off grid ? garage system

i have installed 4.4kw DIY grid tied system that currently, that feeds both my house, office and garage areas. since i have separate sub panel in the garage that serves garage and office, i started playing with idea of adding off grid system for gargage and office. i just started researching this so bear with me.

my idea is to add new solar array, off grid inverter and batteries. then turn existing garage panel into critical loads panel. to do this, i want to disconnect wires that feed this garage panel (the feed comes from main house panel where my grid tied system connects) and use them as AC IN in to connect to off- grid inverter. then use inverters AC OUT and feed into my newly created critical loads panel.

by doing so, i assume that my inverter will switch to grid power anytime i am not producing enough and at the same time there is not enough power in my batteries. and switch back to battery/solar panel use, once there is enough power available. is this assumption correct?

now would this be considered off-grid or grid-tie, since i will still be drawing from the gid? will the inverter also feed to the grid? i dont really want to feed the grid with this system as i dont want to deal with excell on this one.

i have other items i wanted to discuss regarding batteries, but just wanted to make sure that what i am planing makes sense.

brankulo said:

by doing so, i assume that my inverter will switch to grid power anytime i am not producing enough and at the same time there is not enough power in my batteries. and switch back to battery/solar panel use, once there is enough power available. is this assumption correct?

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That is mostly true, but it depends on the inverter and how it is configured. What inverter are you considering?

brankulo said:

now would this be considered off-grid or grid-tie, since i will still be drawing from the gid?

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Traditionally, I considered Grid-tied to be a system that back-feeds the grid and off-grid to be a system completely disconnected from the grid.
A hybrid inverter is what I call a system where it will take power from the grid if needed but its output is independent of the grid.

Unfortunately, the terms Grid-tie, off-grid, and hybrid have become muddled because people use them in many different ways. Sadly, that means we no longer have terms that clearly communicate to everyone how the system is set up.

A term that the NEC uses is Grid-Interactive. A grid-interactive inverter is any system that has its output tied to the grid). This would even apply to inverters that use current sensors to implement zero export. The system you are proposing would not be grid interactive

The other two terms the NEC uses is stand-alone and multimode.

Stand-alone is any inverter where the AC output is not connected to the grid. A hybred inverter would be a stand-alone in NEC parlance.

Multimode is an inverter that can act as a stand-alone or a multimode. These are typically used for AC coupled systems.

I dont have inverter selected i am still diving into this but any advice appreciated. riht now, i am trying to figure out how big of a battery i need. i will try to post in next few days what i come up with based on my expecteduse.

knowing that my plan could work i started figuring out whgat size battery i need.
i listed all the expected loads and estimated hours of usage, then figure out how much will i need per day. some tool hours are hard to guess so i just figured how often i might use during the week and divided by 7 days. some weeks, especially in winter there will be no use at all. anyway, then i took muy daily usage and based on formula i found online calculated battery energy neded and battery bank requirement. i ended up with 48v and 575 ah. i found 12v 200ah battery that i want to use wire in parallel/series combination per attaced diagram.
i would really appreciate if someone could look through my calculations (attachemnts) and let m know if what i am doing seems correct as all this is based on my online research

thanks

Your basic idea is do-able. The sort of inverter you are looking at are probably either an off-grid all-in-one inverter with a grid assist function (e.g. Voltronic family of inverters), or an inverter + separate charge controllers (e.g. Victron). Lots of options in that space from cheap n cheerful to very expensive and built like proverbial brick dunny.

Whether it makes sense to do it is another matter but assuming it is...

Some thoughts:

1. There is a resources section on this forum, so have a look there if you have not already:

2. Your basic battery wiring diagram makes little sense (it's just wrong with 3 x 12 V batteries in series placed in parallel with 2 x 12 V batteries - that would not end well). I note you have referred to using lead acid batteries. In an application where you are looking to regularly cycle your batteries then LiFePO4 batteries would be far superior. There's just no compelling reason to choose lead acid for a new set up in a daily cycling scenario.

3. As far as assessing how much capacity you might need, then frankly the best thing to do would be to install a power / energy meter on your garage/office sub-board and actually record consumption. Note the peak demand and daily consumption.

