DIY grid-tied residential install w/ batteries in Vermont

[MattK]

We've spent the last few years as full-time RVers and are now in the process of buying a plot of land in central Vermont on which we plan to build our own home. We installed a modest-sized solar / battery / inverter setup in our RV back in 2018 and it's been working great the entire time we've been traveling. The system is fully integrated into the home automation system I have built in our RV so we have full monitoring and control of our inverter, batteries, solar etc, as well as automations configured to avoid tripping breakers, use excess solar, etc.

We're keen to take these learnings and apply them to our house build. While I don't have any formal electrical qualifications, I feel confident in my own knowledge and ability to do a safe and effective installation. That said, residential solar introduces some new concepts that I didn't have to contend with in the RV world - e.g. net metering, SRECs, etc. The system will also be significantly larger than our RV (800W solar, 300Ah lithium batteries, Victron MultiPlus 12/3000).

I've been researching the topic and I'm struggling a little to find a way to design a system that fits all our needs. In terms of budget, we don't have a fixed number in our heads, but don't want to needlessly spend more than we have to in order to achieve the capabilities we want.

In short, we intend to be grid-tied on our new property, and have a south-facing orientation in which we can install roof and ground-mounted solar as necessary to achieve our goals. We also plan on building a mechanical shed separate from the main house in which we will terminate all utilities - including grid power and fiber internet. We will be living on-site in our RV once we have utilities installed while we build the house. Our intention is to build the house to passive house standards, and use all electric appliances - e.g. stove, water heater, heating, etc.

Our goals are:

1. Build a scalable system that we can expand as we learn more about our energy needs
2. Achieve (at least) net-zero electrical consumption over the course of a year - likely a 10-20kW system
3. Generate and sell SRECs
4. Have backup power for at least critical circuits (including server rack, LED lights, fridge, 230V well pump and septic pump - so ~5-10kW but with some large motors)
5. Access the data for monitoring via our home automation system (e.g. modbus, MQTT, serial, etc)

Our energy company is Green Mountain Power and they have a BYOD program whereby if we use one of their supported devices to allow them to remotely provide power back to the grid at peak times, they will essentially pay us $850 for every 3kWh of capacity we provide. That would limit us to a small set of inverter systems, several of which are unavailable to DIYers. From the list, I think SolarEdge StorEdge would be the best candidate, but possibly Generac PWRcell too.

I've been researching the StorEdge approach and I think it can do everything I want with the exception of #4 in my list above. The largest StorEdge inverter can only manage 5,000W sustained when on backup power, and although you can install multiple inverters, each powers a separate backup load panel and they cannot be combined. That would mean two inverters, two expensive batteries and two separate load panels with likely an unequal distribution of load.

I'd be very interested in exploring options using 12V LiFePO4 batteries too. I'm reasonably familiar with the Victron ecosystem, but am struggling to find a way to design a system that would fit our needs. In particular, how to wire a Victron system with an AC revenue grade meter on the solar array to accumulate SRECs. This approach wouldn't qualify for the BYOD rebate, but we'd still consider it.

I would love any input from this group on good approaches to designing this system.

Thanks in advance!

 

[12VoltInstalls]

Is $850 correct? That’s like big big dollars!
Not making sense to me.

 

[MattK]

Is $850 correct? That’s like big big dollars!
Not making sense to me.

It's correct, but I should have given a little more context. In return for $850, they have control over your batteries to pull from during peak periods (5-8 times per month on average) for 10 years. They give you ~4 hours notice before a peak period and will try not to deplete your batteries ahead of a storm that may cause a power outage.

They price the scheme at $850 per kW you enroll for up to 3 hours of use, or $950 per kW for up to 4 hours per use. In other words, as I understand it, it basically works out at $850 per 3kWh of battery capacity. So for something like the LG Chem RES10 battery, you could elect to enroll for up to 3kW (3x 3kW = 9kWh and the battery is 9.8kWh) and get a $2,550 rebate. It's a one-off payment but the program lasts for 10 years. If you want to leave early, you basically pay them back the pro rata money owed based on how long you were in the program.

So at face value it sounds great, but you are giving up a lot of control of your batteries in return. You can still use the batteries for your own self-consumption too (e.g. peak shifting) but you need to make sure they're full before a peak period kicks in. If you're only able to charge with solar (e.g. you've claimed the solar FTC) then that could be a challenge.

We're not dead set on joining the scheme because of the issues above. That said, because we can opt-in at any time and opt-out again if it doesn't work for us, I'm leaving the option open for now.

There's more information on the program here: https://greenmountainpower.com/rebates-programs/home-energy-storage/bring-your-own-device/

 

[12VoltInstalls]

Oh! You meant capacity rebate not metered consumption. That’s so obvious but I missed it.
I’m not into those programs here in Vermont. They are way-weighted for the advantage of the power company.
You can do your own load compensation with a solar transfer switch and realize 100% of the ‘bonus’ power AND be autonomous in a 3-day power outage.

I know a guy that sells grid-tie solar. The ROI is most often way out in the future. And not guaranteed. You can ‘sneak up on’ your solar - grid cost offset in many cases at half or a third of the time to ROI. And just use the grid for the stove and well pump. Then if the power goes out you just have a small genny (you likely already have) for when that’s required.

The longest ROI seems like it’s on lifepo batteries because they’re expensive up front. Cost per annum gets pretty low, however. The up-front does stretch the ROI

So I’m not saying you’ll get rich being independent but it just has so many advantages to you instead of going to Grn Mtn Power with your investment in their infrastructure.

 

[MattK]

Haha, sorry, yes, I could have been clearer - apologies!

