Adding Solar to an Existing System to Meet Demand (Part #1)
I put in a solar system 7 years ago. It is a 20 year lease and the payments are reasonable, working out to about $60/month for 20 years. 60 * 12 * 20 = $14,400. Cost is $14400/4680 or about $3.08 per watt. That included some budget panels and an Enphase micro inverter. My next step is to add solar, via ground mount, to offset the many additional electric loads I added (stove, car, more heating), and provide battery backup. Many solar companies are not good at doing what I want to do, because their business model is based on installing 10K or more of solar, and thus they don’t want to deal with the presence of an existing solar system. Especially a leased system! Here is how I am getting around the limitation of keeping the leased system (which could be bought out but that’s not a good deal). My plan is to lower the usage of utility power by moving several of my key circuits to a new subpanel that is powered by an off grid solar setup. This means: an 8K “hybrid inverter” that has power coming in three sources: new battery, new solar panels and the grid. This inverter should be able to power one air source heat pump, one heat pump water heater, the fridge circuit, and some key plugs in the house. In my case I am aiming for about 4400 watts of added solar, 14000 kwh of battery, and a 20 amp grid connection for those rare situations where the there is no sun for 3 or more days at a time. Once I have a “critical loads” sub panel, I will remove several of the key circuits in my house from the main panel and put them into the sub panel. To power the sub panel the inverter I choose needs to support ~ 20amps of solar input on multiple strings, with the VOC between 200 and 400 volts. The inverter will charge a 48 volt battery. If I do most of the work myself to mount panels and run conduit, and have an electrician do the indoor wiring, the system should cost less than the one I bought in 2016. (If I were to just add a battery to my existing system, I have quotes over $20K, which seems ridiculous.) If you are trying to do the same thing or have already done so, feel free to reach out. In my case, it may be necessary, as the electrical panel can handle 200 amps and a home converted to electric heat and electric cars might be pushing the limits of that service.
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It's very important to use the correct units for that battery, which would likely mean you want a "14,000 watt hour" battery, a 14 kWh battery. I see this mixed up all the time, and it's a BIG difference! Similar mixed up units are also used in the media to misrepresent things, sometimes by accident, but often on purpose, either to make something look better or to make it look worse.
I don't see a problem with what you describe, since you would essentially just be building a seperate system, isolated to that one small subpanel. I would advise you against going with a battery system 'off grid' style though -- there are a lot of downsides to a system, some of which are the cost, and the ongoing maintenance required. If you do decide to continue with such a system, I recommend putting the battery string in a seperate fire rated room seperated from the main house, such as a small "battery room" off of the garage.
I deal with a lot of battery systems at work, many of them VERY large systems. Anyone who works with batteries will tell you that they hate batteries.
Bill
We have 2 new fire rated rooms that are 5 x 8 and walls are fieldstone and concrete. Don't worry we have all of that covered! We just need to add one more fire door.
The batteries you deal with -- are they lithium iron phosphate? Wall mounted self contained units? I think the technology of 2023 is much cleaner than the diy type batteries. I am going to stay away from a homemade battery.
I work in the telecom industry. We use mostly VRLA cells, typically variations on the old lead acid batteries (although much fancier than what's used in a car battery). Very little lithium battery chemistry is used in the telecom world.
The problem is all batteries have a downside. Lead acid batteries give off hydrogen gas when overchargered, and hydrogen sulphide gas when deeply discharged. Both are bad, but in different ways. Lithium ion cells can overheat at the extremes of their charge limits, and if it goes too far, they produce their own oxygen to feed their fire. That's bad too! LiFePO4 cells are safer, but spendy.
I really think you're far better off just using grid power when your solar system isn't producing. This is safer, cheaper, and less maintenance issues to deal with.
Bill
Would buying out the lease be a better idea than dealing with batteries?
Walta
no, it would cost more than buying the battery. Battery cost has come down to $4000 and buying out the least would cost over $15000, which is a story in itself.
Maybe your math is different than mine.
13 years of payments remain 60X12 X13 = 9360 total of payments.
If your interest rate is 9% that would make payoff amount would be $5,500 today unless the loan has prepayment penalties.
The fact that you are not allowed to expand the system and it makes the home almost unsellable combined with the high interest rate are the reasons leasing is not very popular.
Seems to me the sooner you come to the conclusion that leasing solar was a bad idea that cost you money every month the sooner you will escape from the lease.
Battey storage is another bad choice unless it is the only viable option.
Walta
if we pay off the loan then we don't own the system and no we can't just pay the principal, the contract didn't allow for that. The contract did allow a buyout at about $8500 at 7 years, however the company that now owns the lease is not honoring the contract. we would need to get lawyers involved to get them to honor that payment. So we made a decision to just let the company be on the hook for fixing the existing system for another 13 years. That might get us warranty replacements on the micro inverters, for example.
I bet if we wait 4 years and then pay off the lease the company won't repossess the panels. If we did it now there is a danger that they could do just that. though I would appreciate any wisdom of others on this issue.
I would not keep batteries in the house. Garage, especially if detached is the best spot for it (this might be code requirement anyways).
I think a simple zero export setup and skipping the battery is even easier. This means more power from the grid when the sun doesn't shine but the ROI on that batteries is probably never.
About the only reason I would add batteries is for backup power, in that case they can be much smaller and since you won't be cycling them, they will also last much longer.
have there been any house fires with lithium ion phosphate chemistry batteries as the cause? I am talking about the all in one units where the batteries are in a case, and not "ternary" lithium ion which are more prone to runaway.
We do have an unfinished basement and can make a fire rated section of it, which attaches to the garage. The garage is a possibility, however a car can drive into the battery in the garage, which seems to negate the supposed safety of putting it there.
