Amps - AC vs DC - Am I doing this right???

[dixonge]

So I've been going through and estimating all of our appliance usage, assuming everything is wired on the AC side through an inverter connected to the 12V batteries. I've been using this formula:

___ watts divided by 120 volts = amps

I ended up with an estimate of 33 or so, which works pretty well assuming I have 240aH of battery.

Now I am reading that this is only for the AC side of things - and that to determine how many amp hours that this will draw off of my batteries, I will need to multiply this number by 10! So now I am looking at 330 amp hours, which would necessitate something like 1000aH of battery capacity. This would probably blow both my financial and weight budgets.

Am I missing anything?

 

[Trebor English]

A hair blower that is 1800 watts at 120 volts is 15 amps.  15 amps times 120 volts is 1800 watts. 

An inverter producing 1800 watts of 120 volts will take in more than 1800 watts of 12 volts.  The inverter gets warm and the little fan blows out the wasted heat.  The loss is usually around 20%.  Twenty percent of 1800 watts is 360 watts.  The input to the inverter will be 1800 + 360 = 2160 watts.  At 12 volts, 2160 watts divided by 12 volts is 180 amps. 

A quick mental arithmetic trick is to skip the calculation of loss and then dividing by 12 but rather take the watts and divide by 10.  Dividing by 10 instead of 12 allows for the 20% inverter loss.  1800 watts divided by 10 is 180 amps on the inverter input.

 

[dixonge]

so a hair dryer at 15 amps (AC) + inverter loss = 180 amps per hour?

a fan at 3 amps (AC) + inverter loss = 36 amps per hour?

As my wife pointed out, this doesn't seem to match all of the many accounts we've read of people getting by on 100-200 aH battery setups. We're basically talking about charging phones and laptops and watching 2 hours of LED TV/day. Warming up a few quick things in the microwave. All lighting on LED. NO A/C, no heavy draws...

 

[dixonge]

Just plugged my watts and hours into a sizing calculator at Renogy. Conclusion: I need over 2000 aH of battery.

:huh:

 

[B and C]

Get away from 120V devices and use propane for cooking. Your laptop and phones will have a car charger so you don't have to use the 120V chargers and 12VDC fans are plentiful. I hardly ever use my fans though.

Baking potatoes? Put a trivet in a pan with a lid over the stovetop. Not as fast as a microwave but what's the hurry? Popcorn can be done over the stovetop too. I wonder what people did before microwaves? Hmmm.

If you must use 120V, use an inverter generator for the time required. Please, not an open frame LOUD one.

You learn how to conserve power when you are the power company.

 

[dixonge]

[font=Tahoma, Verdana, Arial, sans-serif]ok, redid my spreadsheet. I was converting watts to amps, then multiplying by hours of usage.[/font]

[font=Tahoma, Verdana, Arial, sans-serif]When I multiply watts times hours of usage, *then* convert to amps, the numbers look a *lot* more reasonable. Still higher than I'd like, but at least now I know what I need to work on...[/font]

FYI, we're talking about using the microwave for warming up a cup of coffee, 60 seconds. I added extra time to be safe. No cooking. All other cooking is propane, obviously. Charging devices in the car isn't practical given our planned 2-week boondocking stays. Fan use will be high, as the wife is on year 20 or so of hot flashes. Trying to keep generator use to as close to zero as possible...

 

[future_vandweller]

so a hair dryer at 15 amps (AC) + inverter loss = 180 amps per hour?

a fan at 3 amps (AC) + inverter loss = 36 amps per hour?

It's not 180 Amps per hour.  It's just 180 Amps.  Amps is a measure of rate (Coulombs per second) and so, it's like gallons per hour.
If you run the hair dryer for one hr, it's 180 Amp-hr.  You'll deplete a 200 Amp-hr battery in about an hour.

Estimate the wattage of various items you're likely to run and for how long each item will be on.  Then do the math - wattage x time - for all the devices.  Add up the results.  This is the total Watt-hrs.  Double or triple this# to estimate battery bank size.  A 100 Amp-hr battery is about 1200 Watt-hr.

 

[Trebor English]

Why do people go to "amps per hour" when the "per hour" doesn't make sense in this context?

A roast takes 20 minutes per pound at 350 degrees.  Is that 350 degrees per minute?  How hot is a 350 degree per minute oven?  

What kind of scale would you use to weigh a 5 pound per hour roast?  