As long as we can collect the SRECs and sell them, that should be pretty lucrative. Given our climate around here, a rough rule of thumb is that 1kW of panels will generate 1MWh (1 SREC) per year. Let's say we install 15kW of panels (my guess is that's roughly what it'll take to net off our usage, give or take), then we'll also generate 15 SRECs per year. VT takes part in the MA SREC marketplace, and the going rate there is ~$300 per SREC - so that's ~$4,500 gross annual revenue before marketplace fees, taxes, etc. Am I understanding that correctly?

Generating the SRECs is completely independent of whether we install batteries or not. My rationale for doing so is that we need to have backup power for our house - for sensitive networking / IT equipment, but also basic essentials like the HRV/ERV (house ventilation), etc. We may well invest in a generator (3 years of full-time RVing, mostly off-grid, and we've still never owned one) and then we have a decision do we go big (permanently installed standby) or small (portable dual-fuel ~5-10kW). I'm hoping to punt that question further down the line until we can see how stable the grid is - we've been in winter this Vermont and the power has tripped a few times but never for more than a couple of minutes.

Actually, that raises another question. I noticed in the spec sheets for some of the grid-tie inverters that it can take up to 2 seconds to switch to backup power. Is that the case? I guess I've been spoiled with the Victron MultiPlus hybrid inverter! That means we'll need a UPS, at least for the server rack anyway.

 

[12VoltInstalls]

I’m in Vermont too btw

the two second transfer time sounds
normal from what I’ve heard. So small UPS doesn’t hurt much and will buffer fine.

That solar income sounds appealing. Nobody up this way doing that from what I’ve heard. I don’t know exactly what the up-front would be but that’s a good payback. However, winter up here can really kill your daily solar generation. That’s important to keep in mind.

If you have a decent battery bank, a small generator is sufficient when you need to run the stove or well pump. Maybe H2O heater. But if it were me I’d get a propane on-demand hot water heater. Then the only “big” item you couldn’t make do without is the well pump. Also I reason I like gas stoves, too. The small additional cost over time is insignificant compared to the extra batteries and inverter sized to run bigger loads is going to cost. With solar, you could in that case run indefinitely.

I remember in 2003? the power being out six or seven days. Hurricane or rain event. Was as eerie as 9/11
I wasn’t solar then but with some quick adjustments I was the only lights on anywhere I could see at night. (Which made me decide to keep them off anyway LOL)

 

[MattK]

Your input is much appreciated! The up-front investment is basically buying the solar system outright so you own the SRECs, not some solar leasing company.

For various reasons, we're keen to go electric-only in normal operations in the house - the only exception really being a backup generator if we needed it in emergencies. I keep going back and forth on batteries, lol!

One question - is there a way to install a revenue grade meter with Victron equipment in a configuration that would allow the system to operate without grid power? As far as I can tell, to operate without grid power, you need to use solar chargers on the AC-Out side of the Victron equipment, but then you never get the AC output from the solar that the meter needs...

 

[12VoltInstalls]

I know nothing about that. That’s somebody else’s to answer.

My ‘expertise’ by experience- if you can call it that- is 12-volt stuff in boats, utility trailers, RVs, jeeps, small off-grid...
In 1999 I had a VW Westfalia camper with TV, fridge, and a desktop computer- shore power not required! If solar was more readily attainable then I’d probably have used it.

 

[MattK]

That's pretty much my experience too - 12V stuff in our truck / RV! Net metering, SRECs, grid-tie, islanding - all new concepts that I've been trying to understand!

 

[12VoltInstalls]

Well I understand these things a bit. Not as an authority for sure. Enough to know that mostly I don’t like them in Vermont. People talk about 15, 18 years to “pay off the solar then it’s free” and I cringe!

I’m buying my own property in the next 30-40 days.

My very small off-grid system if I think about replacement cost VS grid cost would be like 2- 2.5 years to break even. Maybe 3 years as I’m a low consumer and based on electric rates locally.

Except that I rented a lot for $900/year with my RV trailer on it for three years to save money towards my purchase of property. So if I count the RENT savings alone, my system paid for itself halfway into the second month because it enables/enabled me to essentially live for free 7.5 months of the year.

In your case, that’s going to be an impressive up-front investment to pull down $4500/year on solar.
I guess I just don’t have enough faith or confidence to look to carry “debt” very far down the road. About three years is the limit of my vision. I guess part of that is habit as after a divorce became imminent in 1999 I’ve been a cash operator- I have not used any credit since 1999.

good luck with your project!

 

[Shotgun]

A different tack you might consider is to give up on feeding back to the grid. The big players have too many people working for them to insure that any agreement they make does NOT favor you.

I chose to get an all-in-one system. It will pull from the grid if the solar and batteries are low, but does not feed back. This allowed me to skip the inspections, paperwork, and anti-feedback equipment. For less than $1,700, I have 1.5kW of solar with 1.8kWh of LA battery. I dropped the output of the inverter into a breaker box that I brought several of the low-but-constant house circuits into.

The goal is to cut down on what I pay for energy. My operating logic is that I want to maximize what the panels produce as cheaply as possible. I just got my monitoring software written and running. The results of that will help me decide if I'll double the battery capacity, because I may be dropping photons when the battery gets full and I don't have loads to use the energy.

 

[12VoltInstalls]

I’m in total support of not-feedback solar. But in OP’s situation it sounds like it makes cents. And dollars. When grid is readily available solar is more expensive unless one is gridtie, generally. ^^^the players as mentioned use economies of scale that sortof slant towards the powercos.

once lifepo batteries start sliding down in price from ‘economics of scale’ efficiencies the ‘independent’ model will become more equitably attainable.