I have completed the improvements proposed in this post and wanted to share results. Thanks for all of your suggestions. We ended up with about 5.25kw of solar on top of the existing 4.68kw array that's now 9 years old. Total square footage not including the garage level is about 2300 right now.
1) Creating a battery room indoors. Since the building code in MA allows a battery in the garage, I was able to take the small room linking the basement to the new garage and call it part of the garage. So the auto close fire door is on the house side of this little 6' x 9' room and there is a regular exterior door on the garage side of it.
https://www.greenbuildingadvisor.com/question/repair-or-replace-heat-pump-water-heater?
When I did the insulation for the garage, we used open cell in the main garage but for additional fire resistance we used closed cell in the small room (flash) with fire resistant Rockwool batts for the knee walls and ceiling. The place where the battery is mounted is fieldstone, so no fire issue there.
2) Latest technology inverter: At the time of the post it didn't exist, but fortunately EG4 electronics released its 6000XP solar inverter for $1300 that did exactly what we needed. 240 volts output, ability to have 2 mppts with over 500 volts each, and the ability to have passthru power. That means in the dark winter months the solar power goes directly to battery and the inverter provides grid power to the loads (until the battery fills up in which case it takes over from the grid for a day or so, draining to 20%). We didn't have to modify our existing electrical panel, except we removed some of the larger loads from it and placed them onto a new "backup service panel". The backup service panel is intended to operate whether the grid is working or not. The backup panel does not "back feed" the main panel, so no safety issue there.
3) We had our electrician do the hard parts of the wiring and the roof mounting of the first group of panels. I designed the system myself with assistance from Wattmonk and ordered the hardware from Signature solar.
4) Before the foundation was finished I had the contractor put in a 2" hole in the poured concrete, and from there we laid 1" conduit over to the side yard, ready for eventual installation of more panels.
5) All in the cost of the system was about $18,000 for 11 panels and a 14.3 kilowatt battery.
6) When we completed the addition we didn't need to expand the heating system. The renovation work made the building more efficient so the system we put in in 2023 could handle it. Though we have a new room of 250 sq. feet on top of our new garage, we didn't add heat ducts. Instead, we insulated the heck out of it with about 18 inches of fiberglass in the ceiling and open cell 2x6 bays plus an extra inch of polyiso, with a smart vapor barrier. This means that the room is conditioned as long as we open the door. This January the room served as a large fridge!
7) I really like the Bluesun panels we installed. Made in Singapore, the panels now retail for $177 at Signature Solar:
https://signaturesolar.com/bluesun-460w-half-cell-bifacial-solar-panel-up-to-575w-of-bifacial-gain/
The promo for the panels say that they provide up to 575w if you expose the back of them. We did this and today --- March 2 2025 -- the side yard panels peaked at 541 watts each.
Peak output of the system today, on March 1:
older Enphase panels: 4060 watts (below spec, due to clipping from microinverters)
new Bluesun and Solarever panels: 4907 watts (DC), usable power 4591 watts AC at 9:40am; this does not include the power needed to run the inverter
Total combined peak output (AC): 9498 watts
Total output for today is 56 kilowatt hours.
8) The circuits on the off grid system include one 18000 btu heat pump, our refrigerator, our GE hybrid hot water heater, our Internet router, and an outlet in the garage that can be used to trickle charge an EV at 1200 watts. We will probably add the cable box. In the newly finished room we have a closet suitable for starting vegetables so there will be a grow lamp and a heating pad soon turned on, all powered by solar.
9) With the off grid system handling so much power, we are definitely spinning the meter backward on our Enphase System and could end up with a zero electric bill for March. This was our original intent when we put in the system in 2016, before we got rid of gas and electrified everything. The original solar system was designed to handle 105% of our energy load pre electrification. The new system may be able to handle 100% of our usage, post electrification.
10) Many of the ideas in this post would not be practical if it were not for the large amount of weatherization done on the main house before the addition and new solar were added. Even though the old house still has areas insulated below code, our main accomplishment was to locate and fix the trouble spots: bays without insulation, or areas where rodents cleared out the insulation. Gaps between original house and New Addition. Holes in drywall hidden by bad contractors, etc. We passed a mass save audit to install a heat pump, even when the house wasn't really suitable for a heat pump due to cold spots that are likely typical of a 1930s house. Now, the majority of the walls are above code and the heat pump we put in works very well.
https://www.greenbuildingadvisor.com/question/update-2-on-1930-house-in-northern-ma-adding-many-kinds-of-exterior-insulation-all-at-the-same-time
Note that you should have a remote disconnect at those solar panels to meet code. I always like to install at least two conduits on projects like this, so that you have one for power and one for controls/communication. There are ways to deal with having only one conduit if you need a control wire (shielded wire is one way that often works, but you have to make sure that wire is rated for the highest voltage of anything else it's sharing the conduit with).
If you have a linked fire alarm system in your house, which you really should (they are a good life safety system), be sure to add a heat detector in your battery room. That way, if a fire ever does get going in that battery room, it will trigger ALL the fire alarms in your home to go off, giving the the maximum amount of early warning time to get your family to safety. While the chances of a catastrophic failure of the battery system is pretty low, it's still best to plan ahead and put in some extra safety systems, just in case.
Thanks for posting a followup on your system though! It would be nice if everyone did, but few actually do.
Bill
I believe that the control wire (for tigo, for rsd) is rated for 300 and the voltage of the circuit is around 290. The dc disconnect is installed.
The building department required a heat detector. It was installed and passed inspection.
The EG4 powerpro battery has multiple fire suppression mechanisms in the case. I think it beats powerwall 2 because it is based on LFP and Tesla is not. Also it doesn't support Elon lol!
Thanks for your concern.