# Some basic battery bank calculations

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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

Okay, so now your batteries are flat and need recharging.
I assume you are going to solar or wind turbine recharge these batteries, or
are you intending running an extension lead over the fence and stealing your
neighbour's power to recharge the batteries?

Re: Some basic battery bank calculations

This was drafted out from the thread about solar power installers (my
newsagent sulked). It is to do with battery banking off peak power and
using it during the peak/shoulder periods o run equipment. The batteries
are recharged as off peak equipment.

Re: Some basic battery bank calculations

Don't forget the battery charging losses too, lithiums would be
perfect in that respect only with nearly 100% charge efficiency.

Re: Some basic battery bank calculations

Only on that aspect. Otherwise, they are exceedingly expensive. Lithium
is ideal in instantaneous high pulse power situations, or as a very
expensive weight saving device.

Re: Some basic battery bank calculations

Also they deteriorate rapidly if any air gets into them. :-(

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

You didn't go through anything. You have consistently displayed a lack of
understanding of some basic facts. Hint have a look at your last

is not my figure.

Re: Some basic battery bank calculations

You ignored all the rest.

Where are these batteries?

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

Err, that was the figure after the loan was paid.

Who gives a stuff. I'm only interested in the loan interest rates.

Re: Some basic battery bank calculations

Why? The whole point of the exercise is to increase one's future wealth.
As far as possible you want to factor in all the relevant variables.
Doing otherwise can present the illusion that the path taken is optimum,
when it's not.

Sylvia.

Re: Some basic battery bank calculations

Yep, and if I'm not paying for electricity, that is doing it. You
obviously missed that point.

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?

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.

Re: Some basic battery bank calculations

These are not your figures, but a deliberately misinterpreted set of mine.
Do your own calculations and stop digging yourself deeper into the hole