Comments on: Electricity math! Need help planning for solar...
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Comments on Ask MetaFilter post Electricity math! Need help planning for solar...Tue, 23 Apr 2013 12:41:42 -0800Tue, 23 Apr 2013 12:43:25 -0800en-ushttp://blogs.law.harvard.edu/tech/rss60Question: Electricity math! Need help planning for solar...
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar
So instead of providing an answer, please help me by showing me how you got your result.
I have a 12v device that consumes 1W in idle mode, and 5W in active mode. How many "amp hours" am I looking at over a 24 hour period (always on). I am looking specifically for amp hours... I don't understand the relation between amps and amp hours yet.post:ask.metafilter.com,2013:site.239673Tue, 23 Apr 2013 12:41:42 -0800squirbelSolarelectricityBy: rockindata
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475694
How many hours of active versus idle?comment:ask.metafilter.com,2013:site.239673-3475694Tue, 23 Apr 2013 12:43:25 -0800rockindataBy: squirbel
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475699
Approximately 1 hour out of the entire 24.comment:ask.metafilter.com,2013:site.239673-3475699Tue, 23 Apr 2013 12:46:36 -0800squirbelBy: rockindata
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475712
<a href="http://www.wolframalpha.com/input/?i=%28%285watts%2F12volts%29+*1+hour+%2B+%281watts%2F12volts%29+*1+hour+%29in+ampere+hours">Wolfram Alpha to the rescue!</a><br>
<br>
((5watts/12volts) *1 hour + (1watt/12volts) *1 hour ) = 0.5 Ampere hourscomment:ask.metafilter.com,2013:site.239673-3475712Tue, 23 Apr 2013 12:52:19 -0800rockindataBy: squirbel
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475715
Thank you so much! With that I'll be able to finish the rest of my calculations! :D Doh! Wolfram? XDcomment:ask.metafilter.com,2013:site.239673-3475715Tue, 23 Apr 2013 12:53:26 -0800squirbelBy: LowellLarson
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475716
Except it's 23 hours of idle time, not one. So:<br>
<br>
5/12*1 + 1/12*23.<br>
<br>
I would round it to 1/2 + 2 = 2.5 amp hours.comment:ask.metafilter.com,2013:site.239673-3475716Tue, 23 Apr 2013 12:54:08 -0800LowellLarsonBy: rockindata
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475717
Except, you know, stupid things like getting the numbers right.<br>
<br>
<a href="http://www.wolframalpha.com/input/?i=%28%285watts%2F12volts%29+*1+hour+%2B+%281watts%2F12volts%29+*23+hours+%29in+ampere+hours">Try this!</a> <br>
<br>
((5watts/12volts) *1 hour + (1watts/12volts) *23 hours ) = 2.333 Ampere hourscomment:ask.metafilter.com,2013:site.239673-3475717Tue, 23 Apr 2013 12:54:35 -0800rockindataBy: spacewrench
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475722
Power = Voltage * Current (P=IE), so Current = Power / Voltage (I=P/E)<br>
<br>
At idle, current = 1W / 12V = 1/12A = 83mA.<br>
Active, current = 5W / 12V = 5/12A = 417mA.<br>
<br>
If you're active for 1 hour, that's 1hr * 417mA = 417mAH (about 1/2AH)<br>
If you're idle for 23 hours, that's 23hr * 83mA = 1916mAH (about 2AH)<br>
<br>
You should be able to run your device on a 3AH battery (or larger), as long as you can average enough recharging every day. A bigger battery than necessary can hold extra charge, so even if you don't get 2.5AH one day, your system won't die.<br>
<br>
If you put in a minimum-sized battery, then you can't take advantage of sunny days, and you might run out after a couple of cloudy ones.comment:ask.metafilter.com,2013:site.239673-3475722Tue, 23 Apr 2013 12:56:26 -0800spacewrenchBy: straw
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475723
So amp hours are amps consumed over an hour, 1 amp for 1 hour is 1 amp hour, 2 amps consumed for 5 hours is 10AH.<br>
<br>
For DC, watts is volts times amps, which means amps is watts divided by volts (yay algebra!). So 1 hour in idle mode is 1 watt / 12 volts, and 1 hour in active mode is 5 watts / 12 volts.<br>
<br>
So 1 hour of active mod and 23 hours of idle mode means...<br>
<br>
(1 hour * 5 watts / 12 volts) + (23 hours * 1 watt / 12 volts)<br>
