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Re: Back for more edumacation.....

concretefire wrote: »

Hi guys/gals.

Ok, so this is a two part question.

1) I have a digital camera batter charger. On the back of that charger it reads the following: INPUT: 110V-240V~50/60Hz 0.15A /// OUTPUT: :8.4V=.43A

Question ^^ I have no idea what that means: How can I "do the math" to figure out how many watts I'm using??? I know watts=amps*volts but I just don't understand what that charger is telling me.

In theory:

• 8.4 volts * 0.43 amps = 3.6 watts
• 110 volts * 0.15 amps = 16.5 Volt*Amps (this is not necessarily Watts)

Note that for small chargers, it is not unusual to have 70% efficiency or 0.60 PF (or worse)...

• Volt*Amps * Eff * PF ~ 16.5 VA * 0.70 eff * 0.6 PF = 6.9 Watts output

So even with a lot of guessing--Still a factor of ~2:1 between input based analysis vs output analysis... This just goes to tell you that if you are trying to estimate power usage, you really should do actual measurements with a Kill-a-Watt meter or equivalent.

Note with AC power--There are two important measurement values... One is Watts (the rate of work being performed) and VA (or volt*amps) which is how much current is actually being used for vs watts which is some fraction of current being used for "work".

• Watts = PF (power factor) * Volts * Amps

PF can be the Cosine of the angle of the current with respect to the voltage sine wave... PF can also be a measurement of non sine wave current used by the load (many electronics/battery chargers/etc. only pull current at "the peak of the sine wave"--This means that wiring (and inverters) have to be heavier to carry the total amount of current to supply the loads. Here is a Wiki Article about Power Factor.

So, more or less, the Watts is what you need to know to size the battery bank. PF/VA is what you need to know to size the wiring/inverter/AC generator.

Watts=VA if PF=1.0 (perfect power factor). If PF is less than one (typically 0.5 to 0.7 for many loads), then VA is greater than Watts (and why we need to size wiring/inverter/ac genset for VA). For many system, the wattage of the load is much less than the Inverter/AC Generator ratings--So people can "error" and use Watts for sizing and not get into too much trouble.

I have ANOTHER gadget (modem power supply) and on it is just says 12V 1A. Nothing about 110V and Hz....So that math was rather simple for me. 12V*1A = 12 watts. Right????? But not only do I need to understand how to read the input/outputs of more complicated things (such as my camera charger above) I also need to understand why you "plug" both of these items into a seemingly 12V "outlet."

You have the needs of the equipment (may be 12 VDC at ~1 amp) and you have the fact that it is much easier/cheaper and some ways safer to send power at 120/240 VAC instead--And you need the transformer/AC to DC converter/etc. to take the transmission level voltage and convert it to the local loads.

This is actually quite a complex subject... You have the low power DC stuff (12 volt, 8 volt, 6 volt, etc.)... But you also have things like florescent lamps (arc lamps) that need voltage to start the arc, but something to limit the current flow through the tube once the arc has started--Otherwise the tube would let too much current through and overheat/fail (these are typically called a ballast).

In other words, I see all these guys with "12V" systems and they just have your basic surge protector running and normal things plugged into that strip. Toasters, Fans, cell phone chargers, etc.... Just normal plug in stuff. Well which is it? Is it 110V system or a 12V system? I don't get that part as well.

It is easier to ask one question at a time--Because it is very confusing to address "everything" in one post at one time.

More or less:

• Power = Voltage * Current * Power Factor (PF=1.0 for DC circuits--at least for this level of discussion).

So, you can have a 120 watts delivered to your loads several ways:

• Power = Volts * Amps * PF
• 120 Watts = 120 Volts * 1 amp * 1.0pf (if you have 120 VAC @ 60 Hz US/North American power)
• 120 Watts = 240 VAC * 1 amp * 1.0pf (if you have 230-240 VAC US/European power)
• 120 Watts = 12 VDC * 10 amps (if you are powering from your car/12 volt battery bank)
• 120 Watts = 24 VDC * 5 amps (higher voltage DC battery bank)
• 120 Watts = 48 VDC * 2.5 amps (48 volt battery bank is the standard "high voltage" battery bank for home off-grid power)

So--All of the above are equivalent... But notice the current flow. At "higher" voltages, the current drops down (double the voltage/ one half the current, etc.). It is much easier to send 1 amp 100 feet at 120 VAC using thing/cheap wire vs sending 10 amps at 12 VDC using very heavy wire.

That is why we (typically) use 120-240 VAC for home power use and use 12,000 volts or higher for the Power Lines on top of the power poles.

Then there is how much work is done... Watts is a "rate" (like miles per hour).

If we want to know how much power we used, then we need to multiply by time (hours in our case)... So Watt*Hours (kWatts*Hours is home billing) is the same as MPH *Hour driving to know how many miles you drove.

2) Totally different topic: Battery charging.
I do not understand the BASIC math in charging a battery. Let's just say for the heck of it, I had a 100 AH battery. I'd only run it down to a maximum of 50AH left. So how long in general does it take to recharge to full capacity??? I assume this is where the Production of the PV panels comes into play? Like if I had a 100 Watt PV panel.... how would I do the math to figure out how long it would take to recharge that 100AH battery. But I've only used half of it so I only need to charge it back 50AH, roughly. So how do different size panels play into the formula for recharging batteries/ battery banks?

In theory, if you drain 50 AH from the battery bank, you need to replace 50 AH back... So, if you had a 100 Watt panel:

• 100 watts / 17.5 volt Vmp = 5.7 Amps of current (Imp)
• 50 AH / 5.7 Amps = 8.77 hours of charging time

But, there are a lot of caveats. Batteries typically take the above time plus a couple hours to recharge (i.e., as they approach 80-90% State of Charge, the accept less current). And there is the sun... You do not get 5.7 amps for 8.77 hours... You may only get an average of 5-6 hours on a nice sunny day of "noon-time equivalent sun"--So that 8.7 hours is now 1.5 days or so to recharge the battery bank.

I suggested a link for an AC Watt*Hour meter (the Kill-a-Watt brand above). There are similar meters for DC (here are a couple). And a good way to measure current is with an AC/DC current clamp meter (easier and safer than using a Digital Multi Meter where you have to "cut the circuit" and place the DMM in the current path--DMMs are OK for less than 10 amps and lower voltages, if you are careful). If you are going to do a lot of work with DC (or even AC) Current, get some sort of AC/DC Current Clamp DMM (it is much safer and more versatile).

You have asked a lot of questions... I would suggest that we do this in steps.

First lets discuss your loads--What is it you want to power, get the meter(s) to measure the loads (Watts, Amps, Hours) and define your needs (emergency power, backpacking, off-grid power, RV/Camping, etc.).

Once we have your loads characterized, then we can talk Battery Bank, Solar Array, charge controllers, generators, inverters and such...

Note that with your loads--Conservation is almost always cheaper than generating power (solar, generator, etc.). You may pay around $0.10 to $0.20 per kWH for your utility power--It is not unusual for off grid power to cost in the range of $1 to $2 per kWH -- Or 10x as much as utility/grid power. So wise load management/selection can save you lots of money and heart aches.

-Bill