I would like some real, measured numbers please...

[Marto]

Hello Everyone!

First post here, so let me introduce myself quickly. I have been reading lots and lots lately (including some stickies here) on solar power cause I would like to make the plunge and add a little solar power plant on my stationary RV to extend our really short season here in Quebec, Canada. I want to single pick every components of my system so I can have a better quality system for the price I will invest. But even though I have some basic electricity concepts, the number crunching for my inverter choice is leaving me puzzled. I explain:

Lets take a normal AC household appliance that we all know is 1500 watts like a toaster or an air dryer for example. And let's also say it is connected to an inverter. According to the formula, this same toaster consumes around (1500w divided by 115v) 13 amps AC when working. But inside the inverter, on the battery side, how many DC amps is this inverter pulling on the batteries to supply 1500w to the toaster? More because the voltage is lower? Less because of the RMS equivalent? What's the formula I should go with here? Or has anyone actually measured this current with a meter?

It's that I'm desperately trying to make sense of Sunking's sticky on Inverter size VS Batteries size.

A big thanks in advance to all of you that will take their precious time to answer.

[sensij]

A simple answer would be the power into the inverter equals the power out of the inverter divided by the efficiency.

Let's say your inverter is 90% efficient.

1500 Watts out / 0.9 = 1667 Watts in.

Assuming a 12 V battery:
1667 W / 12 V = 139 A out of the battery.

[Sunking]

Easy Peazy 5th grade math, Simple Ohms law. The common denominator is POWER or WATTS

Voltage = Watts / Current
Current = Watts / Voltage
Watts = Voltage x Current

So for 1500 watts at 12 volts if you ignore conversion efficiency is 1500 watts / 12 volts = 125 amps. Exactly 10 times more than 120 volts. 125 amps is extremely dangerous amount of current to handle with huge losses. Once you account for voltage losses on the wire and inverter conversion losses that current goes up to around 150 amps. That takes cable the size of a big Cuban Cigar and Tractor Tools to work with.

Batteries do not have unlimited amounts of current they can deliver. They are very limited, and that limit is determined by a few factors but most important one is its Amp Hour capacity, and what type of battery it is. Once you try to go beyond reasonable limits, the battery internal Resistance causes the voltage to sag thus limiting the current. The voltag ewill drop so low your inverrter shuts off from Low Voltage. For Flooded Lead Acid Batteries Typical limit is C/8 where C = the battery Amp Hour Capacity. So if you have say a 800 AH / 8 = no more than 100 amps. So you want to run a 12 volt 1500 watt Inverter that draws 150 amps you need a 12 volt 1200 AH battery. Good luck with that. You would need to use 2 volt 1200 AH batteries, 6 of them wired in series to give you 12 volts. That is a 800 pound set of batteries. Additionally you would need at least a 1000 watt solar panel and a $600 80 amp MPPT controller to charge it. Good luck getting all that on your RV.

AGM you can draw C/4, so at 12 volts is a 600 AH battery. It is half the weight, but cost as much as the wet battery. about $2500. How big is your wallet and RV?

[ewarnerusa]

Rough estimate, multiply AC amps by 10 for an estimate of DC amps drawn by an inverter like Sunking describes. Based on your first question about 1500 watt loads, I would say to forget about running high wattage appliances off of battery/inverter power. Air conditioner is out of the question. Even a microwave or toaster is too much for an RV's 12V system unless it has a large battery bank (4+ batteries). See Sunking's description of voltage drop under high load. The higher the AC load the more current the inverter draws from the batteries which causes the battery voltage to drop significantly, then the inverter tries to draw even more amps to maintain the output watts (P=VI), causing the battery voltage to drop still further from the increased load, then your inverter shuts down on low input voltage. If you have a typical 2 battery setup, then keep your inverter designs to 1000 watt or less and try to not ever draw that max amount for very long.

[Marto]

Ok, that is what I was missing: I didn't know we could mix and match the AC and DC numbers in the wattage formula.

Thank you all guys for your inputs.