Voltage, amps, and watts

[clev]

I'm a little confused on inverter use. I know that dealing with AC current, watts = volts x amps and using that formula, when I'm plugged into power, my 1200 watt coffee pot uses 10 amps of my 30 amp system. If I also use my 1500 watt toaster oven, my microwave, and my AC, I'll exceed that 30 amp limit and pop a breaker. My confusion is with batteries. I have 2 6-volt golf cart batteries, rated at 237 amps, hooked into a 2000 watt inverter. The 237 is DC amps, not AC. Does that equate to 23.7 AC amps? If I'm using my tv, satellite, and surround system all rated at about 10 amps, how long will the batteries last? 2.37 hours?

[pianotuna]

Here is a simple flow chart.

Budget-->Energy Audit-->Battery bank size-->number of watts-->PWM or MPPT. What ever type of controller is chosen, make sure it has adjustable set points and a temperature probe that is on the battery.

If you use solar as a battery charger, one rule of thumb is between 60 and 150 watts of panels per 100 amp-hours of storage. The smaller the battery bank the higher the wattage needed (per 100 amp-hours). Here is a link to the rather special spreadsheet which includes an energy audit, that N8GS has created to help size solar battery charging systems!

solar spreadsheet by N8GS

If you full time or use an inverter lots, then populate the entire unshaded area of the roof with panels. I'm considering replacing my awning with solar panels.

For a nice explanation of solar, try this link:

Golden rules of solar

[clev]

Wow! Those are some great responses. We do a lot of dry camping in the high country of Wyoming, in the most secluded places that we can find. It's high enough and cool enough that an AC is not needed. So, we crank up the generator for breakfast and dinner; this is for coffee pot, microwave, induction cooktops, and charging batteries. Other than that, it's strictly battery. We've been doing it this way for a lot of years but I've never really cared about usage. Now, I'm looking into solar panels and want all the info I can get. Thanks for all the help.

[2oldman]

lbrjet wrote:
Run it for an hour and you have used 8.33 amps from your battery, excluding any inverter loss.

8.33 amp-hours

[lbrjet]

The same formula applies whether it is AC or DC: watts = amps X volts. Since the voltage differs by a power or 10 it makes it easy. Just multiply AC amps by 10 to figure DC amps. It takes .833 amps to power a 100 watt light bulb at 120 volts AC. Run it for an hour and you pay for .1 KWH on your electric bill. It takes 8.33 amps to power the same light bulb at 12 volts DC. Run it for an hour and you have used 8.33 amps from your battery, excluding any inverter loss.

[pianotuna]

Hi clev,

I've found the listings on appliances are mostly a "guess" at the wattage.

The big problem is that battery discharge is non linear, extremely load dependent, temperature dependent, and other factors such as the age of the battery bank affect results.

Rule of thumb there are about 650 watts of usable power in a group 29 twelve volt jar. Count your jars, multiply by 650 and divide by the total load in watts. That may give you a rough idea of the maximum time under PERFECT conditions. i.e. starting from 100% state of charge with brand new batteries that have been cycled six to twenty-five times with the temperature at 25 C (77 f).

[clev]

Good grief, sound, for some reason, almost every post that I make, someone with an attitude, such as yourself has to chime in. I have reread my initial post 4 times and it is very clear. I know how to compute usage with AC electricity, listed a formula, and gave an example. I went on to post my confusion on battery usage time, when using an inverter, and also used an example to assist in computing usage time. And you still don't get it. You get PO'd and link in a meter that measures volts, amps, and watts, which has nothing to do with this thread. Using the device will list volts, etc, but that's the same info that's printed on the appliance and we're right back to my initial question; how do you compute usage time and how long will the battery last with the appliance plugged into the inverter. Now, that question has been answered twice, very effectively, I thanked everyone for the input, and I have not complained. Let it go. OBTW; you did post some good relevant information after the meter reference.

[azrving]

SoundGuy wrote:

clev wrote:
I use the inverter just for the tv, TiVo, and dvd. Occasionally, if I'm streaming a movie through my iPhone, I will use my surround system with subwoofer. And there should be no confusion on my initial post. If you read it carefully, I was simply asking how to compute usage time, regardless of what appliance is being used. Others started bringing in different appliances, battery size and number of batteries, and again I tried to steer replies to usage time; as I am doing now.

rjxj wrote:
LOL

That's constructive. :S

To the OP ...

You yourself introduced the topic of inverters in the first sentence of your very first post - "I'm a little confused on inverter use" so naturally that's going to be the reference point in any following commentary by others. :R Now you're complaining that those of us who took the time to reply to your query aren't being specific enough. :S OK, so to me the answer is obvious but if we have to spell it out ...............

Measure each device you're interested in powering with an inverter using a meter such as a Kill-a-Watt Energy Meter to establish it's average power requirements in watts or amps and multiply each result by the time it's being used so you'll know the watt-hour or amp-hour requirement for each device. You already know your pair of 6 volt batteries offer about 230 AH, half of which is usable if you want to draw them down no more than 50% DOD ... at a nominal 12 volts that 115 AH is equivalent to about 1400 watt-hours.

