Wiring a dedicated solar house circuit?

[TheSolarTrough]

Hello, I am new to the forums! I come here in search of insight from some more experienced folks before I make my purchases. Any help or thoughts will be much appreciated!
Here's what I'm thinking:

I can reasonably spend only around 1000-2000 dollars right now - but I want solar power somewhere in my home.

I have several dedicated circuits ran and wired into my breaker box. The circuit I have in mind is a dedicated 4-plug AC wall outlet ran to my living room along side the rooms normal wiring - I've called it the 'clean' circuit. I did this for the home entertainment system, but it was never even installed so now I have a 15 amp breaker wired to a circuit essentially carrying no load.

What I want to do is disconnect the clean circuit from the mains and wire it, with the 15 amp breaker, into a 24V, 900-watt inverter (1800w Surge, or 15 amps at 120VAC). I think this means the breaker should trip if more than the maximum surge power of the inverter is demanded.

My solar array would be two 250-watt panels (perhaps up to 300-watt panels) wired into a a 30-amp MPPT charge controller. The garage roof is south, southwest facing at 55~ degrees with zero shadow cast potential year round.

The battery bank would consist of two 12v, 35Ah batteries (wired parrallel) and hooked to a third 12v, 135Ah battery in series giving me a 24V, 102Ah Bank in total.

I would like to find a way to use the 24V battery bank to charge my power tools lithium ion batteries (both 24V, 2Ah and 4Ah). I realize I could plug the AC powered charging docks that came with the appliance into the power inverter, but I'd like to keep the load off of the inverter (unless I'm drawing power from inside on the living room circuit). I figure hooking them together with a diode to keep them from discharging with the rest of the bank might work? Any insight on this point would be great.

The flow chart of the system w/o the power tool batteries should be from the sun, to the panel, regulated by the controller, throughout and filling the battery banks, into the inverter, powering the clean circuit and whatever I want to run.

I figure the cost to be $500 for the panels, $120 for the Inverter, 380 for the batteries, $100 for the charge controller, $150 for wires, connectors, misc. = $1250 w/ 500W of generation ~$1350 with 600W

--

I plan to use the system to run a small TV, my laptop and few things (about 500W total) during the day for 4-5 hrs at a time, 2-3 times a week as well as to keep my power tool batteries charged whenever I need them. (they see light use).

Please let me know what you think about the feasibility of such a system and my numbers. Thank you!
- Sam, The Solar Trough

[russ]

Why do you want to spend too much for your electricity?

[bcroe]

I want solar power somewhere in my home.
The circuit I have in mind is a dedicated 4-plug AC wall outlet ran to my living room

What I want to do is disconnect the clean circuit from the mains and wire it, with the 15 amp breaker, into a 24V, 900-watt inverter (1800w Surge, or 15 amps at 120VAC). I think this means the breaker should trip if more than the maximum surge power of the inverter is demanded.

My solar array would be two 250-watt panels (perhaps up to 300-watt panels) wired into a a 30-amp MPPT charge controller. The garage roof is south, southwest facing at 55~ degrees with zero shadow cast potential year round.

The battery bank would consist of two 12v, 35Ah batteries (wired parrallel) and hooked to a third 12v, 135Ah battery in series giving me a 24V, 102Ah Bank in total.

I would like to find a way to use the 24V battery bank to charge my power tools lithium ion batteries (both 24V, 2Ah and 4Ah). I realize I could plug the AC powered charging docks that came with the appliance into the power inverter, but I'd like to keep the load off of the inverter (unless I'm drawing power from inside on the living room circuit). I figure hooking them together with a diode to keep them from discharging with the rest of the bank might work? Any insight on this point would be great.

The flow chart of the system w/o the power tool batteries should be from the sun, to the panel, regulated by the controller, throughout and filling the battery banks, into the inverter, powering the clean circuit and whatever I want to run.

I figure the cost to be $500 for the panels, $120 for the Inverter, 380 for the batteries, $100 for the charge controller, $150 for wires, connectors, misc. = $1250 w/ 500W of generation ~$1350 with 600W

I plan to use the system to run a small TV, my laptop and few things (about 500W total) during the day for 4-5 hrs at a time, 2-3 times a week as well as to keep my power tool batteries charged whenever I need them. (they see light use).

