Off grid system to run HVAC only, doable?

[hertfordnc]

I live in North Carolina. We turn on the AC in April and we might still be running it on Thanksgiving Day.

I'm renovating a small house and installing two minisplit heat pumps (five or six zones)

I want to power one of the heat pumps with solar- off grid.

So here's the math as I understand it;

The heat pump draws about 15 amps at 220VAC at full power. Most of the time it will not be drawing that much and I am thinking the 24 hour average will be closer to 3 amps

I'm thinking a refurbished forklift battery for about $1200 and a 24V / 220VAC inverter.

This is the only system I'm interested in. Air conditioning is the one thing that is really hard to reduce the cost on. you can hang your clothes to dry, install LED lights, and heat your house with wood but if you live in the South you will have air conditioning.

I do not trust my state utility commission down the road to not change the rules or add fees so I have no interest in any sort of grid tie system

I'm having trouble with the math. Starting with what solar output actually means? The system I describe above would consume about 15KW / day so in my small brain, 1500 watts of solar times 10 hours of good sun should do the trick, right?

But when I ask solar vendors about that I get different answers.


The vision could be adjusted to accommodate reality. I could just run a small heat pump and only cover one room The main idea is to have a portion of the HAVAC on a stand-alone renewable system,

Look forward to feedback


thanks

dave

[ButchDeal]

First, you don't have 10 hours of good sun in a day in NC.
With Off grid you have to plan for peaks in consumption and valleys in generation.
Your batteries would need to be able to handle the difference.
The best option would be to use a bimodal system such as those from outback. Yes I know you said you didn't want to be grid tied but you can configure the outback systems to NOT feed into the grid. It would just pull extra power from the grid when needed. This would allow you to size the system more reasonably. You could cover both A/C units but during an outage turn one off.
This would be the closest to a system that might save you money. A truly off grid system would not save any money.

[hertfordnc]

Thanks. But I still want to understand what it would take for this vision to work, even if the answer is stupidly expensive and unrealistic.

By building the system around Air Conditioning only, then my demand would tend to align with the available sunlight, right?

[Logan5]

with a grid tied Air conditioning system you will pay around 1 dollar per Ton hour of cooling capacity, with an off grid system 1 Ton will cost 4 to 10 dollars forever. with grid tie there is even a chance that price will go down when new electric rates are implemented, Off grid price is more likely to go up with the cost of lead and labor. If you are serious about Air conditioning with solar battery power, Look at the HotSpot 4812 48 volt direct current variable speed compressor Air condition unit.. It is designed to run off grid on much less power than a 220 volt native AC that requires an inverter.

[ButchDeal]

Thanks. But I still want to understand what it would take for this vision to work, even if the answer is stupidly expensive and unrealistic.

By building the system around Air Conditioning only, then my demand would tend to align with the available sunlight, right?

no the demand will align with A/C usage, till it over runs the available light/battery and your system turns off, or starts the generator.

[SunEagle]

Most people forget that the humidity and heat can get high even if the sun isn't shining. The best way to cool off when it is really humid is to use an AC system.

Relying on the sun to run an AC system will leave you hot and sweaty.

[hertfordnc]

> with a grid tied Air conditioning system you will pay around 1 dollar per Ton hour of cooling capacity, with an off grid system 1 Ton will cost 4 to 10 dollars

Huh? Sounds like you're saying solar never pays for itself? When you say $1/ton what is that based on?

A typical central AC unit (non heat pump) is SEER 10-12. The ductless system I'm installing is SEER 22

The load is 15 amps (3000 watts) peak and 600 watts average.

> Most people forget that the humidity and heat can get high even if the sun isn't shining.

I live at latitude 30 on the coast, I think understand humidity pretty well. (didn't mean to sound snarky)

So the first question is, how much battery do I need to deliver 600 watts for 24 hours and how much solar do I need to replenish that battery on a clear day in summer?

I understand it may be a ridiculous number but that is the starting point for figuring out what problems need to be solved.

I'm thinking that if it works on a hot clear sunny day it will work on days when there is less sun and less heat (they do go together somewhat)

[cebury]

They've answered some questions below, but to fill in a couple easy ones here.
>> A typical central AC unit (non heat pump) is SEER 10-12.
That is efficiency, they are sized by ton which relates to volume of heating cooling ability. A typical newer 1500 sqft home in inland central CA might have a 2.5ton unit.

