# Electricity experts required...

team47b
Posts:

**6,424**
I am having a little trouble getting my head around the mathematics of electrical consumption.

If my iPad takes 10 watts and 5 hours to charge it uses 50 watts - I can do that bit

The question is in two parts and extra points will be awarded for showing your workings out! Feel free to convert from watts to amps if this makes the maths simpler

If the 240v AC socket I plug the iPad into is from a permanently inverted source coming from a 24v DC source, how many watts am I consuming from my DC source? Please ignore the 5% power wasted by the inverter. Probably ignore the fact that the iPad plug is converting to 5v USB too. This is not a trick question

If I drop down (convert) from a 24v DC source to a 12vDC socket and plug in the iPad how many watts does the iPad now use?

I know the answer is really simple but then so am I today

If my iPad takes 10 watts and 5 hours to charge it uses 50 watts - I can do that bit

The question is in two parts and extra points will be awarded for showing your workings out! Feel free to convert from watts to amps if this makes the maths simpler

If the 240v AC socket I plug the iPad into is from a permanently inverted source coming from a 24v DC source, how many watts am I consuming from my DC source? Please ignore the 5% power wasted by the inverter. Probably ignore the fact that the iPad plug is converting to 5v USB too. This is not a trick question

If I drop down (convert) from a 24v DC source to a 12vDC socket and plug in the iPad how many watts does the iPad now use?

I know the answer is really simple but then so am I today

my isetta is a 300cc bike

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## Posts

4,9135,271You can't do it.

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4,736Your iPad consumption module is damaged, or your charger isn't working correctly.

8,4986,424Did you even read the question?

6,424I was not understanding the relationship between amps and amp-hour requirements of 240volt AC items versus the effects on their DC low voltage batteries. For example, say you have a 24 volt nominal system and an inverter powering a load of 3 amps, 240VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah). However, in order to determine the true drain on your batteries you have to divide your nominal battery voltage (24v) into the voltage of the load (240v), which is 10, and then multiply this times your 240vac amp hours (10x 12 ah). So in this case the calculation would be 120amp hours drained from your batteries - not the 12 ah. Or put another way take the total watt-hours of your 240VAC device and divide by nominal system voltage. Using the above example; 3 amps x 240volts x 4 hours = 2880watt-hours divided by 24 DC volts = 120amp hours.

5,271The power consumption is 10 watts, ignoring converter losses, thermal losses etc.

Your original calculation is incorrect. Actually it's nonsense.

10W for 5 hours is still 10W. 10W for 5 hours does however yield 180000 joules total energy...or, if you take into account the 5V iPad connection, 10Ah (amp.hours).

10Ah is quite a lot for a little battery. The battery in my laptop is a little over half that.

What are you trying to figure out?

ETA: Wow, the iPad could easily have a really big battery!

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6,424Sorry for the confusion :oops:

I am trying to work out the total amps used to charge up the iPad and how many amps are discharged from the solar battery bank through the inverter and whether there is a significant difference.

I can plug the iPad into a metered AC plug socket and see the total watts and amps used, it runs at around 10watts and will run for about 5 hours from flat and so will consume 50 watts in total.

In order to produce 50watts how much draw is there from the battery bank that the inverter is connected too? I have to start from this point as the inverter loss on a small item is totally different to large items so will be factored in when the whole house is calculated.

13,532if it's the case that the ipad takes five hours at 10w to charge, full charge == 50 watt hours

so in ampere hour terms, if your primary source were a 24 volt battery, the

totaldrain to charge the ipad would be 50/24, c. 2.1 ampere hourshowever, the

instantaneouscurrent drawn will be determined by how fast the ipad/charger can suck it up, if it's 10 watts then the current will be 10/24, c. 0.42 amperesdo the easy crosscheck, 0.42 * 24 * 5 = 50, yep, back to 50

all this ignores conversion losses and other inefficiences in the supply, if your objective is efficiency then you need to avoid multiple voltage conversion stages and stick to switchmode conversion

6,424That's what I said at the beginning before my question although I agree I should use the correct terms as it is misleading otherwise.

what I can't measure is how many amps are being drawn from my 24v battery that is supplying the power to the inverter so I have to calculate this bit. This is where I get stuck

My house is off grid.

I am trying to calculate the amount of energy required from my solar array to match my usage. Most items in my house run off the AC through the inverter, except my fridge runs off 24v DC. The fridge is easy to calculate the draw from the DC, it's two amps times hours running time over twenty four hours, no loss, no conversion.

But how do I calculate the energy drawn from the DC by the AC items?

