Batteries to panels ratio?

[Angela]

Hi there folks,

I need to install an off-grid system for my home. So far I have had 2 quotes and now I am very confused!

One ccompany quoted for 15x (350watt) panels with 4 lithium batteries and the second company also quoted 15 panels but with just 1 lithium battery. The 2nd guy reckons that 4 batteries will never get fully charged as the first quote suggested.

I am new to this (as you can no doubt tell) but I thought more batteries is better for longer storage of power? But is the 2nd guy right that the batteries will not charge and be a waste of money?

Should I split the difference and have 2 batteries?

many thanks
angela

[SunEagle]

Hello Angela and welcome to Solar Panel Talk

You should not have to decide what the panel to battery ratio is. It comes down to the amount of charging amps to battery Ah rating. But I would think that if you have a couple of days without sunshine a single battery may be overly discharged.

Certainly if you decide to use more of the battery charge then you can put back each day you may have too many batteries but on the flip side if you use more charge then what you can put back maybe you do not have enough panel wattage that can recharge them.

So my first question is what is your estimated watt hour usage each day and what is the watt hour rating of a 4 lithium battery system?

[bob-n]

SunEagle is right. Best to go by math using real numbers for your usage, for your solar generation, and for the proposed equipment.

Solar panels are all quite similar. You'll find 330 watt panels, 350 watt panels, and 380 watt panels. Not much difference there.

But batteries vary tremendously. There are tiny, small, medium, large, and huge batteries. It could be that the two different proposals use very different size batteries but roughly the same final capacity.

Can you ask the two vendors to explain their proposals to you with real numbers? What did they assume you would be using for energy each day? What did they assume that you will generate from the panels each day? How much capacity does each battery have? How many days did they figure into their proposal that are cloudy and not generating any energy?

Energy is commonly expressed in watt-hours. Batteries are rated in amp-hours and volts. Multiply amp-hours by volts and you get watt-hours.

Solar panels generate more energy if they are fully open, less if there are trees or other sources of shade nearby. In the southwest, they have >300 clear days per year. In the northwest, they have lots of cloudy days. One more consideration is that you will get less energy in the winter when the sun is low, compared to the summer when the sun is high. All of this greatly affects the amount of energy you get from the panels each day.

If you are interested in doing some estimates on your own, we can refer you to a website that is easy to use but very detailed. You tell it your location, the number of panels, the power per panel, and the position of your solar panels. It tells you how much energy you get every month. Your vendors may have done this already.

[Angela]

Thanks for your help and advice, I think I understand things a little more.

I use about 11,000kwh per year living on grid, so have asked for quotes based on that. Around 31kwh per day on average, but using more in summer for irrigation, so less in winter.

i have full sun, no trees or shade cast.

system 1 suggests 9.6kwh pylontech US 2000 battery pack
system 2 suggests 5.8kw triple power (this may be the brand?) single battery
both with 5k inverters - which seems small for system 1.

Now I look at that written down I'm more confused by the big discrepancy.

I cannot see any reference to amp hours though.

help

[SunEagle]

Thanks for your help and advice, I think I understand things a little more.

I use about 11,000kwh per year living on grid, so have asked for quotes based on that. Around 31kwh per day on average, but using more in summer for irrigation, so less in winter.

i have full sun, no trees or shade cast.

system 1 suggests 9.6kwh pylontech US 2000 battery pack
system 2 suggests 5.8kw triple power (this may be the brand?) single battery
both with 5k inverters - which seems small for system 1.

Now I look at that written down I'm more confused by the big discrepancy.

I cannot see any reference to amp hours though.

help

Well the first system of 9.6 kW, not kwh, may have the panels facing in different directions which each when powered may only produce 5kw max so the 5kw inverter should be ok. But if all 9.6kw is facing in the same direction you will get "clipping" on that small inverter.

What is the voltage of that US 2000 battery pack? For it to deliver 31kwh per day it needs to be about a 48V 1000Ah system even if they are lithium.

The second system will never delivery your 31kwh a day if you do not have a couple of sunless days due to bad weather.

[J.P.M.]

