New to solar, but not electrical. Thinking of building a 28kw system for my home/work

[Splooter]

Hey all,

Fairly new to the solar world, but with power prices going up 43% here in the last 4 years we've been considering solar more and more lately. I live and work in the same building, which is a large metal building with part of it converted into a home. I have really great south-facing roof with enough sq.ft to house about 140, 400w panels (not that I need that much).

My power usage varies quite a bit, and in this climate we defintely use more in the winter than summer. It ranges from 2,000-4,000 kWh per month, some of it depending on how much work I'm also running that month (CNC machines, plasma, etc.)

I got a few quotes from companies building a system that is 25-28kw in size with anywhere from 10-18kw battery backup, and those quotes range between $90,000-96,000 before federal/local rebates.

I'd like to start exploring the cost to DIY a system as well. I have a good bit of electrical experience and am a fabricator/builder, so I consider myself handy, though I don't relish working on roofs, I would be open to it with a good platform lift and safety equipment to tie off on the roof. IF it was worth the savings that is.

I have 400amp service, so two, 200 amp panels. One of my questions is how I would tie into both, as the only common connection is at the meter.

I'm also trying to better-understand max concurrent output, as with my machines, heat pumps, and potential for well pump/septic, I feel like my concurrent draw may outpace what some systems are set to deliver out of a battery, should it be needed.

I'm also interested in learning what products people prefer, from panels to inverters, to batteries. I currently have two Emporia monitor systems I installed on my panels to get a good sense of where power is going, and I appreciate devices that have decent-to-good software and monitoring capabilities. That info would help me begin to price out the cost of the components and better-understand what I'm paying for in regards to labor, engineering, etc.

I'll continue to watch videos and read more, but wanted to say hi and get this journey started.

 

[sunshine_eggo]

I got a few quotes from companies building a system that is 25-28kw in size with anywhere from 10-18kw battery backup, and those quotes range between $90,000-96,000 before federal/local rebates.

What is your logic for having such a small battery? This would be grossly undersized for a system with 25-28kW of PV.

Your PV power and solar conditions determine how much energy you can use per day.

Your battery capacity determines how long you can go between chargers and are usually calculated in multiples of days.

Applying conventional numbers, you can use 100-140kWh/day (let's say 120kWh). If you need 120kWh/day, then you have enough battery capacity for 2-3.6 hours of operation per day.

If you're trying to size a battery for what you regard as your overnight usage, this is folly. You need a minimum of 24 hours of battery backup.

 

[Splooter]

What is your logic for having such a small battery? This would be grossly undersized for a system with 25-28kW of PV.

Your PV power and solar conditions determine how much energy you can use per day.

Your battery capacity determines how long you can go between chargers and are usually calculated in multiples of days.

Applying conventional numbers, you can use 100-140kWh/day (let's say 120kWh). If you need 120kWh/day, then you have enough battery capacity for 2-3.6 hours of operation per day.

If you're trying to size a battery for what you regard as your overnight usage, this is folly. You need a minimum of 24 hours of battery backup.

These were just battery sizes provided by the two companies I got quotes from to allow me to operate in island mode since otherwise I'd be grid-tied and I wouldn't be able to use my solar when the grid was down. I tried to get like-for-like quotes to make comparisons easier.

Ultimately I'd like to have a system I can grow, at least battery-wise as funds and time permit. I don't often work overnight, so my initial thought was to size just for living/heating conditions long enough to get me from sundown to sun-up conditions. During sleeping hours we burn about 3kWh, so I was thinking long-term we'd probably want between 50-60kWh in battery capacity, likely more considering winter hours.

 

[eppich8479]

If you are thinking 10x 400w panels will get you 4000w for 8hrs a day everyday, that wont happen.

You might want to get panels that, 50% output would cover 50% of your use cases. And then you have battery or grid backup when you exceed 50% case use.

The systems are not hard, the biggest thing to remember is that you never want to go over Voc, and Voc goes up as temp goes down.

I would just check signature solar for prices. Because i think those bids are like 1/4 materials, 1/4 labor and permits and 1/2 profit.

 

[Splooter]

If you are thinking 10x 400w panels will get you 4000w for 8hrs a day everyday, that wont happen.

You might want to get panels that, 50% output would cover 50% of your use cases. And then you have battery or grid backup when you exceed 50% case use.

The systems are not hard, the biggest thing to remember is that you never want to go over Voc, and Voc goes up as temp goes down.

I would just check signature solar for prices. Because i think those bids are like 1/4 materials, 1/4 labor and permits and 1/2 profit.

When you say check signature for solar prices what did you mean?

I'm not basing the calcs on 100% efficiency.

My low-use days are about 50kw, and my high are about 100kw. So based on where I am and the number of sunny hours/days it mathed out to about 30kw producing what I need throughout the year to make me about even, with a good deal of overproduction/credit-building in the summer, and then drawing those credits via net-metering in the winter.

I think nuances like VOC are things I'm still learning about, but also why I'm open to a proven ecosystem, even if it costs a bit more than piecing together things. Would love to learn more about people's proven systems and not try and reinvent the wheel.

Thanks for the info!

 

[BaylentheSky]

I have an 8kW PV system, with a 16kWh battery, and that is adequate for my use (typically around 1200kWh a month). Your usage is 3x mine, so extrapolate that to a 24kW PV system with a 48kWh battery should cover your usage. Your concern regarding concurrent use is valid, so I suggest at least 2 inverters running in parallel to handle the spikes. I don't have a lot of experience with inverters like the other folks on here (sunshine_eggo) but I really do like the set up with EG4 GridBoss gateway unit and 2 EG4 FlexBoss inverters. I do have an EG4 12000XP and it's great! These days you can probably find 24kW worth of solar panels for about $5,000, $26,000 for the aforementioned EG4 setup plus 57.2kWh worth of LFP batteries. So you're looking at $31,000 and some elbow grease in total.