4. Aside from battery storage, you'll also need to assess how much PV you can fit (and whether it will be sufficient), as well as what sort of power demand you expect to be able to service. This will need consideration of the surge power demand of tools which can often be multiples of their running power draw.

I guess the thing missing from your original post is the "why?"

Why are you wanting to do this? What are your objectives?

@wattmatters
1. thanks, i will have a look.
2. in article regarding batteries i read that you can either wire batteries in parallel or series or both, depending on your need. it also said wiring in series keeps amperage same but voltage adds up while parallel keep voltage same and amperage add up. could you elaborate more why my diagram wouldnt work to achieve 48v and 600amps? how would i need to wire it to achieve this with 12v 200amp batteries.
3. i might need to do that, i just did my spreadsheet to get some idea on battery size, i dont need to b to exact here as i will be drawing from grid when battery is not enough.
4. i have a lot of space, my roof is low slope, south facingand minimal shading, i could probably fit 50 panels so space is not an issue.

as to why, well we will need to add another array soon. between me working from home now and running computers at home, plus minisplit for ac/heat, my daughter is growing and uses lot more energy plus our plan to get electric vehicle soon, our cuurent system will not be sufficient.

when i installed my existing DIY grid-tie system, i pre-wired for another string o the same size. so one option, which would be super simple and fairly cheap would be to add another 12 or so panels, 6 microinverters, swap breaker for 40 amps and basically double my system easily. the cons are going through xcell again and still no power during blackouts (quite rare though here)

going with other option would certainly be more complicated, expensive and longer to install, but i wouldnt have to go through excell, and would have backup for those few days we get blackouts. as far as batteries, i would not really need to cycle them daily, i will just need backup during blackout i suppose. during day i could use power directly from panels, at night i am not using computer or office, i would just need to potentially chage car and that could happen with the power stored at excell, produced by my current system. i am not against lifepo4 and maybe i should go that way for flxibility in the future. they just seem quite a bit more expensive.

brankulo said:

2. in article regarding batteries i read that you can either wire batteries in parallel or series or both

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When wiring batteries in series their voltages add. So 4 x 12 V batteries in series forms a 48 V battery.

However when wiring batteries in parallel they MUST be of the same voltage.

In your diagram you have 3 battery banks in parallel:
1 x 12 V
1 x 12 V
3 x 12 V in series (IOW a 36 V battery)

That arrangement would destroy the batteries and possibly injure you if you tried to connect them.

If you want a 48 V system made up with 12 V batteries, then you will need 4 x 12 V batteries in series to create a 48 V battery.

If you want more capacity then you can create a second 48 V battery like the first and then parallel those two 48 V batteries.

e.g. this was the battery set up for my backup system using 2 banks of 12 V sealed lead acid batteries:



Note how the 12 V batteries are first arranged in series to form two 48 V batteries, and then these two 48 V batteries are connected in parallel.

brankulo said:

and would have backup for those few days we get blackouts.

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It would cover the garage/office. How do you intend to back up the home?

brankulo said:

as far as batteries, i would not really need to cycle them daily, i will just need backup during blackout i suppose.

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OK, well if the batteries are not going to be cycled, then lead acid batteries designed for occasional backup duties are fine. It's exactly what I did.

When it comes to sizing the battery then you say you are going to use storage for grid outage backup, not daily cycling.

So you only need to have enough energy to cover essential loads for the expected period of the outage or duration you wish to plan coverage for. An EV's energy needs are not relevant to the calculation of battery capacity for grid outage backup.

ok i see how it works now, thanks again for help. i dont really need to backup the house for few hours of blackout but i do need to keep computers runing during rendering process, as if they turn off during blackout i will have to start over. i can always run extension codr to supply fridge if i have to, or even wire permanent emergency outlet in the kitchen and living room from garage panel. but i am mainly concerned with rendering as it could take from hours to days and if i have to start over its not good.

what do you guys think of this kit?

Another option you may consider is running the garage area off-grid fully, but keeping your existing grid line in a separate panel and either connecting it to the 'generator' input of the inverter OR using it on a single circuit with a 48v battery charger in case your PV can't keep up with load or bad weather drains batteries.

I am currently running my garage fully off-grid. Details are in my signature. The shop is 60x40 with (2) mr cool 36k DIY units, (2) 2-post lifts, Air compressor, 50A EVSE for my model S, and standard shop equipment.