Please let me know what you think about the feasibility of such a system and my
numbers. Thank you! - Sam, The Solar Trough

If you set up a grid tie net metering system, your utilization of your solar
energy generation would be over 90%, and every outlet in your house
would be available. The money saved on batteries & a charge controller
could be used to double the system energy collecting capacity.

Your battery series-parallel proposal would be an operational disaster.
The amp hours would be limited to 70, the total of the smaller batteries.
But since it would be impossible to keep them equalized, they would very
soon be dying. The cost of batteries will multiply the energy cost by 10,
and the batteries will be a constant maintenance project.

Batteries don't give back all the energy you put into them. Then you
lose more energy with an inverter, which will be even less efficient
if lightly loaded. Just having the inverter running all the time would
be a huge energy waste, you would need to turn it on only when needed.

On some days you might not use the solar energy, so with fully charged
batteries, it would be wasted. On other days you might need a lot more
energy than is available, so you would have to change power sources
when the batteries & inverter shut down.

The parts aren't anything like matched to your task. A 24V 70AH battery
could only deliver some 1500 Watt Hours lightly loaded, less if asked to
deliver a KW, and far, far less if operated in a way that won't destroy
them in a few days. Then more power is lost to the inverter. A 5 hour
load of 500W is 2500 WH. 600W of panels will only deliver that figure
a short time on ideal sunny days.

But a net metering system would solve all of these issues. Bruce Roe

[SunEagle]

Sam

I understand your desire to get into solar power. What bothers me that a lot of people come here and think they will save money by using a solar/battery system when they already have grid power. A battery system will cost 10 times what it would cost you getting power from the Utility

If you want to get into solar then listen to what Bruce is suggesting. Go with a gird tie system. It may cost more up front but it will pay for itself over a period of time.

Even if you went with a small off grid solar battery system you will need to better understand how to put one together as well as what it will cost you because your estimates are off. Also using low Ah 12volt batteries wired in parallel is a big mistake.

[TheSolarTrough]

Thank you so much for the replies - they've been extremely helpful!

Sam

I understand your desire to get into solar power. What bothers me that a lot of people come here and think they will save money by using a solar/battery system when they already have grid power. A battery system will cost 10 times what it would cost you getting power from the Utility

I never said I was trying to save money on power - just that I only have 1000-2000 dollars of money to spend on the system as a project. The system isn't going to be used for supplying the houses power - I'd try not to load more than 500w onto the circuit anyways. This is because my home uses 3000kwh per month (mostly because I have a home office and run 7+ computers). It would be completely impractical to reach half of my homes generation through solar power. Especially because I did a solar shadow study, and none of my other roof planes face south AND are free from shadow year round. I could probably fit 6-8 panels on the section of garage roof I'm talking about - maxing me out at a 2kw system or so, but I wouldn't qualify for the 30% tax credit without upwards of a 12kw system (many would not face south and be in shade) and replacing some appliances (something I can't afford right now).

I suppose I should mention what I AM trying to use it for. My office designs homes for track builders and custom clients. I want, in our meeting room, to have an outlet run to power the display TV, my laptop, and a few other small things. Luckily, the clean circuits AC outlet that faces into the living room is on a wall shared with my meeting room, so I can easily flip the outlet around to face into the my conference room.

The idea is to show them the small array, it's potential, and open up the discussion about solar design (passive AND active) in their homes. If I could get it to keep my power tools batteries charged at the same time, I'm all for it. Plus it adds an emergency power backup, wired directly into the house on its own breaker that would protect the inverter from oversurge. Or so I was thinking... it might be better to have an 1800w inverter, so that I can pull almost 15 amps before kicking the inverter into surge mode.

