are my calculations accurate to minimize my electric bill using battery and inverter no solar panel

tglozano

New Member
I don't get enough sunlight in my area, so I would use solar only in an emergency. I live near San Francisco, CA. My energy company, PG&E charges $.28 kWh off peak and $.32-$.42 on-peak hours. My electric bill is usually $200 a month with 90% at higher $.32 kWh. I want to use my 12v, 105 ah lead acid deep cycle battery and a 2000 w pure sine inverter, to run some AC loads about 800w. But, I want to connect my battery to an 120v ac outlet (the grid) using my 10a battery charger. My battery charger "Input 120VAC 0.6A continuous, 3A Intermittent" and of course outputs 10amps. Would this be safe, doable and cost effective? To summarize: I want to use my 10amp battery charger inside the house to continuously charge my 12v, 100a, battery during my electrical peak hours (6 hours).
 

Bubba1

Solar Addict
I don't think the math works out. Your battery has about 600w of usable power. I could be wrong by I think you'll save about$.09 a day.
 

corporate.joe

Solar Addict
Am I reading this right? 90% of your bill($$) is from the peak usage during those 6 hours? Or is 90% of your Kwh measured usage during that timeframe? I might need another coffee first.
 

tglozano

New Member
Am I reading this right? 90% of your bill($$) is from the peak usage during those 6 hours? Or is 90% of your Kwh measured usage during that timeframe? I might need another coffee first.
Yes, reading correctly. I have a 2 tier rate. after 218 kWh the rate jumps to $.32+.
 

Supervstech

Administrator
Staff member
Moderator
Yes, reading correctly. I have a 2 tier rate. after 218 kWh the rate jumps to $.32+.
This implies the rate is usage based, not time of day based.
If usage based, running a charger and inverter will nearly double your usage over peak...
 

schmism

Solar Addict
You can not "mix" AC output of 2 sources at will. They need to be phased match or there are issues. Dumb devices like "normal" inverters are not capable of doing this.

There are inverters that can do this, Solark has time based management built in to the software so you can draw off the grid durring non-peak and then push back to the grid durring peak. However thats also a $7k-$10k investment which given the tone of your post, is not within your budget.

Other turn key solutions include the Tesla power wall which is about the same money.
 

tglozano

New Member
This implies the rate is usage based, not time of day based.
If usage based, running a charger and inverter will nearly double your usage over peak...
Am I understanding by battery charger correctly? It uses "Input 120VAC 0.6A continuous, 3A Intermittent", but outputs 10 amps. Isn't that like a 1:10 ratio?
 

Supervstech

Administrator
Staff member
Moderator
If you are charging the batteries there is a loss from the charger, there is an additional loss from the inverter, so two losses.
Now, if you can charge during off peak time, then use power during peak from the battery that might work... but if your rates are simply raised when you exceed a set KWH, then the battery will waste electricity.
 

boondox

Chief Engineer, RedNeckTech Industries
A 12 volt, 100 amp hour battery has 1,200 watt hours of power. Since you don't want to run it all the way dead, let's say 1000 wH of power if it is a a lithium battery and 600wH if it is a lead battery. We'll go with the lithium battery and say that you have a kWh of storage, or $0.28 to $0.42 worth of power in it. So at most you can save $0.14 worth of power. But then as has been pointed out, you also lose some due to the inefficiency of both charging the battery and converting it to AC when using it. Let's say that runs around 40%. $0.05 worth of your power. So the net savings would be about $0.09.

If you want to save money on power, learn how save energy. Better for your wallet, the grid and the planet.
 

tglozano

New Member
A 12 volt, 100 amp hour battery has 1,200 watt hours of power. Since you don't want to run it all the way dead, let's say 1000 wH of power if it is a a lithium battery and 600wH if it is a lead battery. We'll go with the lithium battery and say that you have a kWh of storage, or $0.28 to $0.42 worth of power in it. So at most you can save $0.14 worth of power. But then as has been pointed out, you also lose some due to the inefficiency of both charging the battery and converting it to AC when using it. Let's say that runs around 40%. $0.05 worth of your power. So the net savings would be about $0.09.