(Ultimately I am trying to calculate battery amp size requirement)

(Ultimately, ultimately I am hoping to combine this thread with the conspiracy thread, do I have to take into account the fact that the sun is a flat disc the size of the moon?)

1,149If your I pad charger uses 10 watts ,ignoring invertor losses, 10 watts are being taken from your power source.If you want the amps,divide the watts by the volts.

Without getting complicated there is no such thing as battery amp size. Batteries are measured in amp hours I.e.lots of amps for short time or little amps for a long time. The amp hours x the battery voltage will give you the watt hours.

I hope that helps.

13,532that's the 0.42 in my example

cleanse your mind of all reference to the voltage/current/ac/dc of the devices you want to power, these things do not matter for the calculation and serve only to muddy the water

for all the stuff you want to connect, determine the wattage either by measurement or seeing what's on the device label etc.

so if you had six things that will run concurrently, with operational wattages of 5, 76, 10, 10, 20, 15, you simply add these up, 136 watts, that's the power the inverter will need to deliver

if your dc source to the inverter is 24 volts, the current will be 136/24, that's c. 5.7 ampere (and the drain is therefore 5.7 ampere per hour)

for efficiency, keep low voltage cabling as short and chunky as possible, and make sure crimped/clamped connections are all good, for a given power the lower the voltage the higher the current, so cable/contact resistance is much crueler to you at low voltages and can waste a considerable amount of energy

6,424My mind was muddied by this learned piece on an American solar web site...

One of the biggest mistakes made by those just starting out is not understanding the relationship between amps and amp-hour requirements of 120 volt AC items versus the effects on their DC low voltage batteries. For example, say you have a 24 volt nominal system and an inverter powering a load of 3 amps, 120VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah). However, in order to determine the true drain on your batteries you have to divide your nominal battery voltage (24v) into the voltage of the load (120v), which is 5, and then multiply this times your 120vac amp hours (5 x 12 ah). So in this case the calculation would be 60 amp hours drained from your batteries - not the 12 ah. Another simple way is to take the total watt-hours of your 120VAC device and divide by nominal system voltage. Using the above example; 3 amps x 120 volts x 4 hours = 1440 watt-hours divided by 24 DC volts = 60 amp .This is either b0llix or a little unfair on its intended audience

13,532it may not be an easy read, but i recalled one of the journalists on el reg has been doing things over the years to reduce energy usage and use solar/other power, might be ideas here you could use...

http://www.earth.org.uk/

6,424Calculations

DC fridge 6.75amps 24 hr

Entire house AC 5amps per 24hr

Total 12amps

Typical average day (sunny ) at this time of year solar produces about 60amps.

I have another question...

2,566Ummm. No. You're mixing up your units again.

You can say "my fridge used 6.75Amps

fore.g. two hours", but not "my fridge used 6.75Ampsper24 hours" (well you can say it but it makes no sense).Therefore, you probably want to say "My fridge uses 6.75 Amps for 2 hours in every 24 hour period" (or something like it)

That'd be steady current multiplied by voltage multiplied by time, giving you 324 watt hours (click that link), or just over a million joules, or the amount of calories in 1000 pints. (ish).

Edit: Your fridge probably doesn't run for 2 hours, nor does it use 7 amps over its duty cycle (I'm guessing)

6,424At the current room temperature it runs for 3.6 hours in every 24 hours at 45watts, which is 162watts which is 6.75amps total (it runs direct from 24volt it does not run from the inverter)

So to phrase it correctly do I say the fridge uses 6.75 amps total and runs for 3.6 hrs in every 24 hours?

DC fridge uses 6.75amps in 24hours?

13,532you've got the right figure, but your persistent mixing of units makes me cringe

it's not amps, it's ampere hours...

ampere (amp)- the current measured at a point in timeampere hour- unit often used to specify capacity of batteries in a user friendly way, denoting the nominal ability to deliver a certain amount of current for a certain periodin an ideal world a 1 ampere hour cell would deliver 1 amp for 1 hour, or 10 amps for 0.1 hour, or 0.1 amp for 10 hours etc.

in practice the usable capacity of cells tends to be lower than rated, especially as loads increase and voltage drops, cells also lose efficiency over time and depending on environment,

never rely on achieving rated capacity!)your fridge, when operating, uses 45 watts with a 24v supply, therefore where operating it draws 45/24 amps, let's call it 2 amps

if it runs for 1 hour, it will use 2

ampere hoursof your battery capacity, for other times it's (hours * current), i.e. if it runs for 14 hours then it will use 28ampere hoursof capacity6,424So, my new question...

(We will have to sort out my use of English later)

How 'big' do my batteries need to be?

