Off Grid System for Man Cave

MrBill-12

New Member
I live in a modest 1600 square foot home in a typical middle class neighborhood in Pensacola, Florida (Latitude 30°). We have an addition off the back of the house, 20’ x 20’ sun room where we spend 95% of our waking hours. We watch TV in the mornings and evenings, and even eat our meals on TV trays. Seems like it would make economic sense to design a small off-grid system to run everything in the room, including a small Midea 8000 BTU window air conditioner (450 watts draw on high setting), as well as two or three LED lights, TV, stereo receiver, and 4 mobile device chargers. Think of it like a small one-room cabin. This way we could leave the main house A/C system set to 80°F-85°F and save a lot of money over our six-month summer season from May through October. During this summer season, I have calculated the system would need to accommodate 7000 watt hours per day with a peak load of 810 watts.

Another use would obviously include emergency power during 5-10 day power outages for hurricane season.

Any thoughts or suggestions? Has anyone else attempted a partial off-grid solar system for a portion of their homes? Do you think it makes sense to do this?

Thanks in advance for reading and commenting!

Bill
 
D

Deleted member 23531

Guest
I think you and the rest of Florida are in the 5-5.5hrs insolation region, so on average you would need only a 1.4 kW system plus some extra margin for loses and cloudy days and the cost of that would not be that high (3kW is probably the smallest it makes sense to have professionally installed; you should consider doing that).

Batteries are way more expensive than solar panels right now. Emergency power for 5-10 days would require a gigantic battery. Even if you conserved your usage slightly during that time, 10 days * 5kWh/day = 50kWh which is 3-4 Tesla Powerwalls, or a massive (by DIY scales) 1000Ah * 48V battery. I think if you're losing power for that many days at a time you should just get a gas generator (or move somewhere else, or go on a vacation to another state when a hurricane knocks out your power, or give us a minimal energy usage number you can see yourself needing during an emergency). EDIT: this estimate assumes no solar production during that time; in reality the sun would still shine even during hurricane conditions, so you would continue to collect at least some energy. Sorry I assumed running everything just off of the battery, like if your panels got knocked offline by a hurricane.

For non-emergency uses, let's assume it is sunny enough and you buy enough panels to run everything and fully charge up the battery each day; this is reasonable given it's Florida, and you don't need that many panels for this usage, and panels are cheap. Then you really just need enough battery capacity to last you through the evening / night when the sun is no longer shining. So let's say that's 8 hours of A/C overnight at your average usage of 291W (7kWh/day / 24 hours/day). So you would want roughly 291W * 8hrs = 2.3kWh of battery. This is 100Ah at 24V and is a reasonable size battery to buy off the shelf, for example here's a 2.6kWh battery. You would want to refine all your energy usage estimates before you purchase to make sure that would be big enough. In particular, assume 90% inverter efficiency, and that you probably only want to use 10%-90% state of charge, or 80% of the total battery capacity so it lasts many years.

810 watt peak load is small and that's no problem for a small inverter or to draw from a battery like this. I bet it's actually higher than that when you factor in your A/C starting up, but it seems like you're capable of measuring it so just go with your measurements (ex: use a Kill-A-Watt or similar device). But anyway this side of the equation is reasonable: a 1.2 - 2.4kW inverter is not that expensive compared to the batteries and solar panels.
 
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svetz

Works in theory! Practice? That's something else
Welcome to the forums!

The place to start is an energy audit. After that I'd suggest a hybrid inverter, possibly the Skybox? Ideally a day's worth of battery and possibly a generator to recharge for day 2 if there isn't enough solar. If you can get natural gas where you live (and it doesn't freeze like Texas), go that way as then you don't have to haul fuel. Or go propane, a few 20 pound tanks (spares for your grill), should last a while and unlike gasoline never goes bad.

... I have calculated the system would need to accommodate 7000 watt hours per day with a peak load of 810 watts.
Assuming you've completed the energy audit and are happy with the numbers, you'll need to know the maximum inrush current for your AC. The inverter will need to supply the inrush or you'll need a softstarter. But check, I believe Midea makes minisplits, so if yours has an inverter the surge might be very small. Also beware that if the Air Conditioner is 120V and you have a 240V inverter, the inverter may only put 1/2 the surge capability on each phase.

You didn't say, but let's assume that 7000w/d calculated is AC power. If the inverter is 94% efficient, the batteries 96% efficient, and the MPPT 98% efficient, then the DC power needed is 7000 W/d AC / .94 / .96 /.98 = ~8000 W/d Solar DC.

Example math:
We'll assume you're optimized for the whole year since Florida has great net metering agreement. A net metering agreement will let you sell excess power to the grid for credit so it's the best bang for the buck.

From the insolation data to the right, 4.51 is the yearly minimum. You want 7000 watt hours per day, so 7000 / 4.51 = 1552 watt array (round up) or about 5x 310 Watt panels.

In April, those same panels would make 6.11 x 1552 = 9,480 wh/d. With a net-metering agreement, you can bank those credits -- which means you don't really need a 1552 watt array. You can split the difference somewhere in the middle.

So how many panels do you actually need? That's up to you, but at least you have some math to play around with it.
1615759240561.png

For more accurate power generation look at SAM or PVWatts.
One last bit on that... those numbers are the average. During a hurricane outage you're guaranteed to be overcast and rainy so you won't be seeing anywhere near as much power. Typically folks will have a one or more days of battery to carry them through, then possibly a generator for extended bad conditions.

