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I am considering a 900 watt cooking power microwave (1350 watt draw). This will be my largest AC load and likely the only other AC load used at the same time would be charging a laptop (laptop charger says input is 1.2A, so if I'm figuring correctly, W=1.2A x 120V, = 144 watt draw). Microwave would likely be in use no more than 5 minutes/day and I could unplug the laptop, if necessary when operating the microwave. Would also have a coffee maker (650 watt draw) which would be used daily but not at the same time as the microwave.

Van has 230 amp alternator with dual starting batteries and I am considering getting a ~50 amp dc to dc charger and 200 amp hours of LiFePO4 house batteries, which would power the inverter. I anticipate using about ~75 amp-hours of DC per day. We would likely be on the road every day or two (or could idle the van in a pinch). I do not plan on adding solar, a second alternator, or using a generator at this time.

Here are my questions:
1.) Would a 1500 watt pure sine wave inverter be sufficient or would the 2000 or 2200 watt inverters that I am considering be better choices (an undersized inverter can obviously cause problems, but I seem to recall reading somewhere that an oversized inverter can also have some negatives, but other than cost, I do not recall what the other problems would be, as I would turn the inverter off when not in use, so there would be no power draw.)

2.) I could go to a smaller microwave at 700 watt (~1050 watt draw) but wouldn't my amp-hour consumption still be about the same? I'm assuming that the 700 watt would run longer to heat a food item to the same temperature as the 900 watt, so the amp-hour usage would be about the same.....or maybe I'm way off base here?

3.) Anyone willing to share your real-world experiences with a set-up similar to what I am considering.....positives and negatives?

Thanks!
 

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I'm not aware of any problems a larger inverter would present. eg. your appliances don't care so long as they get what they need. You're not drawing any more from the batteries, this is all true assuming the inverters are similarly efficient ( You draw 100 watts from your battery, you don't get 100watts out). Just make sure the wiring will support the current drawn.
 

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Oh, I forgot to mention, I would go with the larger , the heat sinks , design etc, it would run cooler even though the smaller could do the job, heat is one of the worst enemies of electronics.
 

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I have a "600" watt Westinghouse microwave that draws 950 watts. House inverter is a 1000 watt Magnum MMS1012 pure sine inverter/charger/transfer switch powered by a 255 amp-hr AGM battery. Combination works very well. Lower wattage microwave requires longer run time. Suspect you are correct that the power used does not change for larger or smaller microwaves.

The disadvantages of a larger inverter are size, cost and larger power usage when on without a load.

I also turn off the house inverter when 120 volt AC is not required. Inverters waste battery capacity if left on without any load.
 

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"larger power usage when on without a load"

This statement doesn't make sense, if no load there is extremely little current drawn.
 

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"larger power usage when on without a load"

This statement doesn't make sense, if no load there is extremely little current drawn.
The inverter draws power when idle without a load. A larger inverter uses more power when idling than a smaller inverter. This is meaningless if inverter is always turned off when 120 volt AC is not required. I once forgot to turn off my 1000 watt inverter overnight and that used 5% of my 255 amp-hr battery capacity.
 

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You certainly have enough power to run almost any sized inverter you want, It has been said that 200Ah of Lithium equals 400Ah of Agm batteries. You got the big alternator. You could go with a even bigger Dc to Dc charger if you wanted, Sterling makes a 120 amp model.

Inverters are more efficient when they are not close to being maxed out, Efficiency means less battery power wasted as heat and less power used to run the inverter.
My Magnum MS2012 is 90 percent efficient at 800 watts output, And 80 percent efficient at 1500 watts output.

I have a 2,000 watt inverter with a 330Ah Agm battery bank. I did not even pay attention about how many watts my microwave uses, It is only On for 5 minutes a day. Cold cereal and sandwiches take care of the rest of the day.
Inverter/ Keurig machine 5 minutes a day, and Tv/Dvd 2 or 3 hours.
I overbuilt my electrical system because of some dream about Inverter powered air conditioning, My original battery bank was 660 Ah.
 

