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Old 03-11-2017, 08:44 PM   #1
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Default Is my solar battery math correct?

Hi folks wanted to confirm if my math skills are up to snuff. Background ...I have a new go power 100 amp agm battery for solar.... a bestek 2000 working watts (4000 peak watts) inverter a new go power 120 watt portable folding solar panels and a 600 watt ac toaster. My question is my math correct ??? DoI divide 600 watts by 12 volts which would make 50 amps that the ac toaster would draw in one hours time? If that is the case if it only took 5 mins to cook toast then 60 dividend 5 equals 12... so 50 amps divided by 12 equals 4.16 amps and say add .5 of a amp for the power draw of the inverter. Then I could say it would cost me 5 amps to cook the toast. With my 120 watt solar panel say in perfect conditions produces 6 plus amps and hour then I would need less than 1 hrs sun ( in optimum conditions) to recharge my single solar battery. Is my math correct. I am using the toaster only as an example as there are other ways to make toast off grid. Thank you for your input.. there is no ego here. Just ensuring I have this down. Regards and thanks..

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Old 03-11-2017, 09:53 PM   #2
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The standard error we see here all the time is confusing amps and amp-hours.

Amps are what is happening right now. Amp-hours are the average amps over a time.

The analogy with plumbing is that amps are gallons and amp-hours are how many gallons you get over time.

Your 600 watt 12v toaster would be 600 divided by 12 or 50 amps, plus the probable 15% inefficiency of the inverter so closer to 60 amps. 5 minutes run time is 1/12 of an hour, so the amp-hours is 60 divided by 12 or 5 amp-hours that your 100 amp hour battery will go down.

Recharging it will depend on how far down it will be. If you are under about 70% full, it will charge at close to what the solar will put out, so yep and hour at 5 amps to give 5 amp-hours back. If it is at 90%, it might take 3 hours because the charging slows way down as AGM batteries near full.

It is also likely that you will see a lot of voltage drop on smallish battery for that size load. Depending on the settings for low voltage cutoff of the inverter, it is likely it would not be able to handle that load at less than 50% full without tripping out.

When calculating power use in amp-hours, like the battery is rated, it is always amp times the hours those amps run, which can be over or under an hour.

Also remember that about 80% discharged is about as low as you want to go with an AGM, and they need to be fully charged every 7-10 days if in cycling use. A full charge can easily take 6-8 hours or more.

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Old 03-12-2017, 05:27 AM   #3
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Awesome thank you!!!
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Old 03-12-2017, 06:00 AM   #4
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Originally Posted by booster View Post
The standard error we see here all the time is confusing amps and amp-hours.
Great correction for this often-made misconception; to go a little further the amp-hour unit is the custom one for the 12V batteries, outside the automotive world the unit for energy consumption used is watt-hour, you house energy consumption is measured in kilo Watt hour [kWh]. So, if you want to go from Amp-hour [Ah] to actual energy unit just multiply Ah by 12V, so 50Ah in energy unit means 50Ah x 12V = 600 Wh.

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Old 03-12-2017, 07:03 AM   #5
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You don't have enough battery to run a 2000 watt inverter. Sure, it will work, for a very short time. This is just an opinion but find another way to make toast.

Use the generator more. If you are outside noise hours in a campground then drive down the road a bit and run your generator.

For what you want to do you need more battery, more solar, and an upgraded charging system. Even a three stage converter is not good enough. This, of course, is just my opinion. I have a single group 27 battery, too, and 100 watts of solar. I wouldn't think of doing what you want to do.
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