"Per" has meaning.  How big a cake do you need to serve one slice per person?  Without the "per person", you need one slice.  With 8 people and two ounce slices you need a 16 ounce cake.

 

[CC58]

Just plugged my watts and hours into a sizing calculator at Renogy. Conclusion: I need over 2000 aH of battery.

:huh:

Consider you only run a hair dryer for 10 minutes at most.  
1800 watts / 12v = 150 amps.

If you run it for 10 minutes then 10min/60min * 150A = 25AH (amp-hours).

You would have to budget 25AH out of your battery power.

 

[B and C]

A car charger plugs into a 12V port. Your camper should have at least one of those. Mine had 4, I added more. No need to use the vehicle to use a "car" charger. A car charger just means a charger for your device that plugs into a 12V power outlet.

 

[RoadtripsAndCampfires]

If using AGM batteries remember they can only be depleted up to 50%, thus a 100 ah battery gives you 50 ah of useable ah. A 100ah Lithium battery, however, gives you almost 100 ah of useable power. Way more expensive but lighter, tend to last way longer.

 

[dixonge]

Why do people go to "amps per hour" when the "per hour" doesn't make sense in this context?

A roast takes 20 minutes per pound at 350 degrees.  Is that 350 degrees per minute?  How hot is a 350 degree per minute oven?  

What kind of scale would you use to weigh a 5 pound per hour roast?  

"Per" has meaning.  How big a cake do you need to serve one slice per person?  Without the "per person", you need one slice.  With 8 people and two ounce slices you need a 16 ounce cake.

Congratulations for being technically correct! However, I use aH because that's how batteries are rated. Well aware of the difference. That's why my spreadsheet has columns for both watts *and* hours of usage. Most devices are in use for multiple hours, some for only fractions of an hour.

Consider you only run a hair dryer for 10 minutes at most.  
1800 watts / 12v = 150 amps.

If you run it for 10 minutes then 10min/60min * 150A = 25AH (amp-hours).

You would have to budget 25AH out of your battery power.

That's exactly how i laid out the microwave usage. I didn't put the hair dryer in yet because the usage is *very* sporadic. I shave my head, and the wife uses the hair dryer maybe once a week.

A car charger plugs into a 12V port.  Your camper should have at least one of those.  Mine had 4, I added more.  No need to use the vehicle to use a "car" charger.  A car charger just means a charger for your device that plugs into a 12V power outlet.

And it looks like that might actually charge devices *faster* - at least faster than using a USB port in a vehicle. Not sure if it's faster than the AC adapter, but I guess less draw since the inverter isn't in play? Good thing to test!

If using AGM batteries remember they can only be depleted up to 50%, thus a 100 ah battery gives you 50 ah of useable ah.  A 100ah Lithium battery, however, gives you almost 100 ah of useable power.  Way more expensive but lighter, tend to last way longer.

I have spent *hours* comparing the pros/cons of all these. Biggest factor is weight. RV payload will be a big factor in our RV purchase decision, as I can't justify the cost difference of LiFePo4 at this time.

 

[Weight]

Trebor has done a fine job of explaining things.

Here is my true experience; 600ah bank. 2000W sine-wave inverter. 1000W microwave. Victron indicates 100% battery. Microwave on high. Battery drain 150 amperes while microwave on. 6 minutes steam in bag potatoes. Victron shows 98.5% battery. battery rest some-minutes, Victron shows 99% battery.

volt times ampere equal watt. watt divided by volt equal ampere. ampere times minutes then divided by 60 equal amp-hour. Battery, being a chemical reaction, doesn't work exactly in linear agreement with the cold math. But close enough for the mortals.

 

[dixonge]

Thanks for the real-world data! I'm actually surprised the microwave only took 1%. Obviously best to avoid without shore power.

 

[B and C]

USB chargers come in different amp ratings too. From 1 amp to 2.4 amps. I put all 2.4 amp USB outlets in (hardwired), I used these Blue Seas ones: https://www.walmart.com/ip/Blue-Sea-Systems-1045-Dual-USB-Charger-Socket-Mount-12-24V-DC/48287015

 

[Weight]

Less money here. Better delivery as well. https://www.amazon.com/dp/product/B0082CXEI8

 

[B and C]

As always, shop around. I just posted the first link I could find to show what I use.

 

[Weight]

Phase I was a bed and place to shit. I started phase II with a need for a microwave. The battery bank, inverter, solar and all the wiring was designed around having the microwave. Phase III is climate control. I guarantee the next build will be complete all together as one.