Angela:

Just an opinion, and not a knock:

Well meaning and well informed folks here can and will help you, but from your comments and what seems to be your current state of knowledge about what you want to do and what is possible vs. what's practical, workable and maybe cost effective, I bet the responses you've got here so far went at least partially over your head, or at least writing things have been written the consequences of which you do not understand, starting with no mention so far of the idea that off grid living is taking on what's usually at least a part time job - a job that needs a fair amount of commitment, not to mention a fair amount of technical knowledge to get safe, workable and right.

You're seeming ill at ease feeling about those two quotes is well founded. But if you had known what was required for what you want to do, you would have been better prepared to see just how disparate those two quotes are. Maybe they did serve a small good in that they seemed to make you more aware of your confusion or at least increase your knowledge that you're in need of education.

Before you do do anything involving commitment of time or resources, I'd respectfully suggest you get informed about the basics of PV, particularly as those basics apply to residential applications, and especially as they apply to off grid applications. Then, know that things are changing fast in many aspects of residential PV, particularly as they are impacted by battery design which seems to be evolving at a rapid pace at this time. Some think that while batteries are coming along, they may not quite be ready for prime time just yet.

I'd start (but not finish) a study of the basis with a download of "Solar Power Your Home for Dummies", a free online PDV for a dated but still useful version or an updated version for ~ $25 @ bookstores/Amazon.

Before any of that, get your annual electrical load as low as possible. That's much easier to accomplish and maintain, and much more cost effective than any onsite generation. Plus, any subsequent PV, grid connected or not, will be smaller and less expensive.

Right now, in your quest for displacing POCO power with alternate energy, your solar ignorance is your worst enemy. It's also the best friend of peddlers whose goal is to put as much equipment on your property as possible, but not necessarily in the safest, most workable (or at least least bothersome), or most cost effective configuration.

If you have POCO power now, one option (if possible) for you (among and including doing nothing at this time), is to start by reducing your annual load by maybe a third and then get a grid tie system that's sized and designed toward a balance in cost effectiveness and adding future storage.

Welcome to the neighborhood and the forum of few(er) illusions.

[bcroe]

Some think that while batteries are coming along, they may not
quite be ready for prime time just yet.

Huh, did someone rattle my chain? Yes batteries are a necessity for going off grid, at
great expense and as much trouble as keeping a dairy herd. They could not even do
what is happening here, using no cost, zero maintenance net metering. For cutting down
electric energy purchases, best return on investment, and least inconvenience, net
metering is the winner.

Using batteries, you must manage your electric energy use limited to their limitations.
Making hay or pumping water when the sun shines can work well, with the least or
no batteries. Do your big jobs at night, or the wrong season, kill the batteries.
good luck, Bruce Roe

[Mike90250]

> I use about 11,000kwh per year living on grid, so have asked for quotes based on that. Around 31kwh per day on average, but using more in summer for irrigation

That's not good !
Living off grid with wife and 3 fridges, little irrigation (winter) is about 7Kw daily, summer with about 1500 gallons daily, about 13Kwh. I have an 800ah 48V battery and 5Kw of PV
on a clear, cool summer day, I could harvest about 23kwh if things were perfect.

I can comfortably go 2 cloudy days in winter before I start up the generator, you seldom want to run batteries flat, because that's the day it's nasty weather, and walking to generator shed and trying to get it to start, is too late !

You size your batteries to run your loads for 5 days, and then size your solar to recharge a dead battery in 1 winter day. Expensive, but that's what off grid is.

[Angela]

Thanks everyone for your help. I realise I have lot to learn, as well as reducing my power consumption, before I invest. I'll read the dummies guide

[J.P.M.]

Thanks everyone for your help. I realise I have lot to learn, as well as reducing my power consumption, before I invest. I'll read the dummies guide

Thank you for the response.
After the dummies book, look into something called "PVWatts". It's a solar modeling program. Read all the help screens a couple of times, get your array orientation(s) approx. correct and do a few runs using a 10 % system loss parameter rather than the default 14 % or so. Just remember that the model's output is only that and not a prediction of any one day's/month's/year's output.

FWIW, if, in your knowledge quest, questions arise that you cannot find answers to on your own, I'm sure at least one person here, and probably several, who will be happy to offer an opinion or two.

Happy knowledge acquisition,

[azdave]

I realize I have lot to learn, as well as reducing my power consumption, before I invest.

It sounds like you already have utility service to you home where grid-tie would be the proper choice to take going solar. Why are you going off-grid or is this for a different home out in the wild?