If you want to save money on power, learn how save energy. Better for your wallet, the grid and the planet.
thanks for your feed back. I do have a lithium iron battery I could use too.
 

boondox

Chief Engineer, RedNeckTech Industries
Some more math...

So lets say you use a Battleborn battery. $800. Those have about 1kWh of usable power and are rated for between 3000 and 5000 cycles. At $0.09 savings per cycle it would take 8,888 cycles to pay for the battery. Of course, it would be dead long before that.

So in short, your scheme just won't work.
 

tglozano

New Member
Some more math...

So lets say you use a Battleborn battery. $800. Those have about 1kWh of usable power and are rated for between 3000 and 5000 cycles. At $0.09 savings per cycle it would take 8,888 cycles to pay for the battery. Of course, it would be dead long before that.

So in short, your scheme just won't work.
ok got it and thanks.
 

RCinFLA

Photon Sorcerer
Do you have a smart meter for your utility meter? Many power companies have converted to smart meters that report useage quite often.

If you do have smart meters you should check utility website. By creating a logon account you may be able to download usage at least to hourly resolution.

Using lead acid batteries is a big net loser if you try to time shift power useage. Besides about 75-85% round trip charge-discharge efficiency you also add the efficiency loss for hybrid grid interactive inverter charger, Typically in range of 92%-95% chg, and discharge. Add in final losses for cabling/current shunt. 1-3%. So summing all the losses is approx. 80% x 92% x 92% x 99% x 99% = about 66% round trip. Just efficiencies make
the cheaper off peak power cost go from $0.28/kWH to $0.28/kWH / 0.66 = $0.42/kWH time shifted cost before adding in cost for lead-acid battery replacement costs every 1 to 3 years.

Changing lead acid to LFP takes just battery round trip efficiency from 80% to 96-99% (LFP @ 0.2C roundtrip = 99%, @ 0.5C = 96% roundtrip). That gets your $0.28/kWH time shifted to $0.34/kWH. Cycle life for LFP will be about 8-10x that of lead-acid. I would use 7-10 years for LFP replacement cost amortizing.
 
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anadiner

Solar Enthusiast
I don't get enough sunlight in my area, so I would use solar only in an emergency. I live near San Francisco, CA. My energy company, PG&E charges $.28 kWh off peak and $.32-$.42 on-peak hours. My electric bill is usually $200 a month with 90% at higher $.32 kWh. I want to use my 12v, 105 ah lead acid deep cycle battery and a 2000 w pure sine inverter, to run some AC loads about 800w. But, I want to connect my battery to an 120v ac outlet (the grid) using my 10a battery charger. My battery charger "Input 120VAC 0.6A continuous, 3A Intermittent" and of course outputs 10amps. Would this be safe, doable and cost effective? To summarize: I want to use my 10amp battery charger inside the house to continuously charge my 12v, 100a, battery during my electrical peak hours (6 hours).

I don't get enough sunlight in my area, so I would use solar only in an emergency. I live near San Francisco, CA. My energy company, PG&E charges $.28 kWh off peak and $.32-$.42 on-peak hours. My electric bill is usually $200 a month with 90% at higher $.32 kWh. I want to use my 12v, 105 ah lead acid deep cycle battery and a 2000 w pure sine inverter, to run some AC loads about 800w. But, I want to connect my battery to an 120v ac outlet (the grid) using my 10a battery charger. My battery charger "Input 120VAC 0.6A continuous, 3A Intermittent" and of course outputs 10amps. Would this be safe, doable and cost effective? To summarize: I want to use my 10amp battery charger inside the house to continuously charge my 12v, 100a, battery during my electrical peak hours (6 hours)
Your question:
Safe? Sure. Im a safety addict. Even use welding blankets. I guess what Im saying is while I see nothing about overheating or overcharging protections, u can encase in nonflammable things in the event of a fire.

Doable? The 2 ac output post ... think meant to say 2 input but I still dont see that either. Correct me if wrong, ur wanting ongrid ac outlet -> battery charger -> inverter -> run some simple appliances during peak times.
Basically u want to charge battery with ac grid in place of solar panel -> controller -> battery.
Yes doable.

