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Old 08-17-2015, 11:28 PM   #1
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110v ac

I've been looking at my options for power. How does this sound?

110v ac input at 15A

Goes via a charger into 12v batteries.

12v batteries feed a 12v system that is of no consequence to this topic .

12v batteries feed 4 x 110v sockets.

I found a 1500w inverter for $37 on eBay. That's 1500 normal, 2000 peak.

Looking at my electrics...
700w rival microwave (I'm a big spender) 1050w
Rival electric kettle 1200w
Rival rice cooker (6 cup,) - unknown
Rival quart crock pot - unknown
Rival coffee maker - unknown
Mini fridge - unknown though it says 315,kwh/year

I'm reckoning that as long as I don't double up on 110v appliances on sockets, the 1500w inverter should be fine at 1 per socket. That's 4 inverters each of which can fail so multiple redundancy.

The water heater is probably going to be a 5 to 10 gallon tank with a 12v element, mounted under the bus. I'll probably have electric hot and cold water pumps running off 12v

Lighting is going to be a battery powered hurricane style lamp. I just don't see the need to run excess cables!

110v ac from outside fills the batteries. Later when its economic to do so, I'll add solar panels.
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Old 08-18-2015, 11:00 AM   #2
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remember that you can only use about 50% of a batteries ah capacity before recharging,

even with that figure you can only draw about 10% off a batteries ah capacity at any one time which means if the battery is a 100ah deep cycle then you can only pull 10 amps out of the battery, if you do pull more than 10% then the volts may drop below 12v and the device and or inverter will stop working, some devices can take as low as 11.5 or even 11v before they stop working.

1 amp AC equals 10 amp DC is an important think go remember when deciding how big to make your battery bank.

Judging from the devices you list you will be needing a pretty big battery bank, probably 600ah at least if you only use one or large draw devices at at time,

The best and most efficient way to make a battery bank is to use bus bars, if you just hook the batteries together with cables from + to + and - to - they wont charge efficiently or quickly and the batteries at the end will get used/abused much more than the batteries in the middle.

bus bars make a quick and easy attachment point for your inverters, charge controllers and solar,
fuses are a necessity between the batteries and the bus bar but breakers are even better since you can use them to quickly disconnect power to the bus bar for maintenance and other stuff.

That 700 watt microwave probably pulls about 5 amps Ac which means it will pull 50 amps DC which means you will need at least a 500ah battery bank connected efficiently with big cables. That being said you probably wont run it for more than a few minutes at time so a slightly smaller bank may work but there is not way to know for sure without experimenting.
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Old 08-18-2015, 11:06 AM   #3
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The microwave and kettle are the biggest power hogs. At 1050w, the microwave uses close to 10a. It's dog slow so 15 minutes for a frozen entree is standard!

The kettle, maybe 4 minutes to boiling.

I'm trying to find if my microwave will work off a square wave because if it can then I can whip up a 6kw inverter in a few minutes.
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Old 08-18-2015, 01:25 PM   #4
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When your plugged in, bypass the battery's, and use the power strait from the source.

Also the AC power your talking about is 120 volts, not 110.

If all your getting is 110 volt, there is a voltage sag somewhere in your source power.

Nat
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Old 08-18-2015, 07:43 PM   #5
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To be honest I thought US voltage was 110v. This is why the US has to have such ridiculously large cables. In the UK where we have 240v, the cables are thinner because the higher voltage gets way less resistance drop.

It's an evolving idea.

The reason I'd feed mains through the battery is that I can have the batteries supping low amperage mains all day then have the capacity for heavy power usage if and when I need.
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Old 08-18-2015, 09:44 PM   #6
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I'm with Nat; if you're plugged in, power your AC plugs directly while charging the battery bank with a converter/charger. Leave the inverter for when shore power isn't available.
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Old 08-19-2015, 07:00 AM   #7
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240 volt is just two wires of 120.

There is not much savings on conductor size.

The only real benefit to 240 volt AC is that it becomes a balanced load for the transformer.

On this big land mass, we use offsets in the main panel to balance all the 120 volt loads.

After wiring a main breaker panel, I always come back a month later when the home owners are using the power under normal loads, and make sure the two 120 volt legs are balanced. If not, I move a large load breaker or two till it evens out. You don't want the common having to compensate to much.

One customer of mine had a DIY guy wire up his panel before calling me. In a year he burned on leg out of the main breaker on the power pole, just after the transformer. This caused only half his main breaker panel to work.

I rewired his main breaker panel, and now all is well.

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Old 08-19-2015, 07:07 AM   #8
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Should have seen a house I lived in here one time. All the lights flickered. Turned out all circuits ran through just 3 breakers!
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Old 08-19-2015, 11:42 AM   #9
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Quote:
Originally Posted by nat_ster View Post
240 volt is just two wires of 120.

There is not much savings on conductor size.

I think Zephod was referring to the 240vac/50Hz typical wall outlet current in the UK, which uses 3 wires in the same way as we do in North America: hot, neutral and ground. In the same place where you would use 12AWG (3.3mm˛) wiring for our 120vac system you could be using 15AWG (1.7mm˛) wiring for a 240vac system. I can definitely see the benefits. Light weight extension cords, cheaper due to less copper, easier to route...

^ Those wire sizes in mm˛ aren't real, by the way.. I think they'd be rounded up to the next full or half mm˛. It gets the idea across, though.

In North America we get 240vac by combining 120vac, as Nat mentioned.
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Old 08-19-2015, 11:53 AM   #10
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But, yes, bypass the batteries when you're plugged in. There are several benefits to this.

First, you'd be wasting electricity going through the batteries. You'd have a conversion of 120vac -> 12vdc -> 120vac. Those conversions all come with an efficiency loss. Additionally, some of your devices will be doing an internal conversion once again to DC. USB wall plugs will convert 120vac to 5vdc. Laptops will convert 120vac to ~20vdc. Small to medium sized LCD tvs will bring 120vac to ~12-24vdc. That's a lot of converting.

Another issue is that you'll be needlessly taxing your batteries. Batteries have a finite number of times they can be charged and discharged. You can cut this number down significantly by bypassing the batteries.

Lastly, you will be able to run more appliances at once while plugged in if you bypass the batteries. A 1500w inverter won't provide as much power as a direct plug into a 15 or 20amp outlet. You could also still kill the batteries while plugged in if you always went through the batteries. The charger might not put in as much power as you could potentially take out, so it's possible to drain the batteries with heavy use even while plugged in.
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