How many batteries?
Let's assume you want one day of batteries and will use a generator to augment the solar recharging beyond that, so you need 8000 wh of DC. You don't want to discharge the battery to deeply, otherwise it can impact the lifespan. For LiFePO4, you can typically go with a depth of discharge around 80%, so 8000 / .8 = 10 kWh batteries. Kilovault sells batteries for about $1/W. The Tesla powerwall is a turnkey solution for a bit more. If you're a DIYer check out Will's videos for how to build one. Check out the battery FAQ too, these numbers are representative of LiFePO4, if you use lead acid there are a number of other factors to consider.

If you're only using your batteries for emergency purposes, you might not want LeFePO4. LiFePO4 doesn't like to be fully charged all the time, lead acid does. Again, look through the Battery FAQ and this thread, ask any questions you have. Finally, check out the FAQ index, lots of examples that will answer a lot of questions.

Hope that helps and hope to hear a lot more from you!
 

rcrracer

Solar Enthusiast
I live in a modest 1600 square foot home in a typical middle class neighborhood in Pensacola, Florida (Latitude 30°). We have an addition off the back of the house, 20’ x 20’ sun room where we spend 95% of our waking hours. We watch TV in the mornings and evenings, and even eat our meals on TV trays. Seems like it would make economic sense to design a small off-grid system to run everything in the room, including a small Midea 8000 BTU window air conditioner (450 watts draw on high setting), as well as two or three LED lights, TV, stereo receiver, and 4 mobile device chargers. Think of it like a small one-room cabin. This way we could leave the main house A/C system set to 80°F-85°F and save a lot of money over our six-month summer season from May through October. During this summer season, I have calculated the system would need to accommodate 7000 watt hours per day with a peak load of 810 watts.

Another use would obviously include emergency power during 5-10 day power outages for hurricane season.

Any thoughts or suggestions? Has anyone else attempted a partial off-grid solar system for a portion of their homes? Do you think it makes sense to do this?

Thanks in advance for reading and commenting!

Bill
Another Lat 30 in FL here and owner of a Midea 8000 BTU u-shaped . Using it to cool a 1450 sq. ft. home. (450 watts draw on high setting) isn't how inverter units function. Last summer when it 102F my Midea used 750 watts, as the specs state it will. Typically you set a temp, auto fan, eco, cool and let the Midea computer program decide what to do. It will maintain a temp. within a 1.5 degree or so range. Normally central heat pumps have the thermostat positioned near a return air. Mideas thermostat is in the remote, so I set mine by the air intake. At the lowest wattage usage while the compressor is on , the Midea uses around 100 watts. The delta T is around 13 degrees. If that output is 4K(?) BTUs then that is 25 watts per BTU. When working at max 750 watts and output of 8K BTUs, that's 94 watts per BTU. So try to keep whatever inverter tech, be it a window AC or a mini-split, working at a low wattage. Don't turn the unit off, allow the room to reach a high temp, and turn the back on with a low temp setting. Just inch the temp down to the your low setting, while keeping the wattage low. Another quirk about the Midea. When running at low wattage every few minutes it will ramp up to 410 watts for a minute or so. Apparently not enough lubricant circulates when the compressor is running at a low frequency so the unit ramps up to 60 cycles or so the distribute the lubricant.
I live in a modest 1600 square foot home in a typical middle class neighborhood in Pensacola, Florida (Latitude 30°). We have an addition off the back of the house, 20’ x 20’ sun room where we spend 95% of our waking hours. We watch TV in the mornings and evenings, and even eat our meals on TV trays. Seems like it would make economic sense to design a small off-grid system to run everything in the room, including a small Midea 8000 BTU window air conditioner (450 watts draw on high setting), as well as two or three LED lights, TV, stereo receiver, and 4 mobile device chargers. Think of it like a small one-room cabin. This way we could leave the main house A/C system set to 80°F-85°F and save a lot of money over our six-month summer season from May through October. During this summer season, I have calculated the system would need to accommodate 7000 watt hours per day with a peak load of 810 watts.

Another use would obviously include emergency power during 5-10 day power outages for hurricane season.

Any thoughts or suggestions? Has anyone else attempted a partial off-grid solar system for a portion of their homes? Do you think it makes sense to do this?

Thanks in advance for reading and commenting!

Bill
Another FL 30 Lat, but using a Midea 8000 BTU inverter AC to cool a 1450 sq. ft. house. (450 watts draw on high setting) isn't close to true and not how inverter units function. Last summer when it was 100F plus my Midea drew 750 watts, or about 95 watts per BTU, which is what the specs say it will draw when working at it's max, 120Hz(?). Where inverter technology shines is when it is working at the lower cycles, like 10Hz. 100 watts produces 4K(?) BTUs or about 25 watts per BTU. These different efficiencies lead to LG, then Midea, petitioning the govt. to rate the inverter ACs differently than non inverter ACs. https://www.federalregister.gov/doc...sa-inc-from-the-department-of-energy-room-air where the Hz is mentioned. Then Midea requesting the same. https://www.federalregister.gov/doc...co-ltd-from-the-department-of-energy-room-air The low Hz lead to a quirk. When running at low Hz, probably not enough lubricant is circulated, so the computer program ramps the AC up to 410 watts. It stays there for a minute or so, and then ramps back down to the previous wattage. For ease of installation, inverter window AC with DC pedestal fan to distribute the air into other rooms. I cool the main room in the center of my house and if necessary, I use a DC fan to blow cooled air into the room I'm in.
 

Bubba1

Solar Addict
During a hurricane your air-conditioning needs will be less but you will need to power fridge and freezer. Use a fan instead of AC until the sun shines. Most hurricanes are a few cloudy day event and you still have power the first day or so. The grid issue starts when storm nears landfall and could last for weeks but often only days. The sun seems to shine a day or so after landfall. I would get a small Honda eu2000 generator and good 70 or 80 amp charger that you could charge batteries a few hours a day if needed. Have 15 or 20 gallons of gas and you should be good to go as long as solar panels don't blow away.
 
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