First Post, New House Build - Review my assumptions and point me in the right direction

voland

New Member
Joined
Apr 18, 2023
Messages
7
Location
North Texas
Good day,

This is my first post so please forgive the wall of text…

Im primarily looking for validation of my assumptions and some opinions if I am not thinking about this correctly. Keep in mind that I am fairly new to this and I will be doing a lot of reading and watching youtube for information to gain better understanding of these systems.

Background and data Ive gathered so far

Im building a new house in a very rural part of Texas and I am not 100% sure how reliable the power situation is going to be. My goal is to have a solar system that can run a large portion of the house during the day to offset my electric usage, under optimal conditions. Additionally, I would like to have enough battery capacity to heat/cool a bit of space for 3 days, in case of an emergency/power down type situation. Assuming some sunny days, I would like to have this system run pretty reliably "indefinitely". By that, I mean as long as the battery life cycle will permit. Yes, the pessimist in me wants to have a plan B…

Under normal conditions, my current and fairly inefficient house, used 7.2 kWh during the highest use hour of the highest use day which used a total of 129.8 kWh for the day. I expect this house to do a lot better since it will be better insulated, have more efficient HVAC systems (likely heatpump) etc. Nevertheless, I am basing my calculations on this…

1682011130605.png





The house plans allow for roughly 22 panels to be deploy with a south/south-east exposure(~150 degrees).

1682011145782.png


Using 400w panels, I should be able to achieve a peak ~8800w give or take a few. I can see about getting higher producing panels but lets use this for the sake of my calculation… I would like to have some extra panels in the garage that I could add in case of a prolonged grid down scenario to augment the system so I would get larger inverters than needed to handle the extra panels. Eventually, I will have an electric car, so I would like to wire up the charger to allow to backfeed from the cars battery too.

Below are my estimated loads I got using a simple load calculator and Im not sure how accurate are the ratings for the devices…

This is what I would consider to be a high running load that I could see during the day. Looks like my max load would be approximately 5.2kWh

1682011164802.png



This is what I would expect to run during a grid down scenario where I would have to rely on battery power to provide coverage after "dark". Looks like I would be using approximately 4.7kWh per night.

1682011174901.png


Based on these assumptions, here's the system that I am proposing…

22 400w panels on roof
8 400w panels on ground (if needed)
12 kWh inverter
14 32 kWh battery bank
charge controller (not sure what kind yet)
My "off grid" loads would be wired to a critical loads panel that would be energized by the battery bank
Optional 14kw propane generator on a 1000 gallon tank to charge up/run the house to help run the house while the propane lasts?



Questions:

1 - Am I even close to on target or am I talking nonsense?
2 - I was going to use a high efficiency 12000 btu heat pump for my heating cooling needs. I was thinking of doing a dedicated ductless system just for the master bedroom which is the room Ill want to keep heated/cooled. Would it be better to use a ducted system and heat/cool the bathroom as well? Should I look at other options?
3 - Fridges and Freezers don’t really run 24x7. They cycle. Does the kWH rating means its running non stop? Should I be using a % of that load for my calculations?
4 - What should I reasonably expect to generate from a 400w panel on a 12-12 slope facing south/south-east on a sunny day?

5 - If you read to this point, and you had similar requirements, would you do anything differently?


Thank you in advance for everyone's time and comments!
 
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Im in NE Texas. I have off grid 12kw and a 30kwh batt bank. using a 2 ton mini split it is sometimes hard to make it 1 night. not uncommon to use the grid to supplement during summer and even winter using heat
I suspect you are undersized on both panels and batts
being rural, can you do a ground mount instead of roof ?
 
Im in NE Texas. I have off grid 12kw and a 30kwh batt bank. using a 2 ton mini split it is sometimes hard to make it 1 night. not uncommon to use the grid to supplement during summer and even winter using heat
I suspect you are undersized on both panels and batts
being rural, can you do a ground mount instead of roof ?