Of course, for larger scale applications using the grid as your 'bank' is optimal in the current economy, and I would encourage clients to look at larger systems w/o battery back-ups (or with small backups), but because I have a high demand and limited potential for generation (either by roof sq/ft or money) I want to keep the system isolated to my office (and my garage where the inverter and breaker box are; directly below the optimal roof plane). My desire may be misguided, however, because I'm unfamiliar with the practical costs of net metering. Ultimately, it's likely a more cost effective path to take, but not my direct goal.

Your battery series-parallel proposal would be an operational disaster.
The amp hours would be limited to 70, the total of the smaller batteries.
But since it would be impossible to keep them equalized, they would very
soon be dying. The cost of batteries will multiply the energy cost by 10,
and the batteries will be a constant maintenance project.

This is extremely helpful! I should say that I didn't intentionally use different types of batteries, but I can easily return the lower Ah ones. If I have two identical, 12v 135Ah AGM batteries, I should a have 24v, 135Ah Bank wired parallel or ~3200Wh potential. Correct? That jumps my battery costs from 380 to 520, but AGM should last a little longer than sealed lead acid, or so I've read.

I wouldn't want to drain the battery banks entirely (in my mind beyond 80% discharge), so around ~5 hours of 500w draw would be safe from a full charge. Longer if I use the system during the day when the array is refilling the batteries by ~300-400W per peak hour (4hrs per day in virginia). I take this to mean I could realistically see 1600W+ of generation on any given day, or half my battery banks potential storage (minus transmission losses and inefficiencies of the charge controller). Because I don't have client meetings every day, and they run 2-4 hours on average, this small system should work fine for those purposes and could keep my power tool batteries charged in the process.

Any insight on how to wire the power tool batteries (small lithium ions, 2Ah and 4Ah) into the battery bank? I could, like I said, plug them into the inverter with the included AC plug, but I'd rather not seeing that incurs its own losses and strain on the AC side of the system. Surely there's a way to use a big 24V bank to charger smaller 24V batteries without compromising the bank? And without draining the small lithium batteries when I drain the larger batteries...

Bruce, how practical is a 500-600W grid tied system? I wouldn't expect more than 45kw a month, or <5 dollars of savings (@.11kwh). I suppose going for a 1.5kw, grid-tied system would be better, but like I said - no tax credit for me to offet installation costs.

My adjusted costs are: $500/600 for panels, $130 for the Inverter, $520 for the batteries, $200 for the mppt charge controller, $150 for wires, connectors, misc. = $1500 w/ 500W of generation ~$1600 with 600W

I am confident of my ability to install and properly seal two panels, run the wires to my garage, hook up the charge controller and revised battery bank. I could hook the inverter up to the bank and draw from it fine, but wiring the smaller batteries into the system and wiring the inverter into my breaker box is a bit beyond me. For the small batteries, I'm hoping to find assistance online, but I'd hire an electrician to review the hookups to my breaker box, so add on another $250 for that. Still brings me in under $2000 dollars. I might also spring for a 40amp charge controller so I can expand the solar array in the future.

Edit: could my system charge my bank, be wired into a home circuit, AND have a back flow meter for the excess? Because that would net me the losses incurred from having no load on the system and the batteries fully charged on a sunny day, even if it was minimal.

[bcroe]

I never said I was trying to save money on power - just that I only have 1000-2000 dollars of money to spend on the system as a project. The system isn't going to be used for supplying the houses power - I'd try not to load more than 500w onto the circuit anyways. This is because my home uses 3000kwh per month (mostly because I have a home office and run 7+ computers). It would be completely impractical to reach half of my homes generation through solar power. Especially because I did a solar shadow study, and none of my other roof planes face south AND are free from shadow year round. I could probably fit 6-8 panels on the section of garage roof I'm talking about - maxing me out at a 2kw system or so, but I wouldn't qualify for the 30% tax credit without upwards of a 12kw system (many would not face south and be in shade) and replacing some appliances (something I can't afford right now).

The idea is to show them the small array, it's potential, and open up the discussion about solar design (passive AND active) in their homes. If I could get it to keep my power tools batteries charged at the same time, I'm all for it. Plus it adds an emergency power backup, wired directly into the house on its own breaker that would protect the inverter from oversurge. Or so I was thinking... it might be better to have an 1800w inverter, so that I can pull almost 15 amps before kicking the inverter into surge mode.

