Electricity experts required...

You know the bit you think you can do?

You can't do it.

team47b wrote:

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 you're ignoring the inefficiencies of the inverter then the answer is 50watt. As p=va you can vary the v as much as you like - the a drops and rises but the p stays the same ....

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

that'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

the cousins could have worded that better!

it 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/

team47b wrote:

DC fridge 6.75amps 24 hr
Entire house AC 5amps per 24hr

Ummm. No. You're mixing up your units again.

You can say "my fridge used 6.75Amps for e.g. two hours", but not "my fridge used 6.75Amps per 24 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)

team47b wrote:

My fridge is a DC 45watt brushless motor, no power surge when the compressor kicks in.

At 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?

you'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 time

ampere 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 period

in 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 hours of your battery capacity, for other times it's (hours * current), i.e. if it runs for 14 hours then it will use 28 ampere hours of capacity

if you mean the average current over the course of a day is 15 amps, then your daily usage 15 * 24 = 360 ampere hours

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

i really do recommend getting professional help to design and install a system of this capacity, the stored energy is sufficient to cause extremely violent 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 fails

team47b wrote:

So to clarify I will attempt to use the correct terms, please allow a little leeway...

I draw 15 ampere hours from my system.
(This is the total I use - 360Wpd in my language)

hmmmm, 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 should know this and add in appropriate breakers/fuses

team47b wrote:

Nothing is being increased.

I 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.

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

team47b wrote:

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

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

wattage is an instantaneous measure 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