Unfortunately, its not going to be possible. As far out as it is, this group of homes has an HOA (its a lake subdivision) so there are restrictions.

One think to add is that we would likely not keep the ac at 71 when the power is out. We would likely keep it at 77 just to make it livable... Same thing for heat. Would likely keep it at 60 degrees. Hopefully that will reduce enough to make it work. This is the system I was looking at... https://signaturesolar.com/eg4-hybrid-ac-dc-solar-air-conditioner-1-ton/


ETA, I already found a mistake. I sized the battery bank to fully discharge and it needs to be closer to 30% discharge...
 
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the easiest way to get true load data is use a kill-a-watt meter, run the items you use for a week and you will have real results

on larger loads you will have to use a amp meter like your a/c unit the kill-a-watts only good for 15 amps max
 
the easiest way to get true load data is use a kill-a-watt meter, run the items you use for a week and you will have real results

on larger loads you will have to use a amp meter like your a/c unit the kill-a-watts only good for 15 amps max

I have one on order to figure out my small devices. For the larger appliances, since its a new build, ill have rely on manufacturer specs to start. We are still in design phase so tough to know what we will end up with.
 
You don't say where you are. Suggest PVWatts as a tool to give you location specific PV power estimates - https://pvwatts.nrel.gov/pvwatts.php

I have a 13.7kw pv array (e.g. 48 panels @ 285w each) and my PVWatts Estimate is this:
1682012549818.png

And my actual for 2021, and 2022 numbers are like this. The Green column is the useable power out of the inverter the home consumed.
1682012767643.png

A really big issue is do you want to overpower to be 100% offgrid in winter (if you have winter) and perhaps waste power or sell back to the grid in summer OR hit somewhere in the middle. For me, I get 25% in winter of what I get in summer. 25% in winter is not nearly enough for me to power my home. Just look at December - 262kwh and 397kwh for the month respectively vs 1700 and 1900 kwh/month in summer time.
 
You don't say where you are. Suggest PVWatts as a tool to give you location specific PV power estimates - https://pvwatts.nrel.gov/pvwatts.php

I have a 13.7kw pv array (e.g. 48 panels @ 285w each) and my PVWatts Estimate is this:
View attachment 145677

And my actual for 2021, and 2022 numbers are like this. The Green column is the useable power out of the inverter the home consumed.
View attachment 145678

A really big issue is do you want to overpower to be 100% offgrid in winter (if you have winter) and perhaps waste power or sell back to the grid in summer OR hit somewhere in the middle. For me, I get 25% in winter of what I get in summer. 25% in winter is not nearly enough for me to power my home. Just look at December - 262kwh and 397kwh for the month respectively vs 1700 and 1900 kwh/month in summer time.

Yeah... I guess Im way off base here...

This is what I came up with for an 8kw system... Im between Corsicana and Palestine, TX with a 12/12 pitch roof or 45% with the azimuth of 150... Booo...

1682014051517.png
 
Yeah... I guess Im way off base here...

This is what I came up with for an 8kw system... Im between Corsicana and Palestine, TX with a 12/12 pitch roof or 45% with the azimuth of 150... Booo...
I'm going to agree with @Texican that you are likely undersized. It's a crap shoot when you don't have live data from actual use and have to go with estimates and manufacturer specs, but I think it's still doable. Depending on your budget, my advice is to figure out what your bare minimum requirements are going to be for both overall production and battery storage then add at least 50%. IMO, most of us will never achieve 100% offgrid but having the basic necessities covered for three days is way better than having nothing, or only a generator.

I'm close to Livingston so I'm not too far south of you. I put my panels on the shop roof with maybe a 1/12 pitch, almost flat, but I still get pretty decent production. But it sounds like your HOA is going to hamper any efforts at expanding beyond what you've described.

Whatever the case, there are a ton of people on here who are willing to help you try to figure out the best path forward.
 