How you use that reserve is up to you but rather than try to pre-calculate this out I'd find it much simpler to just do a test run, powering what you want for however long you want until your battery bank is drawn down to 50% DOD, keeping in mind that in a normal situation those batteries are not just powering an inverter and it's associated loads but also the rest of the trailer itself. If the results of such a test meet your requirements then you know your setup is sufficient for your intended use, if not you then know you either need more battery reserve OR you need to cut back on usage OR both.

If this isn't the answer you were looking for, my apologies - but at least it's more constructive than "LOL". :W

Boohoo hoo. Waaaaaaa. Lol.

[SoundGuy]

clev wrote:
I use the inverter just for the tv, TiVo, and dvd. Occasionally, if I'm streaming a movie through my iPhone, I will use my surround system with subwoofer. And there should be no confusion on my initial post. If you read it carefully, I was simply asking how to compute usage time, regardless of what appliance is being used. Others started bringing in different appliances, battery size and number of batteries, and again I tried to steer replies to usage time; as I am doing now.

rjxj wrote:
LOL

That's constructive. :S

To the OP ...

You yourself introduced the topic of inverters in the first sentence of your very first post - "I'm a little confused on inverter use" so naturally that's going to be the reference point in any following commentary by others. :R Now you're complaining that those of us who took the time to reply to your query aren't being specific enough. :S OK, so to me the answer is obvious but if we have to spell it out ...............

Measure each device you're interested in powering with an inverter using a meter such as a Kill-a-Watt Energy Meter to establish it's average power requirements in watts or amps and multiply each result by the time it's being used so you'll know the watt-hour or amp-hour requirement for each device. You already know your pair of 6 volt batteries offer about 230 AH, half of which is usable if you want to draw them down no more than 50% DOD ... at a nominal 12 volts that 115 AH is equivalent to about 1400 watt-hours.

How you use that reserve is up to you but rather than try to pre-calculate this out I'd find it much simpler to just do a test run, powering what you want for however long you want until your battery bank is drawn down to 50% DOD, keeping in mind that in a normal situation those batteries are not just powering an inverter and it's associated loads but also the rest of the trailer itself. If the results of such a test meet your requirements then you know your setup is sufficient for your intended use, if not you then know you either need more battery reserve OR you need to cut back on usage OR both.

If this isn't the answer you were looking for, my apologies - but at least it's more constructive than "LOL". :W

[azrving]

LOL

[clev]

I use the inverter just for the tv, TiVo, and dvd. Occasionally, if I'm streaming a movie through my iPhone, I will use my surround system with subwoofer. And there should be no confusion on my initial post. If you read it carefully, I was simply asking how to compute usage time, regardless of what appliance is being used. Others started bringing in different appliances, battery size and number of batteries, and again I tried to steer replies to usage time; as I am doing now.

[SoundGuy]

clev wrote:
These are a lot of good replies, thank you all. But guys I am not running coffe pots, toasters, etc. I merely used them for example purposes only. If I use that stuff, I crank up the generator.

So that leaves me confused about your original post which began with - "I'm a little confused on inverter use." If you're not using or intending to use an inverter what's the question? :h

[clev]

These are a lot of good replies, thank you all. But guys I am not running coffee pots, toasters, etc. I merely used them for example purposes only. If I use that stuff, I crank up the generator.

[SoundGuy]

2oldman wrote:
Don't expect to be running any toasters or MW's on just 2 batteries. They'll be flat in a couple minutes.

A microwave oven, I agree ... but I with my 8 yr old G27 Interstate battery at full charge I can easily power a 2-slice toaster that pulls a measured 696 watts from my 1000 watt PSW inverter. In a few weeks I'll be replacing that single G27 with a new pair of G31s which together will more than double battery capacity and allow the inverter to easily power any device within it's ratings. 🙂

As for a coffee machine there's no need to draw anywhere near 1200 watts ... with this same inverter I can also easily power a Cuisinart 5 cup coffee machine that draws a measured 563 watts ... 6 minutes tops and it's done. 🙂

[RoyB]

I have been doing these 50% to 90% cycles since 2008 alot. My three batteries are just now staring to fall off on their power performance. Even if I do a bunch of full charge states... So I guess they are getting closer to their end life...

Now if you do this to your standard car/truck start battery you find you really only get one cycle of going down real low on the battery charge.

I usually have to replace my truck start battery here about every two years and with my Ham Radio setup I am forever running the start battery down in the truck. Over this last winter I have having to jump start my truck several times over the months. My 2KW generator is secured to the truck bed so I have 120VAC on me all the time and can hook up my portable battery charger and get some juice back in the battery so I can start the truck haha... They gettin used to me doing this in my local WALMART as i sit out there waiting for the wife alot and can't leave my Han radio alone...