Of course, for larger scale applications using the grid as your 'bank' is optimal in the current economy, and I would encourage clients to look at larger systems w/o battery back-ups (or with small backups), but because I have a high demand and limited potential for generation (either by roof sq/ft or money) I want to keep the system isolated to my office (and my garage where the inverter and breaker box are; directly below the optimal roof plane). My desire may be misguided, however, because I'm unfamiliar with the practical costs of net metering. Ultimately, it's likely a more cost effective path to take, but not my direct goal.

This is extremely helpful! I should say that I didn't intentionally use different types of batteries, but I can easily return the lower Ah ones. If I have two identical, 12v 135Ah AGM batteries, I should a have 24v, 135Ah Bank wired parallel or ~3200Wh potential. Correct? That jumps my battery costs from 380 to 520, but AGM should last a little longer than sealed lead acid, or so I've read.

I wouldn't want to drain the battery banks entirely (in my mind beyond 80% discharge), so around ~5 hours of 500w draw would be safe from a full charge. Longer if I use the system during the day when the array is refilling the batteries by ~300-400W per peak hour (4hrs per day in virginia). I take this to mean I could realistically see 1600W+ of generation on any given day, or half my battery banks potential storage (minus transmission losses and inefficiencies of the charge controller). Because I don't have client meetings every day, and they run 2-4 hours on average, this small system should work fine for those purposes and could keep my power tool batteries charged in the process.

Any insight on how to wire the power tool batteries (small lithium ions, 2Ah and 4Ah) into the battery bank? I could, like I said, plug them into the inverter with the included AC plug, but I'd rather not seeing that incurs its own losses and strain on the AC side of the system. Surely there's a way to use a big 24V bank to charger smaller 24V batteries without compromising the bank? And without draining the small lithium batteries when I drain the larger batteries...

Bruce, how practical is a 500-600W grid tied system? I wouldn't expect more than 45kw a month, or <5 dollars of savings (@.11kwh). I suppose going for a 1.5kw, grid-tied system would be better, but like I said - no tax credit for me to offet installation costs.

My adjusted costs are: $500/600 for panels, $130 for the Inverter, $520 for the batteries, $200 for the mppt charge controller, $150 for wires, connectors, misc. = $1500 w/ 500W of generation ~$1600 with 600W

I am confident of my ability to install and properly seal two panels, run the wires to my garage, hook up the charge controller and revised battery bank. I could hook the inverter up to the bank and draw from it fine, but wiring the smaller batteries into the system and wiring the inverter into my breaker box is a bit beyond me. For the small batteries, I'm hoping to find assistance online, but I'd hire an electrician to review the hookups to my breaker box, so add on another $250 for that. Still brings me in under $2000 dollars. I might also spring for a 40amp charge controller so I can expand the solar array in the future.

Edit: could my system charge my bank, be wired into a home circuit, AND have a back flow meter for the excess? Because that would net me the losses incurred from having no load on the system and the batteries fully charged on a sunny day, even if it was minimal.

So you want to spend a couple $K learning about solar, fair enough. My first rule is
use batteries only if there is no other way. Every region has its rules for net metering,
its a good project to see what your options are. Your output is limited by your panels,
most start at multi KW. Clouds will kill the days output. So will snow, if you don't
clear it, one reason I prefer ground mount.

If you want to learn about batteries, you aren't concerned about saving money. Probably
only half your panel output will get to the appliances, at 10 times the cost. But to avoid
a total wipe out, read up on the care of batteries. They don't like 80% discharge, they
REALLY don't like being left in a mostly discharged state for very long, they
need to be periodically equalized, and you will need a charger to bring them back the
first time there is a cloudy week. The harder you discharge them, the less of the charge
energy will be returned. Don't parallel batteries, and be prepared to keep a close watch
on their condition. You need to hear from the real battery experts and off grid types here.

If you have groups of panels, each group should have the same voltage and shading.
Otherwise each group will need another set of charge controls. That problem goes
away with net metered micro inverters (one per panel).