>>> Consume 15 kwhr per day
15kwhr/day needs a much larger than 1500watt system. Thats closer to a 2500watt system at a Lattitude like San Diego, during summer. During winter, or any day with clouds, it's much much less than 15 kwhr/day. This still doesn't suggest a simple 3KW system is appropriate to your situation. Here's partially why:

You don't need 15kwhr per day, you need it during the time period when the heat pump is running. SolarPV generates power from sun availability. Again, at my location during summer only, that's 730am to 8pm ish. My A/C unit runs from Noon and peaks at hourly usage about midnight. Many nights It doesn't get cool enough here until a couple hours right before sunup. Spring and winter is much different. Your location is different.

[hertfordnc]

I checked out the HotSpot 4812 48 volt direct current variable speed compressor Air condition unit.
It looks like the heat pump I'm installing. I'm kinda thinking the inverter heat pumps are using DC to run the motor at variable speeds and that the HotSpot is doing that without the AC conversion - just a hunch

[jflorey2]

Sounds like you're saying solar never pays for itself?

Solar+battery never pays for itself due to the ongoing cost for batteries.

So the first question is, how much battery do I need to deliver 600 watts for 24 hours and how much solar do I need to replenish that battery on a clear day in summer?

600*24/.95 (inverter efficiency) = 15.2kwhr

So for lead acid you need 30.4kwhr per day (no more than 50% discharge.) That means about 90kwhr of batteries total for 3 days of autonomy (an often-used standard.)

Minimum solar for that battery size will be about 9kw (for C/10 charging.)

[Logan5]

A solar PV system with hard cycled batteries will not only never pay for it's self ever, but will actually always cost many times more forever as long as you run that system. 1 dollar per Ton hour is an example only. It's probably much worse. unless you are cooling a very small place, your battery need could be the size of a mini van and cost 10's of thousands of dollars and have to be replaced every year to 3 years or so. I would get a fork lift for the annual ritual.

[hertfordnc]

Finally ! I've asked this question a few times and never got a clear answer. It's the batteries that wreck the math!

I'm still not interested in a typical grid-tie but maybe a grid switch? Solar panels to a small battery to handle startup and then switch over to the grid when the sun goes down?

[ButchDeal]

This is what I suggested in post #2 BTW.
Any time the solar/battery is not enough it will pull from grid. You could set the discharge limit for the battery that it would be allowed to discharge to before pulling from the grid as well. It can be programmed to never back feed as well.

[hertfordnc]

Yeah but at post #2 I still had a deathgrip on NOT tying to the grid

[ButchDeal]

it is not cheap and you will by cycling the battery a lot more than grid tie but much less than off grid completely.
It is pretty much my set up with a different program configured on the inverter.
because of the battery use it will be the best option for working without a net meter configuration but still very much what SunEagle is talking about for costs.

[SunEagle]

Yeah but at post #2 I still had a deathgrip on NOT tying to the grid

Unfortunately when you add batteries of any size to a solar pv system (even a hybrid connected to the grid), the cost of those batteries ends up costing you much more /kWh then to just run directly from the grid.

Unless you are paying above $0.30/kWh a battery system will cost you much more and is not financially smart thing to do.

[Logan5]

Yes, if you are trying to save money grid, tie is the only way to go. Almost anything is possible with enough money, so yes it can be done. Very few solar, battery salesmen will tell you this though. Until a new battery storage options come to market it's grid tie, or spend more, I have seen solar hybrid split AC units that run on 220 VAC and have built in solar assist. so not to feed back grid but assist a known heavy load.

[Sunking]

Finally ! I've asked this question a few times and never got a clear answer. It's the batteries that wreck the math!

Big time. Battery cost alone will be 5 to 7 times more than you pay the mean ole power company. Size of bank you would need requires a permit because it contains over 500 pounds of toxic material. You get to go through that pain of inspections and paper work with the Employment Prevention Agency. For the quantity of batteries needed will require its own room with reinforced floor. You are talking 5000 to 6000 pounds of batteries costing $15K to $20K every 5 years.

You got that kind of money to throw away?