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**posted on**

March 25, 2009, 4:48 am

Power industry isn't doing it because their size in batteries can not be

purchased at the local garage.

Okay, taking my fileserver, which requires 96 watts max and my LG995E CRT

monitor which requires 72 watts max, that means 168 watts I need out of

the battery bank, or 14 amps @ 12Volt.

So, to cover 7am to 10pm, I need to store 15x14 = 210 amp hours. In lead

acid, this means 420amps hours.

Practically, that equates to 5x100AmpHr batteriess hour at $200 (old

price), or $1,000. 10 year life span means costs 28c per day in

depreciation. Loan costs is 6% atm or 20c per day.

Electricity savings are 15(0.1575-0.0554) =$1.53c. Net savings are $1.07c

per day. <BLINK> or $3,905.50c over ten years.

Re: Some basic battery bank calculations

I think you lost a factor of 100 there. It's 3905.5 cents, or just over $39.

10 year life with a 15 hour discharge to 50% capacity and 9 hour charge

back to 100% on a daily cycle?

What about the charger, the inverter, and the timer? They cost money too.

Sylvia.

Re: Some basic battery bank calculations

Nope, but I think it proves that you didn't do any back of envelope

calculations as you completely missed my senior moment that left out the

correct factor and used elapsed time.

In any case, it is currently break even if you can get 15.75c/kwh as your

max power cost.

A charger is the only other piece of equipment you need and a good one

lasts for decades. Computers can run directly off 12v these days. The

time is the off peak switch.

Re: Some basic battery bank calculations

I didn't go through your calculations with a fine toothcomb. I don't

know what you meant by "nope". 1.07cents per day is 390.55 cents per

year, or 3905.5 cents in ten years = about $39.

And subject to getting a ten year life out of lead-acid batteries used

in this regime. Where are these 100 Amp-Hour batteries for $200 that

will last 3650 cycles?

BTW, what about the efficiency? The charge discharge cycle of a battery

loses quite a lot of energy.

How must does it cost?

Computers can run directly off 12v these days.

Mine can't.

Did you include the cost of the switch? Is that a practical solution?

Sylvia.

Re: Some basic battery bank calculations

As you pointed out, you've omitted the factor of 0.168, reducing the

gross saving to $0.257, and meaning your net saving is negative.

If you're going to do this properly, then you should assume that you're

paying back the loan from the electricity cost savings. You should also

factor in inflation. In practice, as long as both interest rates and

inflation rates are reasonably small, you can subtract the inflation

rate from the interest rate to give a "real interest rate", and then

treat the inflation rate as zero.

The daily saving is 0.168kW

*** 15 hours ***$0.1021/kWh = $0.257 per day.

Treated as 12 equal length months per year, that's $7.83 per month

that's available to pay off the loan.

Run the numbers on that with 6% interest and 3% inflation, and monthly

repayments, and after 10 years, you still owe $289 (in present day dollars).

To break even, you'd need to get the batteries and other equipment for

$791. And this is still assuming that batteries will last for 10 years,

that nothing simply breaks and has to be replaced, and ignoring any

losses in the charge-discharge cycle.

Sylvia.

Re: Some basic battery bank calculations

If you're not paying for (or rather in the battery bank case, paying

less for) electricity but are paying more for the equipment required

than you are saving, then your future wealth is reduced.

I'd still like to have a link for these alleged 3650 cycle batteries.

Sylvia.

Re: Some basic battery bank calculations

But I'm not, for which i produced figures to show this. If you want to

discuss this further, then produce your own set of figures.

Do you have trouble using a search engine for basic searches?

Read here, then progress.

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

Basically, if you do not see the benefit, then don't do it.

Re: Some basic battery bank calculations

Your electricity savings were $0.25 per day, not $1.53 per day, as I

thought you'd realised, because you're drawing 0.168 kW, not 1kW. That

saving is completely wiped out by your own estimation of interest and

depreciation.

The true situation is not so bad, as I indicated, because the interest

drops as you pay back the loan. But there's still a net loss over 10 years.

And the place where you intend to source the 10 year life batteries at

the prices you stated?

Sylvia.

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