.

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I'm expanding my fleet of EVs and California power prices are my motivation. I have a pair of Tesla PowerWalls running my house but are limited to 10kW combined output, which the solar installers had to re-arrange my breaker panel to exclude the high amperage air conditioning and 40A Level 2 EV charger. Plugging in after work is peak time for SDGE and their abominable rates - but my fleet currently is short-range EVs, a plug-in Sonata Hybrid and a Chevy Spark - less than 30 kWh combined. They come home with dead batteries so waiting for off-peak doesn't work for me. I'm building a 28kWh (expandable) LFP (LiFePO4) 48V battery array, adding 12.3 kW of ground-mount solar panels, a pair of off-grid inverter/chargers, dual Level 2 chargers, and even a rain water collection system for the array and solar shed with a well pump for irrigation... and maybe a home car wash some day!

The story so far... 

I have been engineering my system for months, trying to learn as much as I can from YouTube about good sources for panels, inverters, batteries, battery management, etc. Big thanks to Andy at the Off-Grid Garage and the DIY Solar Power with Will Prowse channels, among others. 

After buying a house in rural San Diego county in 2019, we immediately expanded our solar power, knowing the meager 4 kW array it had would never be enough for our needs. We beat the rush to get Tesla PowerWalls and that has turned out to be a wise choice, as recent wildfires have scared the utilities into emergency cutoff plans when fire danger is high and high winds are predicted. We added 12kW of microinverter solar panels and two Tesla PowerWalls, which provide 28 kWh of storage and 10 kW combined off-grid power capacity. 

Unfortunately, 10 kW capacity from battery did not allow for the PowerWalls to have enough capacity left for running a Level 2 EV charger, but that project set us back $52000. We probably goofed in several areas on that design as well. We upgraded the old DC string from dual 1800W grid-tie inverters to a single 3800W inverter with monitoring capability. It probably would have been wiser to remove the old panels and use the newer SunPower microinverter panels. This would have allowed a bigger capacity, uniform appearance, and a single point to monitor. It probably would have cost a bit more, though, exceeding our solar loan amount we qualified for. Meh.

After studying the DIY channels and researching sources, I was inspired to do a system myself, and also avoid tying to the grid for the sake of simplicity. Brand-new bifacial 410W panels by the pallet load (30 panels) were only $185 each + freight. 12kW off-grid inverter, $2049. Freight $570 from Signature Solar in Texas to SoCal was $570.... plus a U-Haul truck rental when it turned out the lift gate truck could not get to our house. Add another $335 for renting a U-Haul + fuel costs to DIY the delivery. 

Words for the wise: the pallet weighed about 1800 pounds, plus about 200 for the crated inverter. This requires a heavy lift gate truck, which has poor ground clearance and bottomed out on the highway when hitting the slope of the driveway. Light lift gates have delivered to my house successfully, so I wasn't expecting this. U-Haul trucks are easy to find in the Western US states, but Penske trucks have been hard to find since the pandemic. Penske is more expensive to rent, but you want their trucks because they have standard loading dock deck heights, which allows a forklift to drive right in and deposit the pallet. Since I could only get a low-deck U-Haul, the very helpful delivery guy was able to back his liftgate truck up to my truck, lower the gate to the U-Haul height, and use a pallet jack to transfer the 2000 pounds of equipment over. Sounds simple, right? This was dangerous AF in terms of personal safety and risk of damage to trucks and cargo. Solar pallets are huge, like 40" x 85". Since we had to use a pallet jack (with 48" forks), the pallet had to be loaded wide-ways, which barely fit in the U-Haul, and we had to remove the "helpful" rental hand truck to get the pallet to fit. This cost me a $30 rental fee they wouldn't take off my bill. Sigh.

Finally got the panels and inverter home, where it took a bit of brainstorming to figure out how to get the 200 pound inverter out of the truck, not an easy task since the crate was full of splinters and razor-sharp metal trim holding it together. It is intended to be lifted by forklifts, not soft fleshy bits. Once done, we dismantled the 30-pack crate of panels and carried them by hand into storage. 

Inverter Rear Wall.pdf

To-scale view of inverters, combiner boxes, battery shelves, and breaker panels for sizing/placement analysis. Also note Mr Reddy Kilowatt holding the kid at knifepoint. This is integral to the design and it's on order from Etsy.

Adobe Portable Document Format - 619.27 kB - 01/27/2022 at 03:25


Shed Wall Studs.pdf

Illustrates wall stud placement to align with James Hardie fiber cement exterior paneling.

Adobe Portable Document Format - 293.46 kB - 01/27/2022 at 03:25


Blue ECO Carpet Just Because.pdf

Everything EV-related has to be blue, hence blue carpet. Wanted something to protect the OSB flooring from damage, also protect my knees. Note the carpet is cheap but the glue isn't.

Adobe Portable Document Format - 317.13 kB - 01/27/2022 at 03:25


Battery Shelves Overhead.pdf

Illustrates how much space the 16S 2P arrays of 280Ah LFP batteries will consume.