<br>
or 5/12+23/12 = 28/12 or 2.3333... amp hours at 12 volts to drive this thing for a day.comment:ask.metafilter.com,2013:site.239673-3475723Tue, 23 Apr 2013 12:56:37 -0800strawBy: squirbel
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475726
My device is a 12VDC and it has a power brick that allows it to plug into AC...comment:ask.metafilter.com,2013:site.239673-3475726Tue, 23 Apr 2013 13:00:41 -0800squirbelBy: sebastienbailard
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475757
The water analogy:<br>
<br>
Amps are chunks of electrical charge per second. Like 100 golf balls per second. Or 100 liters per second.<br>
<br>
Amp hours: So many amps times so much time.<br>
100 liters per second * 1 hour = ( 100 liters / second ) * 3600 seconds = 360,000 liters.<br>
<br>
(Only, amps are <a href="http://en.wikipedia.org/wiki/Coulomb">Coulombs</a> per second, where a coulomb is 6.2 ×10e18 electrons.)<br>
<br>
So if it was <br>
100 Amps * 1 hour = (100 Coulombs / second ) 3600 seconds = 360,000 Coulombs.comment:ask.metafilter.com,2013:site.239673-3475757Tue, 23 Apr 2013 13:33:04 -0800sebastienbailardBy: squirbel
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475767
Imagine swirly, confused eyes. Thanks for the explanation!comment:ask.metafilter.com,2013:site.239673-3475767Tue, 23 Apr 2013 13:39:06 -0800squirbelBy: tommasz
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475780
Just to be pedantic, if it's a 12V device that can be powered by a car battery or through an accessory outlet (AKA cigarette lighter), it operates at a nominal 13.8V. Just substitute for the 12 in the formulas above.comment:ask.metafilter.com,2013:site.239673-3475780Tue, 23 Apr 2013 13:54:00 -0800tommaszBy: PercussivePaul
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3475814
It's confusing, I know. If you use the water analogy, Amps might map to something like 'garden hoses', as in 10 hoses is a 10X bigger flow than 1 hose. And then hose-hours means 'enough pressurized water to run one hose for one hour'. You fill your water tower up with many hose-hours worth of pressurized water. With electricity amp-hours means 'enough stored electrical energy to push 1 Amp worth of electricity for one hour'. You fill your battery up with many amp-hours worth of electrical energy. <br>
<br>
(at a given 'pressure', or voltage, which is 12V in this case. The higher the pressure, the faster you go through water, so the more water you need to make up one hose-hour at that pressure. Thus one amp-hour in a 12V battery is a smaller quantity of energy than one amp-hour in a 24V battery -- half, in fact.) <br>
<br>
When doing your planning be sure to overdesign for your requirements. A 3Ah battery may only give you 2Ah in practice, and there may be extra drains and losses that increase the amount of energy you need.comment:ask.metafilter.com,2013:site.239673-3475814Tue, 23 Apr 2013 14:27:36 -0800PercussivePaulBy: hattifattener
http://ask.metafilter.com/239673/Electricity-math-Need-help-planning-for-solar#3476241
Since you're planning for solar, PercussivePaul's last paragraph is a good one to keep in mind. A battery's amp-hour rating is usually a best-case number— it's based on whatever situation gives the best number, typically a fairly slow discharge. (A good battery spec sheet will list different AH numbers for different rates of discharge.)<br>
<br>
Also, you probably don't want to plan to fully discharge your battery very often — lead-acid batteries, in particular, <em>hate</em> deep discharge cycles (deep-cycle marine batteries or AGM cells or etc. do better than car batteries, but deep-cycling is still not something you want to do regularly unless you plan to replace the battery a lot). Some other chemistries, like lithium-ion, don't mind being deep-cycled as much, but might have other drawbacks, like cost. So you might need to size your battery so that you don't expect to draw it down below, say, 40% very often. Yes, this is all super complicated. :/comment:ask.metafilter.com,2013:site.239673-3476241Tue, 23 Apr 2013 23:24:00 -0800hattifattener