On again, Off again. Technical

[Convert]

Somewhat new to solar but have a pretty good understanding of how it all works.
My hope is to install 400 - 800 watts worth of panels and to have them power a few circuits of appropriate size.
I'll also install batteries of similar size.
My question regards switching on and off the grid.
I would like to power these circuits as long as my batteries hold up then switch to the grid and let the solar recharge the batteries. Of course, the solar array would be contributing to powering these circuits as long as there was sufficient light.
Once recharged, I'd like to switch back to the batteries/ solar.
I believe a relay (magnetic or solid state) will handle the switching but my problem is building/ buying the appropriate electronics to handle the relay.
I'll need to de-energize this relay at 11.6 volts and re-energize around 12.8 volts.
A thought involves one of these battery voltage indicators/ IC's where the relay is energized by the 11.6 v output but how to keep it off until the voltage rises to 12.8v?
Thanks for your input.
Alex in Myrtle Beach

[jflorey2]

I would like to power these circuits as long as my batteries hold up then switch to the grid and let the solar recharge the batteries. Of course, the solar array would be contributing to powering these circuits as long as there was sufficient light.
Once recharged, I'd like to switch back to the batteries/ solar.

That's a great way to kill your batteries as quickly as possible (hard cycling from close to 0% to 50%, never recharging fully.) You would be MUCH better off floating the batteries with a smallish charger (but sufficient to charge them to 100%) and using the available solar to power the loads.

You would be even better off doing a grid tie installation. Cheaper, safer, easier.

I believe a relay (magnetic or solid state) will handle the switching but my problem is building/ buying the appropriate electronics to handle the relay.
I'll need to de-energize this relay at 11.6 volts and re-energize around 12.8 volts.
A thought involves one of these battery voltage indicators/ IC's where the relay is energized by the 11.6 v output but how to keep it off until the voltage rises to 12.8v?

A window comparator and an RS flip flop or latching relay. The lower comparator turns on the relay; top one turns it off.

Again, however, you will burn through batteries quickly that way. I'd be surprised if you got even 100 cycles (3 months) out of them that way.

[SunEagle]

As jflorey2 stated that type of system will kill off your batteries quickly. It will also be much more expensive then just having a grid tie system.

You have to understand that cost of electricity from a solar / battery system could be almost $1.00/kWh which is much more then purchasing power from a Myrtle Beach POCO.

Get rid of the battery part and just go with solar grid tie. It will eventually pay for itself.

[Sunking]

Somewhat new to solar but have a pretty good understanding of how it all works.

No you do not, or you just have more money than you can spend. Which is it?

Please explain in full detail why you want to pay 5 to 10 times more for power and destroy batteries.

[Convert]

So, the consensus is that I should just stay on the grid until the grid fails?
I have little interest in attempting to sell excess power to the utility (not that there would be much excess). Here, the utilities payout about 25% of the going rate and charge a monthly fee on top of that.
Could you explain why this setup would be so detrimental to the batteries? Don't electric vehicles do essentially the same thing?
Perhaps I've got the wrong idea altogether.

[SunEagle]

So, the consensus is that I should just stay on the grid until the grid fails?
I have little interest in attempting to sell excess power to the utility (not that there would be much excess). Here, the utilities payout about 25% of the going rate and charge a monthly fee on top of that.
Could you explain why this setup would be so detrimental to the batteries? Don't electric vehicles do essentially the same thing?
Perhaps I've got the wrong idea altogether.

The batteries in an EV are designed for multiple cycles yet they too will lose the ability to get back to 100% charged after they have been discharged a number of times and will need to be replaced. A deep cycle solar pv system battery will only last a number of cycles per the manufacturer and you will use them up faster with your plan.

As for not wanting to give excess power back to the POCO. Fine there are grid tie inverters that will allow you to set them up for Zero Output so you don't sell the POCO one kWh. But the real question is why would you want to pay all that money to the battery seller?

It is real simple math. Add up the cost of the panels, charger, batteries, wiring & inverter. Then determine the number of "cycles" that battery system can provide you and how many kWh it will provide you each "cycle". Total up the kWh that the battery can provide and divide that number into the cost of you solar pv system. Depending on what you have paid and the quality of the battery you will come up with a number somewhere between $0.50/kWh and $1.25/kWh. I am sure you are paying less than $0.25/kWh so why would you pay 5 times the cost to generate your own power from batteries?

[Sunking]

Perhaps I've got the wrong idea altogether.

You got it all wrong like I said earlier. You just had 3 professionals, not DIY morons, told you to stay away from your idea as you would be throwing money away and not achieving your goals. That is the bottom line in summary.

Here is your issue. You imagined everything. If you were to limit daily discharge to 20 to 25% and treat the batteries with TLC will maximize your battery investment dollars which is not an investment at all because you loose money. But if you did that, and you can't, but if you could just in battery cost alone will be paying roughly $1.00 to $1.25 per Kwh instead of buying from the Power Company for 12 to 15 cents per Kwh. Only person that is getting screwed is you paying 10 times more for power than you have to. You just volunteered for a 1000% rate increase. Something only a democrat would like.

But no you want to pay even more by discharging your batteries every day like an EV which does not use the same batteries as you will use. Instead of replacing them every 3 to 5 years, you want to replace them every year or two which means you are now paying even more than 10 times, more like 20 times what the power company charges you.

Like I said, you are the only one getting screwed, and if you do this means you like it. Basically for every 10 -cents you save on your electric bill will cost you $1. So build you a system that will save $10 a day on your electric bill to make up for the losses, All you gotta do is burn a $100 bill everyday for breakfast to get it.

