Some basic battery bank calculations

> - if you

>>> can find an economic way of doing it. However, even running the >>> numbers on the back of an envelope will quickly show that it's not >>> just uneconomic - it's hugely uneconomic. >> >> Care to share them? > > I lost the envelope. But calculate the cost of the batteries you'd need > to support your day-time load. Then calculate the amount the interest > you'd earn on the money if you didn't buy batteries. Then calculate the > diffence in cost of the electricity you achieve through changing the > times you draw power from the grid. > > Finally, take into account the very limited life of batteries. > > The numbers show that it's a no-brainer. > > Which is why the power generation industry isn't doing it already.

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. or $3,905.50c over ten years.

Reply to
terryc
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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?

Reply to
Alan Rutlidge

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.

Reply to
Sylvia Else

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.

Reply to
terryc

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

Reply to
Mark Harriss

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.

Reply to
terryc

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.

Reply to
Sylvia Else

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.

Reply to
terryc

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 electricity bill and you will realise where you made your mistake.

is not my figure.

Reply to
terryc

You ignored all the rest.

Where are these batteries?

Sylvia.

Reply to
Sylvia Else

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.

Reply to
Sylvia Else

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

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

Reply to
terryc

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.

Reply to
Sylvia Else

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

Reply to
terryc

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

Reply to
Alan Rutlidge

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.

Reply to
Sylvia Else

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.

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

Reply to
terryc

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.

Reply to
Sylvia Else

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

Reply to
terryc

Explain how your figures should be interpreted then.

Sylvia.

Reply to
Sylvia Else

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