Ok, so let me rephrase the question... JUST to run one of these small ductless minisplit systems going for 4 hours per night for 3 nights?

It looks like they use about 1kWh so I would need 3x400kW panels tied to a battery system that could provide 12kWh or really 24kWh with a 50% discharge. During the day, if I went with 6 panels, I could theoretically be charging the batteries AND running the AC at the same time.

Does this check out?

I think I would still want to install a full array like I indicated in my OP, but I would cut down my after dark use to strictly the AC, I should be in an OK shape. With an 8kW array, I should be making enough during the day to do what I want.
 
Ok, so let me rephrase the question... JUST to run one of these small ductless minisplit systems going for 4 hours per night for 3 nights?

It looks like they use about 1kWh so I would need 3x400kW panels tied to a battery system that could provide 12kWh or really 24kWh with a 50% discharge. During the day, if I went with 6 panels, I could theoretically be charging the batteries AND running the AC at the same time.

Does this check out?

I think I would still want to install a full array like I indicated in my OP, but I would cut down my after dark use to strictly the AC, I should be in an OK shape. With an 8kW array, I should be making enough during the day to do what I want.
Consumption for the mini split is going to vary depending on a number of factors - the area you're cooling, obviously the time of year/ambient temperature, the thermostat setting, etc. I have a 1 ton (12k BTU) Mitsubishi on the garage and it pulls around 5 to 6 amps when it's under full load but drops way down most of the time - less than 1 amp - but I haven't ran it during the summer yet. That said, an 8kw array isn't going to have a problem carrying it when it's sunny. If you go with LifePO4 batteries you can discharge 80% vs the 50% on lead acid. The battery capacity needed is going to vary based on all those same factors. The ideal solution would be if you already had the mini split and could put a meter on it for a week or two during peak usage. Since that's not an option it's going to be a guess no matter how you go about it. A 24kWh battery bank could last multiple days without sun if all it was running was the mini split and temps were moderate. Rainy days in a row in August will eat into your battery charge relatively quickly.

None of this gives you a firm answer but that's kind of the nature of the beast when you don't have usage data to calculate needed production and battery capacity.
 
2 examples.....

Trailer: I run a 9000BTU Senville Minisplit on my 7 x 14 cargo trailer -> camper. It has 1" insulation (R5) all around (walls, ceiling, floor, nooks/crannies) and no windows = R5. The unit maxes out at 1100w. For mild temps 50Flow and 90F hi - it takes ~8kwh for 24hrs. As the temps get more extreme to 20F low... it runs 1100w all the time and get's into the 22kwh for 24hrs range.

Home:
Here's a climate zone definition - https://inspectapedia.com/heat/Heating-Cooling-Climate-Zone-BTU-Requirements.php
1682177922476.png
I run a hi efficiency 4-ton heat-pump for a 2600sq foot home that's reasonable insulated - let's say R25. I'm in zone 4 climate - e.g. winter lows of 30F and 50F hi are routine. At 30F ambient + R25, the house looses a degree of heat per hour kind of thing. The unit burns 1300-1500kwh a month for the 3 winter months - e.g. ~45kwh/day - to maintain 70F at night and 75F during day.

----------------------------
- "Rural Texas covers 3 different climate zones - can you describe you're climate zone 4, 3, or 2?
- Sq Ft + insulation level? *Insulation (and passive solar design) really make a difference. Don't know the curves but R50 vs R25 would use way less power - maybe 30-40% less? (just making that up) but it's significant.
- Do you plan heat-pump as the primary heat/cool?

Based on this I think one can start to make realistic operating estimates for design purposes.
 
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I run a 9000BTU Senville Minisplit on my 7 x 14 cargo trailer -> camper. It has 1" insulation all around (walls, ceiling, floor, nooks/crannies) and no windows = R5. This maxes out at 1100w.