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I have a 700-Watt microwave that pulls just over 1,000 Watts, a low-power Keurig coffee maker (~700 Watts), and a small air conditioner that normally pulls between 600 to 700 Watts. I can run any two of the three from an 1,800 Watt continuous / 2,200 Watt peak inverter generator.

Because there isn’t that much cost difference, I’d go with 2,000 Watt inverter instead of 1,500 Watt unit. Efficiency should be as high or higher and due to higher surge capacity compared to an inverter/generator should allow running microwave and coffee together in case you choose to. We never planned to, but often make extra coffee while using microwave because we can.

Compared to home units, both microwave and Keurig take longer by roughly 50%. Not a big deal when making coffee, hot chocolate or oatmeal, etc.


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I have a 2,000 watt inverter with a 330Ah Agm battery bank. I did not even pay attention about how many watts my microwave uses, It is only On for 5 minutes a day. Cold cereal and sandwiches take care of the rest of the day.
Inverter/ Keurig machine 5 minutes a day, and Tv/Dvd 2 or 3 hours.
I overbuilt my electrical system because of some dream about Inverter powered air conditioning, My original battery bank was 660 Ah.
Coachmen Cross Trek runs A/C for a limited time (maybe 1~2 hours) using 3,000-Watt Xantrex inverter powered by either one 330 Ah or 4X100 Ah AGM batteries. The motorhomes at Tampa Super Show were designed to function without a generator. It’s just long enough to leave pets in motorhome for short time, or to cool inside at night enough to fall asleep. They could use more battery capacity for sure, but shows the “dream” isn’t that far fetched. Just needs a bit more work, or $$$$.
 

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Why risk undersizing? Get a 3500-5000w inverter and be done with it. Also get at least 1500ah worth of battery bank. You'll never have "ah anxiety" again.
 

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Discussion Starter #10
Thanks for all the responses! Now I'm also looking into the inverter/charger option.....so many different ways to get the job done.....my brain is full!!!
 

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@Calothrix you also might want to check out the owner's manual for a Renogy inverter. You can view the manual under the downloads section while reading the inverter's details page. They have a calculated step-by-step on how to size a battery and inverter using a microwave as an example.

Also be sure to check that the battery's max discharge is greater than the max draw from the inverter. This is not an issue with my 200Ah battery and 2000W inverter since the max discharge of the battery is something like 24,000 W for 5 seconds but I have seen others report much lower discharge rates for their battery.
 

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I have a lot of experience in a camper that runs a microwave and a hot water kettle. My opinion is you should not use the 1500 watt inverter, the 2000 watt would be okay, the 2500 watt would be better. Your battery size should be adequate, since it is LiFePO4. Don't go to a smaller microwave. More power, more better. I would recommend adding a solar panel. A solar panel is good for topping off the batteries. This was more important with lead acid batteries, but I think it's a good idea for the lithium batteries as well, and it will keep them topped off if the van is parked (outside) for an extended length of time.
 

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Hi,

My thoughts.

1) Spend the money once on a larger inverter. Worth it not to fret that someone is charging their device while someone else is watching TV or charging some device.

2) I'm with Orton, smaller microwave that has to run longer doesn't save energy. Especially if you turn your inverter off when not needed.

3) We have 3 100 AH lithium batteries and a 2000 watt inverter. We run a 1500 watt electric tea kettle every day with no problems. Yesterday afternoon through this morning used 15% of our battery capacity, ~45 AH. Power usage was our inverter, 12V Dometic refrigerator, CPAP, phone chargers, various lights, boiling a pot of water for coffee. We do have solar as well; without driving we got to 100% batteries by early afternoon.
 

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Like the others above have said, go with the larger inverter. You won't regret it. We have a Magnum MS2812 2800 W inverter, a AGM 4D battery and two 115W solar panels. We run a 700W microwave to warm up water for coffee and tea each morning (and sometimes to reheat food) and run a small 300W rice cooker for 20 minutes on a regular basis. They both have an impact on the battery but the solar panels recharge it pretty quickly. We have never had any issues with a lack of energy. We only fire up the inverter for the microwave and rice cooker. Everything else can run off the 12V system.