My net metered array generated more energy in June, than you used. It won't in Dec.
Solar makes a lousy backup, the sun doesn't shine when YOU need it. A battery limits
you to a microscopic amount of energy. Get a generator, I did 20 years ago. If power
really has an extended outage, you will need the generator to save the batteries. A
transfer arrangement to the rest of your house requires special equipment and
expertise; it generally wouldn't done on such a small battery system.

Many battery operated appliances have available chargers that work directly off a (car)
battery. If you can't get one with 24 VDC input, a very small (and efficient) DC to DC
converter can supply 12V from your 24V bat when needed. Your expectations of
running appliances off a modest battery are very optimistic. good luck, Bruce Roe

[SunEagle]

Bruce

Nice response. You did a much better job explaining the facts of life of a battery system than I could have.

What the OP wants and what can be achieved are very far apart. I hope he learns and understands what he is trying to do will not work or will cost a lot of money. But again it is his money to burn.

[bcroe]

Bruce
Nice response. You did a much better job explaining the facts of life of a battery system than I could have.

What the OP wants and what can be achieved are very far apart. I hope he learns and understands what he is trying to do will not work or will cost a lot of money. But again it is his money to burn.

I'll bet Sunking is chuckling at all this. I have learned enough about batteries
here to realize, all my recharge/float battery chargers need to redesigned. Bruce

[Sunking]

I'll bet Sunking is chuckling at all this.

Correct sir. What the OP wants to do is laughable.

[TheSolarTrough]

So you want to spend a couple $K learning about solar, fair enough. My first rule is
use batteries only if there is no other way. Every region has its rules for net metering,
its a good project to see what your options are. Your output is limited by your panels,
most start at multi KW. Clouds will kill the days output. So will snow, if you don't
clear it, one reason I prefer ground mount.

If you want to learn about batteries, you aren't concerned about saving money. Probably
only half your panel output will get to the appliances, at 10 times the cost. But to avoid
a total wipe out, read up on the care of batteries. They don't like 80% discharge, they
REALLY don't like being left in a mostly discharged state for very long, they
need to be periodically equalized, and you will need a charger to bring them back the
first time there is a cloudy week. The harder you discharge them, the less of the charge
energy will be returned. Don't parallel batteries, and be prepared to keep a close watch
on their condition. You need to hear from the real battery experts and off grid types here.

If you have groups of panels, each group should have the same voltage and shading.
Otherwise each group will need another set of charge controls. That problem goes
away with net metered micro inverters (one per panel).

My net metered array generated more energy in June, than you used. It won't in Dec.
Solar makes a lousy backup, the sun doesn't shine when YOU need it. A battery limits
you to a microscopic amount of energy. Get a generator, I did 20 years ago. If power
really has an extended outage, you will need the generator to save the batteries. A
transfer arrangement to the rest of your house requires special equipment and
expertise; it generally wouldn't done on such a small battery system.

Many battery operated appliances have available chargers that work directly off a (car)
battery. If you can't get one with 24 VDC input, a very small (and efficient) DC to DC
converter can supply 12V from your 24V bat when needed. Your expectations of
running appliances off a modest battery are very optimistic. good luck, Bruce Roe

As a back up, generators can't be matched - I've got a 17.5kw gas generator and an automated switch for real emergencies, so I can't REALLY justify the small bank as a back up, but it would still be available if gas wasn't. Of course, if i'm having trouble buying gas there's likely more immediate problems than a 3kwh battery bank could handle...

Unfortunately for me, in Virginia net metering is available to customers of investor-owned utilities (including competitive suppliers) and electric cooperatives, but not to customers of municipal utilities (like I use). Looks as if I'll waste generation from any small system, unless I can get over 4kw of panels AND qualify for a limited program. Oh well, it's not a complete deterrent

I have the house, terrain, and trees modeled in 3D and I've run a simulation to see where sunlight and shadows will be cast over the course of a year.
I used this information to determine the optimal location for my panels; only one was clear of shadows year round AND facing south. That, plus my budget limitations, keep me from springing for a large enough system to tie it to the grid, but even then because of my energy usage I'd fall short of the 30% Federal Tax Credit because it requires half of my homes energy use be from solar (and that would require using most of the roof).

