Any FL off grid folks?

[Bobthebuilder352]

So it’s a bit off topic but I’m building a barndominium. Walls, slab, roof are done. Approx 4000sqft. I’m in duke service area and instead of bothering with a grid tie I’m thinking off grid. This is ag so I can skirt permitting a bit. I mostly want to power HVAC (est 100,000btu in summer) and the second largest use would be charging station for an electric truck. To power the HVAC in summer will take a ton of power but essentially none starting this time of year.

PV panels are the easy part. anyone have any suggestions, mostly as it relates to storage? Given my huge swing in use anyone want to talk me into a grid tie? Duke will cap my excess power and will require specific inverters, by my rough math that’s an extra $10k. I’m trying to DIY this. I would like to tie my house into this down the road which is 400’ away.

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[ajdelange]

Approx 4000sqft. I’m in duke service area and instead of bothering with a grid tie I’m thinking off grid. I mostly want to power HVAC (est 100,000btu in summer) and the second largest use would be charging station for an electric truck. To power the HVAC in summer will take a ton of power

Well 100 kbtu is a little over 8 tons which is about 10 kW (SEER 10 system i.e. pretty good). In 24hrs that's 240 kWh. Assuming 7 hours FSE (Full Sun Equivalent) per day that means you will need 240/7 = 34 kW peak system or about 132 panels in the summer. In the winter you will need more. For the 16 hours the sun is not shining you will need to provide 16*10 = 160 kWh from batteries. That's 160/13 = 13 PowerWalls. Then there are the other loads. Shall I go on?

 

[Bobthebuilder352]

Well 100 kbtu is a little over 8 tons which is about 10 kW (SEER 10 system i.e. pretty good). In 24hrs that's 240 kWh. Assuming 7 hours FSE (Full Sun Equivalent) per day that means you will need 240/7 = 34 kW peak system or about 132 panels in the summer. In the winter you will need more. For the 16 hours the sun is not shining you will need to provide 16*10 = 160 kWh from batteries. That's 160/13 = 13 PowerWalls. Then there are the other loads. Shall I go on?

No please stop. Anyone with actual off grid experience?

 

[skyote]

No please stop. Anyone with actual off grid experience?

His point is that it would not be feasible from both a solar & battery perspective to run the AC.

My $.02 - You'd be better off to install a propane generator with a large tank for running the AC or anything with a larger draw. Smaller solar & more modest battery storage could run lights etc.

 

[skyote]

Just read your comment about grid tie for $10K. Simple answer is do it, that's your most affordable & flexible solution.

 

[ajdelange]

A couple of comments:

Unless you are Henry David Thoreau (actually, he faked all that Walden stuff) a totally off grid solution is going to be tough. I am in the planning phase for a totally off grid garage only for housing and charging the cars. It will be air conditioned in the summer by the solar system alone but heated with propane in the winter as it will have workshop and storage space but the heating and cooling will not be to the extent of residential space. It's been an interesting excercise and it's going to cost a bunch (5 power walls). This is the reality of solar. In June on a clear day there's plenty of juice to run A/C, charge cars and operate lights and tools around noon. Around midnight there isn't and in the winter the story is altogether different.

WRT air conditioning: The main house here has well more than 8 tons of air conditioning but at least 8 tons of it is geothermal. The load is not 8 tons continuous by any means i.e. the duty cycle is well below that. In the summertime a 58 panel (13.9 kW peak AC) system carries the whole house load in a good month and a good 80 - 90 % of it in cloudier months. But my "battery" is the utility. In the winter months the solar system may only provide 16 - 20% of the load (the majority of the heating load comes off that same 8 ton heat pump which seldom runs at the full 8 ton level).

In considering going to a generator for an off grid system one needs to understand that the "backup" generators sold for residential or even commercial use are rated for just that i.e. backup. They are not to be used as the primary source of power. The implications are obvious: a generator intended for primary use is going to be appreciably more expensive.

Building codes may or may not be a consideration here. In many jurisdictions you cannot get an occupancy permit if grid is available and you are not connected to it.

At the bottom line it is pretty clear that as much as you may hate the utility and capitalism in general the only practical solution for you is a grid tie-in.

 

[Bobthebuilder352]

Okay guys. Your first problem is referencing power walls. They’re pretty but you can store for a LOT less especially when not having to worry about interconnection to the grid. 90% of my draw will be for cooling and for charging. In Central Florida that’s 9 months of the year and at night summer 50% cycle rate. No heat in winter but I will need some dehumidification. Code and permits are a non issue for me though safety is. I will ground mount for cost and simplicity and could cover an acre and so long as it’s for ag purposes I’m fine . My math is I can do this for about $35k (excluding the car charging). If I hire someone to do this soup to nuts I’m looking at roughly double which is what I’m trying to avoid not just because of cost but I’m trying to ensure expandability down the road.

I’ve got a background in utility scale PV but none in storage (other than forums and google) so I was hoping to get some real world knowledge and best practices on applicability. I’m spray foaming the shell in a few weeks so just trying to get the ducks in order.

 

[ajdelange]

Power walls are the first think I look at because they represent the gold standard. Accordingly they are expensive ($700/kWh before incentives in a time frame when a kWh in the cars probably costs you arounf $200) but of course they do other stuff besides store like charge and invert. There are other manufacturers of solutions (such as Enphase) but I don't think they can handle capacities such as what I suspect you will need. If you decide to go fully DIY then you will have to make determinations as to whether to use bulk or micro inverters for the arrays and the inverter topology for the house loads. Before you can do any of this you need to
1)Do your Manual J for the building
2)Get the NREL data for your area

The better you know the temporal behaviour of the sun in you area and the temporal characteristics of your load (ideally histograms of consumption but, of course, it is impossible to get that on a building you haven't built yet - use histograms of the loads in your current residence as second best) the better you will be able to determine how big an array you will need and how much storage you will need. The latter depends on how long you are determined to go before the backup generator (a necessity) kicks in.

I'll say again that the cheapest battery is the utility.

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