Cosr effective? No.
Think simple cuz its obvious.
1. Maybe if u wanted to go opposite route during peak times = panels->inverter->ac output. Run simple appliances off strictly solar. BUT u said gets cloudy and this would be daytime only so not what ur wanting.
2. Seems obvious to me the grid power consumption to keep batteries charged is gonna be greater than output power from batteries.

Common sense says no to this project. Can make it safe. Yes doable. Worth time and trouble, no.
 

anadiner

Solar Enthusiast
Btw u would probably get tired of this project within few months.

If u go ahead: recommend trickle charger (can be solar). Keeping batteries charged for occasional use is difficult... they drain during nonuse periods)
 

12VoltInstalls

Solar Addict
your scheme just won't work.
It never does

middle-US powercos are experimenting with gigawatt inverter stations that buy off-peak grid watts and return it to the grid during peak. Somehow this fractionally works for them apparently.

the only time it works for kilowatt level systems is if solar enables a situation where savings in another area are possible
 

anadiner

Solar Enthusiast
Do you have a smart meter for your utility meter? Many power companies have converted to smart meters that report useage quite often.

If you do have smart meters you should check utility website. By creating a logon account you may be able to download usage at least to hourly resolution.

Using lead acid batteries is a big net loser if you try to time shift power useage. Besides about 75-85% round trip charge-discharge efficiency you also add the efficiency loss for hybrid grid interactive inverter charger, Typically in range of 92%-95% chg, and discharge. Add in final losses for cabling/current shunt. 1-3%. So summing all the losses is approx. 80% x 92% x 92% x 99% x 99% = about 66% round trip. Just efficiencies make
the cheaper off peak power cost go from $0.28/kWH to $0.28/kWH / 0.66 = $0.42/kWH time shifted cost before adding in cost for lead-acid battery replacement costs every 1 to 3 years.

Changing lead acid to LFP takes just battery round trip efficiency from 80% to 96-99% (LFP @ 0.2C roundtrip = 99%, @ 0.5C = 96% roundtrip). That gets your $0.28/kWH time shifted to $0.34/kWH. Cycle life for LFP will be about 8-10x that of lead-acid. I would use 7-10 years for LFP replacement cost amortizing.
Oh I completely forgot about a watt meter. I bought one for like $15 off Amazon. They can be fun. Just go around house plugging it in... fan, laptop, etc showing u watts being used.
 

coolbz

New Member
I don't get enough sunlight in my area, so I would use solar only in an emergency. I live near San Francisco, CA. My energy company, PG&E charges $.28 kWh off peak and $.32-$.42 on-peak hours. My electric bill is usually $200 a month with 90% at higher $.32 kWh. I want to use my 12v, 105 ah lead acid deep cycle battery and a 2000 w pure sine inverter, to run some AC loads about 800w. But, I want to connect my battery to an 120v ac outlet (the grid) using my 10a battery charger. My battery charger "Input 120VAC 0.6A continuous, 3A Intermittent" and of course outputs 10amps. Would this be safe, doable and cost effective? To summarize: I want to use my 10amp battery charger inside the house to continuously charge my 12v, 100a, battery during my electrical peak hours (6 hours).

I don’t think it’s cost effective. There will be waste during battery charge and discharge, inverter idle watt, plus your inverter and battery cost, even with 26% federal credit (you have to claim this as solar power system by adding a bit solar component ), you may never break even.

in my area, where the rates can be more extreme like $0.5 vs $0.09, it may worth to arbitrage using a battery inverter system.

I just hate my power company. They raise rate at their will to cover cost from, free power to “poor” people, fire damage due to their neglect maintenance, etc.
 

sunshine

Solar Addict
Just on maths:
The 14% difference between your 28c off peak and 32c peak doesn't match the 40% loss from the (20% Charger & 20% Inverter) and this doesn't include the battery losses- in & out!

However your thinking is taking you along the right path.
Tesla is doing similar here in Australia with their giant batteries at wholesale level, the prices between high and low are greater due to grid stabilisation issues, Changes to billing cycles in big batteries favour will ensure we consumers will be paying more and Musk getting richer!
 

time2roll

Photon Sorcerer
How much power do you use during on-peak? I think you will need 10 batteries to properly accomplish what you are looking at. Need 100+ amp charging to get back ready for the next day.
 
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