There's enough room for two, or even four, solar panels - so up to 1kw of panels or so - all on the same roof plane in direct sunlight. I could use two smaller charge controllers for each panel, but the voltage should be fine because of the limited potential for shadows (clouds and planes). I read about products from companies like SolarEdge that use a controller for each panel, but Id have too few panels to justify the expense of the better monitoring system (plus the gains id have would be lost after charging the bank at float).

I appreciate the insight y'all have into battery tech, but I suppose I'm left scratching my head. I'd expect to use 500w for 2-4 hours, 1000-2000w drains of a 3200W bank or 30-60 percent discharge at any given time (never more than 80%). This would also be during the day while PV generation is happening, so wouldn't even less of the battery actually be discharged if I limited the use to daytime?

I would expect to generate about 40-80% of my battery banks capacity (1500-2000w) on any given day, so wouldn't it follow that using half the bank one day and then not the next would give the battery an opportunity to recharge? If the circuit was left unused, wouldn't the solar array keep my batteries charged until I need them? (while wasting the excess due to a lack of net-metering options w/ my utility company)

Why would I need a charger to bring them back after a cloudy week? wouldn't the panels generate enough voltage at high enough current to more-or-less fully recharge them? Is there a limitation to the charge controller; can't most overcharge lead acid batteries that have been left discharged for an extended period? Do they make charge controllers that have 'equalization' modes? Aren't AGM and other sealed batteries not supposed to be equalized because of the potential to explode?

Oh well, I can't expect you to take all the aspects and types of equipment I want to use into account (especially since I am changing them with your help!). I'll have to read more about sulfation and battery types to determine all this for myself.

Thank you for your insight and time, you've given me a lot to think about.
-Sam

[daz]

As a back up, generators can't be matched - I've got a 17.5kw gas generator and an automated switch for real emergencies, so I can't REALLY justify the small bank as a back up, but it would still be available if gas wasn't. Of course, if i'm having trouble buying gas there's likely more immediate problems than a 3kwh battery bank could handle...

That would definitely be true!

I appreciate the insight y'all have into battery tech, but I suppose I'm left scratching my head. I'd expect to use 500w for 2-4 hours, 1000-2000w drains of a 3200W bank or 30-60 percent discharge at any given time (never more than 80%).

Lead acid batteries should never be discharged more than 50%. Going over the 50% shortens the battery's lifespan more than the power you get out of the battery. A lot of people design for 20% or 30% discharges, as the battery lasts alot longer! Have you had a read in the batteries section of the forum? If not, I would recommend that you go and have a read there, as there is a lot of information on batteries there. Also have a look at the stickies there, they have a goldmine of information that will help you understand battery bank design and use.

Why would I need a charger to bring them back after a cloudy week? wouldn't the panels generate enough voltage at high enough current to more-or-less fully recharge them?

Simple answer....solar systems are not the world's most reliable systems (well not without spending a fortune). Solar depends on the sun...which means you are subject to the weather.....
When there is insufficient sun, your batteries will not be charging, and if you are using your system, your batteries will be depleted. Solar panels rely directly on the intensity of the sun, so cloudy days equals less power (and raining days pretty much equal no power).

To counter the dependency on the sun, people with solar systems generally have a generator, or if mains power is available, a charger. This is to ensure that you can fully recharge your batteries during periods where the solar panels are unable to.


I would recommend that you go to the off-grid section of the forum and have a look at the stickies. You will find a lot of useful information there, and it will make what everyone is mentioning here more understandable.


PS...a solar system equals another full time job!

[bcroe]

PS...a solar system equals another full time job!

I do hope, that only applies to battery systems, once a system is fine tuned. The snow is
still a project, but I'm already out there cleaning walks & drives anyway.

Wanted to mention, there is no absolute requirement that panels face south. My panels
face several directions, raising the daily energy collected by 50% both under good sun,
and esp under clouds. Simulations can predict what the performance of each group will
be, a more fun project than batteries. Bruce Roe