Not wanting to sell back to the grid is as silly as your battery idea. Again this astonishes me how ignorant people are today. Lets ignore the fact a grid tied system can possible have a payback which you hate and want no part of. So I guess you have more money than you know what to do with is the only other explanation I can come up with.

If you are not demanding as much power as the GT system can generate, and do not sell back is money/credits lost forever. You cannot store that power, it has to go somewhere or you loose. So say you are using 3 Kw but your system can generate up to 7 Kw selling the excess 4 Kw back to the POCO for credit, you loose forever. No problem for the POCO, you can throttle your system back to limit to just your needs and loose that credit to build build up so when you demand more power than your system can generate, you have to buy it from the POCO because you are not selling credits to use latter like when the sun sets or winter. YOU GET A LARGER BILL and the POCO could not be happier about it. Not their fault you do not know WTF you are doing.

Tell you what. I got a great investment tip for you. Give me $10,000, and I will give you $100 per year for the rest of your life. At least it is a beter investment than you are looking at.

[ButchDeal]

So, the consensus is that I should just stay on the grid until the grid fails?
I have little interest in attempting to sell excess power to the utility (not that there would be much excess). Here, the utilities payout about 25% of the going rate and charge a monthly fee on top of that.
Could you explain why this setup would be so detrimental to the batteries? Don't electric vehicles do essentially the same thing?
Perhaps I've got the wrong idea altogether.

You are a bit confused about the grid interconnect situation.
FL has NET metering. net metering means that you TRADE at full retail value kWh. Excess Generation is the power you generate beyond your consumption in a period, usually a year. Excess generation is payed out at wholesale rate or roughly 25% retail.
Using Net metering is 100% efficient and can allow you to get very close to a zero dollar bill at a much lower equipment cost.

Battery, particular off grid battery systems are far from 100% efficient and the batteries will not last that long, especially at maximum cycling as you proposed.

[azdave]

So, the consensus is that I should just stay on the grid until the grid fails?

Yes. Buy a small backup generator if you need some A/C power when the grid is down. Far easier and cheaper than what you are planning.

[Convert]

The batteries in an EV are designed for multiple cycles yet they too will lose the ability to get back to 100% charged after they have been discharged a number of times and will need to be replaced. A deep cycle solar pv system battery will only last a number of cycles per the manufacturer and you will use them up faster with your plan.

As for not wanting to give excess power back to the POCO. Fine there are grid tie inverters that will allow you to set them up for Zero Output so you don't sell the POCO one kWh. But the real question is why would you want to pay all that money to the battery seller?

It is real simple math. Add up the cost of the panels, charger, batteries, wiring & inverter. Then determine the number of "cycles" that battery system can provide you and how many kWh it will provide you each "cycle". Total up the kWh that the battery can provide and divide that number into the cost of you solar pv system. Depending on what you have paid and the quality of the battery you will come up with a number somewhere between $0.50/kWh and $1.25/kWh. I am sure you are paying less than $0.25/kWh so why would you pay 5 times the cost to generate your own power from batteries?

Thank you SUNEAGLE for your explanation. You've certainly got me (re) thinking this premise.
I'll still want the batteries but, perhaps, only for those (very few) times the grid fails. Actually, I may be able to utilize the batteries in my golf cart to cover those times.

[Convert]

Not their fault you do not know WTF you are doing.

Tell you what. I got a great investment tip for you. Give me $10,000, and I will give you $100 per year for the rest of your life. At least it is a beter investment than you are looking at.

SUNKING. Thank you for your insights.
Could you be a little more condescending, please. I'm still feeling excited and inquisitive.
Seriously, if I knew "WTF" I was doing, I wouldn't be here asking for your help!

[Sunking]

Seriously, if I knew "WTF" I was doing, I wouldn't be here asking for your help!

You said and I quote you directly:

Somewhat new to solar but have a pretty good understanding of how it all works.

You clearly do not understand anything about solar and economics.

[Convert]

You are a bit confused about the grid interconnect situation.
FL has NET metering. net metering means that you TRADE at full retail value kWh. Excess Generation is the power you generate beyond your consumption in a period, usually a year. Excess generation is payed out at wholesale rate or roughly 25% retail.
Using Net metering is 100% efficient and can allow you to get very close to a zero dollar bill at a much lower equipment cost.

Battery, particular off grid battery systems are far from 100% efficient and the batteries will not last that long, especially at maximum cycling as you proposed.

BUTCHDEAL. Thank you for your input. Unfortunately, my utilities company will only credit $0.04/ Kwh but I suppose that's better than nothing. My supposition was to utilize all that I generate (thus the batteries) without dealing with the provider. Obviously, that's not going to 'cut it'. Guess I'll need to look into a meter and an appropriate inverter.

[Convert]

You said and I quote you directly:



You clearly do not understand anything about solar and economics.

True!
But I'm learning.
I only meant that I understand how a solar panel (and the associated support gear) works.
I'm happy to have the input on 'systems'. Never considered most of the factors therein.

[J.P.M.]

BUTCHDEAL. Thank you for your input. Unfortunately, my utilities company will only credit $0.04/ Kwh but I suppose that's better than nothing. My supposition was to utilize all that I generate (thus the batteries) without dealing with the provider. Obviously, that's not going to 'cut it'. Guess I'll need to look into a meter and an appropriate inverter.

Either you're confused or I'm reading FPL's website wrong, or FPL is not your POCO, but FPL claims to be paying full retail on what looks like the usual net metering arrangements. Excess generation is at ~ $0.04/kWh, pretty much like elsewhere.