For mild temps 50Flow and 90F hi - it takes ~8kwh for 24hrs. As the temps get more extreme - say 20F low - it runs 1100w all the time and get's into the 22kwh/24hrs range.
That's good real time data. (y)
 
2 examples.....

Trailer: I run a 9000BTU Senville Minisplit on my 7 x 14 cargo trailer -> camper. It has 1" insulation (R5) all around (walls, ceiling, floor, nooks/crannies) and no windows = R5. The unit maxes out at 1100w. For mild temps 50Flow and 90F hi - it takes ~8kwh for 24hrs. As the temps get more extreme to 20F low... it runs 1100w all the time and get's into the 22kwh for 24hrs range.

Home:
Here's a climate zone definition - https://inspectapedia.com/heat/Heating-Cooling-Climate-Zone-BTU-Requirements.php
View attachment 145949
I run a hi efficiency 4-ton heat-pump for a 2600sq foot home that's reasonable insulated - let's say R25. I'm in zone 4 climate - e.g. winter lows of 30F and 50F hi are routine. At 30F ambient + R25, the house looses a degree of heat per hour kind of thing. The unit burns 1300-1500kwh a month for the 3 winter months - e.g. ~45kwh/day - to maintain 70F at night and 75F during day.

----------------------------
- "Rural Texas covers 3 different climate zones - can you describe you're climate zone 4, 3, or 2?
- Sq Ft + insulation level? *Insulation (and passive solar design) really make a difference. Don't know the curves but R50 vs R25 would use way less power - maybe 30-40% less? (just making that up) but it's significant.
- Do you plan heat-pump as the primary heat/cool?

Based on this I think one can start to make realistic operating estimates for design purposes.
I would be classified in Zone 3. Im between Corsicana and Palestine. Richland Chambers Lake area.

The house exterior is going to be a 2x6 with 4" of spray foam insulation which typically ends up being more like 5.5" due to expansion. I dont know what the actual R value would be on that.

The entire house will be ~3500 square feet with ~3000 being downstairs and 500 up. The space Im primarily interested in cooling is < 300 square feet. See image below: Its a ~16.5x17 room. The French door that you see there will be replaced with a traditional door. The fire place will be replaced by a wood burning stove of some sort so it will not leak air so bad. This is why my focus is primarily on cooling...

1682437679035.png


Ideally, Id like to have this set up so the system can offset my daily usage under normal circumstances but if the grid was down, Id be able to get a little AC going during the hottest months and keep the freezer and refrigerator going as well. That's really my goal.

Thank you for jumping in!
 
Consumption for the mini split is going to vary depending on a number of factors - the area you're cooling, obviously the time of year/ambient temperature, the thermostat setting, etc. I have a 1 ton (12k BTU) Mitsubishi on the garage and it pulls around 5 to 6 amps when it's under full load but drops way down most of the time - less than 1 amp - but I haven't ran it during the summer yet. That said, an 8kw array isn't going to have a problem carrying it when it's sunny. If you go with LifePO4 batteries you can discharge 80% vs the 50% on lead acid. The battery capacity needed is going to vary based on all those same factors. The ideal solution would be if you already had the mini split and could put a meter on it for a week or two during peak usage. Since that's not an option it's going to be a guess no matter how you go about it. A 24kWh battery bank could last multiple days without sun if all it was running was the mini split and temps were moderate. Rainy days in a row in August will eat into your battery charge relatively quickly.

None of this gives you a firm answer but that's kind of the nature of the beast when you don't have usage data to calculate needed production and battery capacity.


This is all really good information. Thank you!
 
Welcome to the forum, and congratulations on your new house build in rural Texas! It's awesome that you're considering a solar system to offset your electric usage and ensure reliable power, even in emergency situations. While I'm not an expert, it seems like you've done some solid research and calculations. However, for more accurate information and guidance specific to your needs, I recommend consulting professionals or specialized forums. Websites like getmanandvan.co.uk might be a good resource for finding experts who can provide insight into solar systems and energy solutions for your area.
 
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