Adobe Portable Document Format - 362.51 kB - 01/27/2022 at 03:24


Roof Truss Gussets.pdf

Optimal cuts for making rood truss gussets from OSB sheets. Want to be efficient with the expensive carbohydrate foam.... scrap can be used to build battery box.

Adobe Portable Document Format - 242.54 kB - 01/27/2022 at 03:24


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  • Well, this happened...

    Bryan Williams04/23/2022 at 07:06 0 comments

    Nothing much to update on the project as yet, I've had a few distractions. I just noticed I had not visited the project since I had scooped up a Tesla 2021 Model 3 Performance. So now I have a long range charger burden... better get cracking on the project, especially since my net metering trued-up this month and my electric bill was $3268.72! 

  • S/P Nomenclature

    Bryan Williams01/27/2022 at 04:38 0 comments

    Just reading some forums, realized I should probably clarify my baseline battery would be called 2P 16S, implying a single 48V battery and BMS, with 2 cells paralleled then strung in series.

    Apparently the ? common ? nomenclature would mean 16S 2P would be two 1P 16S batteries in parallel, implying the need for two BMSs, and corresponds to using something like a pre-build off the shelf battery like SoK or EG4. Note the EG4LL 51.2V 100Ah rack mount batteries at Signature Solar et al are rated to be paralleled up to 16 packs -- not sure about others like SoK. 

    Also, not sure if Daly Smart BMS 250A model which I have on hand can be paralleled. It would be awkward to build 4P 16S with one BMS as the layout I have planned requires 2 shelves to do that - bus bars and balance leads would be all over the place. Anybody know if you can take two 250A Daly BMS batteries as "2P 16S 2P" ?  

    I know series stringing isn't allowed on a lot of 12V batteries because the balance circuits in certain conditions can see the full series voltage, e.g. 48V, while their controller chips can only handle much lower voltages, since they were designed as stand-alone 1S 12V batteries.

  • Inverter Selection

    Bryan Williams01/22/2022 at 15:21 0 comments

    After comparing a great many recommended units, I settled on the GroWatt 12 kW 48V 150 VDC 120A Off-Grid Inverter, which is actually a rebranded Sunrino SP Series low frequency split phase inverter. GroWatt has their own designs that use high frequency conversion technology, but have their own quirks. Decoding the part description, the primary inverter in my system is 12 kW continurous output with split-phase 120/240 VAC output as is the standard residential service voltage in the United States. This allows 120V or 240V circuits at the output. Battery nominal voltage of 48V (Lead and Lithium are supported), 150 VDC solar input (open circuit voltage of string), and 120A battery charging circuit. 

    The 150VDC input limitation is one of the biggest drawbacks to this particular model. My bifacial 410W panels are 49.9V open circuit maximum. This means I am maxed out at 3 series panels, which is unfortunate as it means using combiner boxes and paralleling strings. This model supports two independent MPPT tracker inputs.

    The battery charger of 120A x nominal 48V battery works out to 5760 Watts maximum being pushed into the battery storage. The maximum PV array power for this model is 7000 W. Recall I have a full pallet of 30 x 410W panels. It took reading this spec to realize that I wouldn't be able to get by using a single inverter with my 12,300 W panel array! I went back to Signature Solar's website to find the 12 kW models were sold out, but an 8 kW was available. I figured this would not be too much of a compromise, as this could be dedicated to my 32 Amp EV charger. Both units have the 7000 W solar input limit, 120 A battery charger circuit. Both units have the ability to charge the battery from the AC utility, with the significant difference between the units being a 70 A limit on the 8 kW model vs a 100 A limit on the 12 kW. 

    I don't intend to use AC charging unless absolutely necessary, but it is a nice-to-have that I can use a relay control to allow charging if the battery gets low at night and control the time of charge so only "super off-peak" 12am to 6am rates are used. In case of a cloudy week with poor solar output, I can still use grid power and use the battery to time shift the load. This is still an off-grid use, as the inverters cannot supply power back to the grid.

    The Lithium Iron Phosphate cells I am using are EVE 280 Ah cells, with a standard charge rate of 0.5C, or 140 A. The charge rate of the battery is really what drives a lot of the ratings specs it appears, from 7000 W limit to the solar array to the 120 A, 5760 W charging circuit.

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David Sadler wrote 07/15/2023 at 11:19 point

Expanding your fleet of EVs in California, considering the high power prices, sounds like a smart move. It's unfortunate that your current setup with Tesla PowerWalls is limiting due to its combined output and the need to exclude high-amperage devices. To address the peak-time charging costs and the short-range of your EVs, it seems like you have a comprehensive plan in place. Building a 28kWh expandable LFP (LiFePO4) 48V battery array, adding ground-mount solar panels, off-grid inverter/chargers, dual Level 2 chargers, and a rainwater collection system for the array and solar shed shows a proactive approach towards sustainability and cost efficiency.
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