First dumb question of the day

[quickburst]

Ok, I'm fooling around with Solar panels and the things that go along with that. My first system is an Off Grid system and designed to provide enough current to power two porch lights (LED or CFC bulbs), a hot water circulating pump and our entertainment center. A total load of about 12 amps, AC.


Now for the dumb question, is a DC amp the same as an AC amp, or is there a difference. I'll slip out on the limb and say that I believe that an amp is a amp, DC or AC, am I correct?

 

[JimmyL]

I have been in the electronics field for 40+ years and never heard mention of amps being different for ac or dc. There is of course ac amps and dc amps. Ohms law has no mention of being ac or dc. A quick on line check found many definitions of ampere and no mention of ac or dc. I, fwiw, would say you are correct. Good luck on your solar power. I work at an electric cooperative and we are starting to see a few systems come on line. We recently added a tariff for buying back any excess power they generate. They were not real happy to find out we buy it back at the wholesale rate and not the rate we charge for it. The next few years should be interesting as more people go solar.

 

[bobkrech]

Dave

You are correct that an amp is an amp, however appliances designed to run on DC may not run on AC, and vice versa.

Usually solar panels have a regulated converter/inverter to convert a variable DC solar panel output to a stable 120 volt 60 Hz AC power output used by household appliances. These inverters are fairly efficient but it's not 100% so you loose some power in the conversion. Some inverters are syncronized to the powerlines and they can feedback unused power generated by your panels to the power grid and actually run your electric meter backwards.

Bob

 

[Handeman]

Bob is right about the less then 100% efficiency when converting. The loss is the heat generated in transformers and such.

An Amp is and Amp whether AC or DC, but you need to do the power calculations to see what the conversion is. It's all about the wattage.

12 VDC at 10 amps, converted at 100% to 120 VAC would drive 1 amp. If your conversion is only 90% efficient, you would get 120 VAC at 0.9 amp.

If you want 120 VAC at 12 amps, and your converter was working at 90%, you would need to supply the converter with 12 VDC at 133.33 amps.

 

[Rocketjunkie]

Ok, I'm fooling around with Solar panels and the things that go along with that. My first system is an Off Grid system and designed to provide enough current to power two porch lights (LED or CFC bulbs), a hot water circulating pump and our entertainment center. A total load of about 12 amps, AC.

You are talking about a *big* solar panel here. 12 amps AC at 120 volts is 1440 watts. With the loads you describe, actual volts * amps will be higher as electric motors and switching power supplies don't make perfect use of AC current (see "power factor") and you probably will need about 1800 watts of solar cell capacity. Low cost cells are only about 15% efficent so a square meter will provide 150 watts in full sun. Lets say you want to power the devices 3 hours per day and the solar panel will see the equilivent of full sun for 6 hours per day. You will need 900 watts of cells, a panel 2 x 3 meters (about 6.5 x 10 feet). You also need batteries for storage and a DC/AC inverter.

 

[shrox]

...You also need batteries for storage and a DC/AC inverter.

My Sarah Connor-like sister's house in Arizona is all solar. Imagine the desert compound in T2, that is like my sister's place. She has some special batteries she ordered from China that are not lead/acid and keep a charge longer and have a long service life. They do have a temperature max of 110f I think, so she has them passively cooled.

 

[Sleepy_Steve]

I'd look into making the home entertainment setup bigger. 12 amps isnt THAT much.

A 2KW array is probably the size you're looking for if you want it to be 'effectve' as an off grid system.

 

[Breeze1913]

I'd look into making the home entertainment setup bigger.

YEAH Baby, I love the sounds of shells falling on the pavement all around me when watching a shootout in the movies. Or the sound of footsteps on the grass sneaking up behind me in a scary movie..... Awfully hard to go too big in home entertainment.

 

[n5wd]

...We recently added a tariff for buying back any excess power they generate. They were not real happy to find out we buy it back at the wholesale rate and not the rate we charge for it. The next few years should be interesting as more people go solar.

Tariffs like that are where there's a law being proposed at the federal level that would require utilities to offset power bought by the customer with the power generated by the customer at the retail price up to the point where they zero out each other. At that point the util's can start paying the wholesale rate. As you might imagine, your employer probably wouldn't like something like that.

Utilities have a hard time understanding having their customer be somewhat independent of them is actually good for the utility as a whole - saves on having to keep building those big ol' coal fired plants.

 

[quickburst]

I'd look into making the home entertainment setup bigger. 12 amps isnt THAT much.

A 2KW array is probably the size you're looking for if you want it to be 'effectve' as an off grid system.

A 2KW system would run my whole house and cost thousands of dollars ($30K?). Cool idea, but a little rich for my blood.

For now I'll be happy to do a little experimenting/learning and chisel the light bill down a little. If what I have in mind works, I'll be saving about $30.00 a month off the old light bill. Thirty bucks isn't much, but it's the gift that keeps on giving. Also our government is willing to rebate 1/3 of the cost.

Very Cool.

 

[quickburst]

I'd look into making the home entertainment setup bigger. 12 amps isnt THAT much.

A 2KW array is probably the size you're looking for if you want it to be 'effectve' as an off grid system.

12 amps is about three amps more than I need. I'm saving some for later.

 

[RimfireJim]

A 2KW system would run my whole house

Hardly. As Rocketjunkie points out above, 12 amps @ 120V is 1.44kW, not much under 2kW. An electric toaster or iron is typically 1.2-1.5kW.

and cost thousands of dollars ($30K?). Cool idea, but a little rich for my blood.

For now I'll be happy to do a little experimenting/learning and chisel the light bill down a little. If what I have in mind works, I'll be saving about $30.00 a month off the old light bill. Thirty bucks isn't much, but it's the gift that keeps on giving. Also our government is willing to rebate 1/3 of the cost.

Very Cool.

Using the Texas average electricity rate of 10.39 cents/kW-hr, your $30/mo works out to 289 kW-hr/mo, or 9.6 kW-hr/day. So if your load is on for 10 hours per day, that works out to an average load of just under 1 kW, or 8 amps @ 120V. So that seems about right, but if you need 1 kW for just those few things, how do you figure 2 kW would power your whole house? Or cost many more times than what you are doing?

 

[quickburst]

Siorry, missed a decimal or two.

I stand corrected. I'd need 100 Amps or 12,000kW to run the entire house.

I'll only be pulling 10 amps about 4 hours a day (TV time). The only appliance that will be running 24/7 is a hot water circulating pump. Eventually it will be running two of them. I'm adding a batch solar water preheater too.

Hardly. As Rocketjunkie points out above, 12 amps @ 120V is 1.44kW, not much under 2kW. An electric toaster or iron is typically 1.2-1.5kW.


Using the Texas average electricity rate of 10.39 cents/kW-hr, your $30/mo works out to 289 kW-hr/mo, or 9.6 kW-hr/day. So if your load is on for 10 hours per day, that works out to an average load of just under 1 kW, or 8 amps @ 120V. So that seems about right, but if you need 1 kW for just those few things, how do you figure 2 kW would power your whole house? Or cost many more times than what you are doing?