Watts needed

StirCrazy wrote:
for a 6V GC battery the recomended charge amprage is 10% of your total amp hour capacity at the 20h rate of your entire battery bank.

With Li's you can go way over that.

I learned a new method of sizing yesterday while I was reading. for a 6V GC battery the recomended charge amprage is 10% of your total amp hour capacity at the 20h rate of your entire battery bank. now how you adjust this for other loads I havent figured yet but from a charging stand point for your batteries you want to get about 21 amps from your panels. for myself, I would probaby double this and call it good.

Steve

cliffy49 wrote:
I have read where folks suggest any where from 1 to 2 watts of solar per ah of battery. My question, is this based on battery rating (ie 215 AH) or is it based on useable amp hours(105)?

All replys are greatly appreciated.

A poorly thought out rule of thumb. As likely to give you an undersized system or an oversized system. Pretty good chance, you are left unhappy. If you are drawing negligible watt-hours from the battery bank each day, a very small solar array can handle a large battery bank but then why bother dragging around a huge battery bank.

The question is how many watt-hrs do you use per day (w-hr = amp-hr * voltage).

A solar panel will produce around 4-5 w-hr per name plate watt rating...so a 100w panel will generate around 400-500w-hr on a typical day.

Then how many overcast days with minimal solar do you want to be able to go before the usable battery capacity is gone.

Let's say your fridge uses 50w and is on 50% of the time, so you need 50w * 24 * 50% = 600w-hr to service the fridge. (repeat for the other items that will be run of battery power and add them up to get the total w-hr).

If you want to go 3 days with overcast days, that's 1800w-hr you need in usable battery storage. For lead acid, you will need an overall rating of 3600w-hr (limit usage to around 50% of the rating) which translates to 300amp-hr @ 12v. Lithium you can use around 80% of the rated capacity.

Assuming you want to recharge the bank completely the next sunny day (1800W-hr), you will need around 450w of solar panels.

If you are only off grid for a couple days at a time or you are willing to use a generator occasionally to top up the batteries, you can reduce the size of the battery bank and solar panels..

When you come up with your final sizing, it's not a bad idea to upsize by 10-20% as the systems aren't 100% efficient.

battery rating

I think the battery size relatively to energy needs is important. Other than weight, there is no penalty for oversizing the battery.

No penalty for oversizing solar, either - when the weather is not cooperating you can play with laptop or turn all the lights on to lift your spirits, without worrying about poor solar harvest from a small panel. Or if you park in a partial shade.

Bare minimum is 60 watts per 100 amp-hours. After 150 watts per 100 amp-hours, there is little point in having more as the acceptance rate for lead acid at 85% state of charge is about 12.5 amps per 100 amp-hours.

So for 215 amp-hours between 130 watts to 323 watts.

For SiO2 (100 amp-hour) the 323 could be doubled as the acceptance rate is 25 amps. That makes the wattage about 600

This may help: https://freecampsites.net/adding-solar/

1W per 1 AH is a very broad guideline. There is no such thing as too much solar.

Geographic location matters. Also, how big is your battery bank relatively to your daily energy needs.

Flat 100W solar would harvest ~35 AH on an average day in June at latitude of Portland OR. More in San Diego, less in Seattle. Tilted panel (ex. portable solar suitcase) will harvest more than flat panel. If your 100 AH battery is discharged 50%, you need ~55 AH solar harvest because of all the losses, so flat 100W solar won't cut it. Even 150W might not cut it because the last 20 AH will go in slowly.

It is easier to get away with less solar wattage if your energy use is low and battery bank is bigger than you need, for example your energy use is the same 50 AH a day and you have 200 AH battery bank. Then you won't need more than flat 150W solar because you'll start the day with battery 75% full and in a few minutes it will slow down and won't need more than a few amps.

When you're using energy mostly in daytime, having more solar than, say, 1.5W per 1 AH helps "better" because solar is feeding both loads and the battery, so it's less important that the battery is slow on those last 20% of SOC. Whatever solar energy battery can't take, loads will.

If you drain that battery every day you need to error toward the high side. If that battery goes a week then a little less solar would be needed.

Nothing better electrically than glancing at the controller or remote at 11 a.m. and seeing “batteries 100%.” I don’t know either but prefer to base watts on total battery amp hours. The item I need to remember is to exercise the generator.

good question, I don't know.