I need to draw 15amps per 24hours, my solar produces about 60amps per day at this time of year. I would like 4 days autonomy and no more than 50% depth of discharge. 4x12volt wired in series two banks of two series/parallel giving me 24volt and double amps. How many amps is each battery?

13,532for 4 days to 50% discharge, that's 2880 ampere hours, assuming you intend using car batteries it'll take a

lotof batteriesi really do recommend getting professional help to design and install a system of this capacity, the stored energy is sufficient to cause

extremelyviolent and nasty things to happen in the event of failure, so the design needs to maintain safety under a range of fault conditions, because stuff fails1,149Good luck b47b.☺

6,424I draw 15 ampere hours from my system.

(This is the total I use - 360Wpd in my language)

I realise this is probably viewed as impossibly small!

4 days autonomy, 50% DOD.

I have a 24 volt battery bank, these are deep cell, flooded. They are not starter batteries or hybrids.

I have been using this system for about 14 years, so it is a safe system, and was installed by a qualified person who went to a college.

All I am trying to do is establish that the battery bank that I am about to replace is the best size and type for my requirements over the next seven years. Then I can order the correct size batteries and call in my qualified engineer to take off the 8 standard battery connections and bolt them back onto the new batteries.

Out of interest I have 4x12v 130A wired in series/parallel in two banks of two giving me 24v and 260A

EDIT: if you think i am difficult to understand, you should see the reaction when I discuss this in Portuguese with solar companies and with my German engineer! Electronics is not the only language barrier!

2,566Why not just go and read the stickers on them?

13,532hmmmm, this is still tangled up

do you mean the average current over the course of a day is 15 amps? (same as saying the average load is 360 watts for a 24v system)

if so, then your daily usage 15 amps * 24 hours = 360 ampere hours battery drain per day

or to calculate from the wattage: 360 watts / 24 volts is 15 amps, times 24 hours is 360 ampere hours

...i inserted units to make it clear, i hope, what is going on, with 360 and 24 each being, coincidentally, the value of two different parameters it's best to be certain whether it's watts or ampere hours and volts or hours involved

so four days automony will be 4 * 360 == 1440 ampere hours, with 50% margin taking it to 2880 ampere hours

if the "130A" on the existing cells is 130 ampere hours, then your current system has capacity of 260 ampere hours as configured, so you'd need to embiggen it by a factor of 11 to reach your target automony and margin

i hadn't realised the system is in place, makes it easier and with less chance of molten metal based screaming acid death, but...

adding the extra capacity will dramatically increase fault current (and indeed charge current) so it's vital to have overload protection on each pair of series connected cells - in the olds days i saw the outcome of a paralel/serial connected battery bank without such protection, it was mostly melted and not at all pretty

...however your local chap

shouldknow this and add in appropriate breakers/fuses6,424I use a very small amount of electricity.

I realise I am using the wrong terms so this leads you to conclude I use a large amount, I don't.

I use 360w in 24 hours. Not per hour. I use15 watts per hour average over a 24 hour period, if you prefer.

So in 4 days I use 1440watts, 50% DOD, therefore I need to be able to use 1880watts, 1880watts dived by 24volts equals 78.33amps, my bank could be 4 x 12 v 85Amp series/parallel connected batteries giving me a 24 volt system with 170 amps

I am trying to work out if my current (sorry) batteries are too big.

8,498Seriously, you are so muddled up you'd be better off just taking Sungods word for it ...

6,424I know I am muddled but I also know I don't use 360 ampere hours per day as Sungod said, I use 15 as I said

Was hoping for confirmation of required battery bank size based on this figure.

13,532like i said, you are causing confusion with abuse of units

wattage is an

instantaneousmeasure of power, you do NOT use "360 watts in 24 hours"watt hours is a different unit with totally different implication, 360 watts is not 360 watt hours

i assume you mean you use 360 watt hours every 24 hours, as if you use 15 watts average for 24 hours that is 360 watt hours

in a 24v system that means average current of 0.625 amps, over 24 hours that is 15 ampere hours drain on your batteries

for four days it's 60 ampere hours, with the 50% margin it's 120 ampere hours

which i think is what your current system capacity is

4,825Watts is power, AmpHour is energy

I think...you need 360W continuously over 4 days x2 for redundancy. So total power required is 2880wh (not 1880wh).

For a 24v system that equates to 120Ah - you need a 24volt battery rated at 120Ah.

Your suggseted setup using 4x12v batteries in a 2S2P setup would give you 24volts @ 170Ah, so you got that number right at least.

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6,424I understand now.

I am working on another question...