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Old 08-30-2018, 09:11 AM   #1
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Help: How much power will I need?

Hello everyone,

A skoolie newbie and first-time poster here. For the past week or so, I've been scouring the web for info on skoolie power sources, and I've found some very helpful information but need help determining exactly how much power I'll need.

I'd like to avoid diving into solar right now and stick with a battery bank, which I'll charge with my alternator. I don't plan to stay in one place with the bus off for more than 24 hours, and if I do, I'll just cut power so I don't drain the batteries below 50%. Here are the devices I plan to power with the batteries:
1. A mini fridge, probably one just like this: https://bit.ly/2okiC0m. I'd like to have this run 24/7, if possible. Details say it has a wattage output of 47 watts, but that can't be that same as consumption, right? I can't find consumption information anywhere.

2. A fan, like this: https://bit.ly/2Nuz9cN. It would run a maximum of 8 hrs per day. Details say power consumption is 48 watts.

3. Two cells phones, for charging. Approximately 3 hrs per day, using ~5 watts each.

4. One laptop, for charging. Approximately 3 hrs per day, using ~60 watts.
So, my questions are...
What battery arrangement should give me the power I need? (2x12V batteries in parallel, 4x6V batteries in series parallel, or some other combo?)

What kind of batteries would you recommend? My bank will have to be inside the bus.

What kind of inverter would you recommend?
Thanks in advance for helping me out, and please let me know if I've left out any crucial information. I'm totally new to this!
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Old 09-01-2018, 12:55 PM   #2
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Power consumption is measured in watt/hours. A 1 watt load run for one hour has consumed 1 watt/hour.

A 1000 watt load run for 1 hour has consumed 1 kilowatt/hour. 1 kwh.

A 250 watt load run for 4 hours also consumes 1 kwh. (250w x 4h = 1000watt/hour)

So, if you take the wattage of the loads and multiply by the hours a day that load will operate that will give you the daily consumption (in watt/hours).

Add up all of the loads and you will find your total daily consumption. From that you can size your battery.

Keep in mind that your inverter will not be 100% efficient. Loads like the refrigerator(120v) that run from the inverter will consume 5-20% more from your battery due to inverter losses.

Take your total daily watt/hours and divide by your battery voltage (12) and that will give you amp/hours needed from your battery.

As you want to limit your batteries depth of discharge to 50% you will want to buy a battery with at least 2 times your calculated consumption.

I would recommend going with a pure sine wave inverter. I have a similar set of loads in my travel trailer and run Xantrex 1000 watt inverter and it supports the refrigerator well.

Also recommend that you get 12v laptop and phone chargers.

Hope that all made sense.
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Old 09-01-2018, 04:22 PM   #3
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Hi Steve,

Thank you! This makes it much easier for me to determine what I need. Here are my calculations so far:

1. Fridge: 96 watts x 24 hours = 2,304 watts/hour (2.3 kwh/day)
*Add about 460 watts/hour (20% for the inverter) to this to be safe. (.46 kwh/day)
2. Fan: 43 watts x 8 hours = 344 watts/hour (.34 kwh/day)
3. Phones: 10 watts x 3 hours = 30 watts/hour (.03 kwh/day)
4. Laptop: 60 watts x 3 hours = 180 watts/hour (.18 kwh/day)

Total Consumption = 3,318 watts/hour. Round that and we get roughly 3.5 kwh/day...does that sound correct?

Divide the rough total 3,500 watts/hour by 12 and I get ~ 292 amp/hours. So, if I'm following correctly, I'll need a battery bank that provides me with roughly 600 amp/hours?

Thanks again for your help.
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Old 09-01-2018, 04:32 PM   #4
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Looking good!

One item to adjust. Your refrigerator does not run all of the time. The compressor cycles on and off.

Duty cycle depends on ambient temperature, fill and how often you open the door.

I have a refrigerator in my travel trailer that had similar Energy Star numbers that I tested with a kill-a-watt meter to test real world conditions.

Over the course of a month I averaged about 1kwh/day.
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Old 09-01-2018, 04:58 PM   #5
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Awesome!

So I'll say 1,500 watt/hours (1.5 kwh) for the fridge just to be conservative, and add 20% for the inverter to make it 1,800 (1.8 kwh).

That makes my total 2,354 watt/hours -- or roughly 2.4 kwh per day and about 200 amp/hours. Now we're looking at a battery bank that provides 400 amp/hours. That sounds better!

I believe I can get that if I wire two of these (https://bit.ly/2wK1iWv) in parallel. What do you think? And just out of curiosity, what batteries do you have?
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Old 09-03-2018, 11:53 AM   #6
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If you are intent on a 12 volt system then a pair of those in parallel should do the job.

Personally, I am not a fan of parallel batteries. Sometimes your options are limited.

I am installing 4 x L16's in series in my bus for a 24v/370ah bank.

My travel trailer has a pair of GC2's in series for a 12v/215ah bank.

A great resource for solar and battery info is: Forums - Solar Panels - Solar Panels Forum
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Old 09-15-2018, 06:47 PM   #7
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Batteries inside the bus- bad idea- hydrogen gas is explosive when they are charged and even equalized that will be an issue- Lots of sailboats have burned because of this- Contact the folks at Outbackpower.com. They have charts and great products. Isolator for your two engine batteries are necessary- they switch off the engine battery when they get to a preset level so you can still start the engine. Some transmissions can be fitted with a pto and you could run the generator with the engine.
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Old 09-15-2018, 08:03 PM   #8
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I'll add that on our little camper boat, I run the small fridge on an inverter. It is a 3.7 cuft haier. It pulls 5-7 amps on start (about 1 second), but only 1.1 running. With only two adults, it is not opened and closed constantly, and may go for hours without cycling. A little honda EU1000i runs the whole show, runs the 5000 btu AC, and a little portable electric 1000 watt heater when needed. Sometimes we go for days without cranking the little generator. The large marine battery does a good job.
I use a Black and Decker 750 watt inverter. It is the first one that I found that the cooling fans do not run except when the inverter has a large load. I'm on my second one, the first started making fan noise, so I replaced it. They don't make this exact model anymore, but I found a new one on ebay and carry it as a spare. Plus, if it comes to it, I can try to replace the fans in the first one.
This is over 12 years. Anytime the generator or the outboard motor is running, the battery is being charged. The inverter sounds an alarm just below 12v. It has worked out great for us. I have 2400+ hours on the generator, only oil changes every 100 hours, nothing else. Only 1160 hours on the outboard, so there is more "sitting" than "traveling"....
This little generator weighs 29 lbs. The next one up, the EU2000i weighs 50. When and if I have to replace it, I will go to the larger just so it will run the microwave on high power... Popcorn and such...
I also have the TV and antenna powered from same inverter, sometimes we watch TV waiting for the real STARS and moon to come out...
Mainly, try not to open and close the fridge more than necessary. Either put cold stuff in to start with, or have a long run before you cut power. We freeze water bottles and fill as much as possible with them, leaving room for beer of course... and food...
I do wish I could find a newer super efficient inverter, one that uses about nothing until it gets a load.
Good luck! Woody
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