However, with that being said, if I was gonna do it over again today the only thing I would change is to get the Lithium battery in lieu of the AGM 4D. Our little 700W microwave is small but is more than sufficient for our needs.
 

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I've sampled a number of pure sine wave inverters and this is what I found out: A 1000W inverter draws close to 1A standby (no load). A 2000W inverter draws close to 2A standby. 12 hours overnight (no solar charge) would draw 24AH with a 2000W inverter. I happened to have extra inverters laying around so I have 1000W inverter always on (to power my dorm fridge), and I turn on the 2000W inverter just when I need higher wattage need. Odd set up but works for me and conserves energy. FYI.
 

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I've sampled a number of pure sine wave inverters and this is what I found out: A 1000W inverter draws close to 1A standby (no load). A 2000W inverter draws close to 2A standby. 12 hours overnight (no solar charge) would draw 24AH with a 2000W inverter. I happened to have extra inverters laying around so I have 1000W inverter always on (to power my dorm fridge), and I turn on the 2000W inverter just when I need higher wattage need. Odd set up but works for me and conserves energy. FYI.
Does the ~1Amp difference carry over to when both inverters power the dorm fridge, or does the difference become smaller? Or maybe larger though doubtful.

Just curious because, for example, in “theory” a 2,000 Watt inverter at 1,000 Watts (50% load) is suppose to be more efficient in most cases than a 1,000 Watt at 100%; provided they are similar. Just wondering at what point the curves cross over. I’d guess a dorm fridge isn’t enough load, but may reduce the 1-Amp difference some.
 

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All inverters have some standby draw. They are essentially a computer + amplifier + signal generator + filters + other stuff. When they are "on", they will draw some power.

There is a tendency for larger inverters to use more power in standby mode than smaller ones. Back in the days when solar panels cost $1000 / each and people budgeted how many electrons they could use that day, it made a bigger difference.

If your van has 400 watts or so of solar and 400+ amp-hrs of AGM or LiFe batteries, the standby draw isn't that significant. If it really bothers you, just turn off the inverter when you aren't using it.

If you want to get decent life out of your battery pack (AGM or LiFe) , then try to have 1 battery per 500 watts of the max load. In other words, if you think your max load will be 1500 watts, then use 1500/3 = 3 batteries minimum.

That might sound like an wives tale but it is based on battery discharge curves.

Just as an example, the MAX rating of the battle born BMS is 100 amps. (the point where I have personally accidentally blown some out) Ideally you want to stay far away from this max and operate at 50 amps. ( 3 ) x (50 amps) x (12 volts) ~ 1500 watts.
 

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As I said before, best to not try to make a system that is "just enough"; spend the extra few thousand dollars and make it almost double what you "think" you need.
 

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If you want to get decent life out of your battery pack (AGM or LiFe) , then try to have 1 battery per 500 watts of the max load. In other words, if you think your max load will be 1500 watts, then use 1500/3 = 3 batteries minimum.

That might sound like an wives tale but it is based on battery discharge curves.
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Doesn’t make sense to me because it’s an oversimplification. Early in my career there were many “rules of thumb” as they were called then that often led to bad designs because they were taken out of context.

This would be OK if all AGM and LiFe batteries were around 100 Ah, and granted most are, but all are not which opens door to mistakes. An 80 Ah AGM is far from a 255 or 330 Amp-hour AGM. I know what you mean, but the new guy may not.

For that matter, we need to get people to stop thinking of battery capacity solely in Amp-hours, but that’s a different issue; and one that may take years to change if it ever does.
 

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As I said before, best to not try to make a system that is "just enough"; spend the extra few thousand dollars and make it almost double what you "think" you need.
My thoughts exactly, $2,000 dollars for the inverter with options, Another $1,200 dollars for six 110 Ah. Agm batteries.
(I paid list price for the inverter and I got the batteries Wholesale for almost half the price of List)
Probably triple of what I need now but starting out the system was undersized for what I wanted back then.
 
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