power formula

[marigold]

I'm trying to calculate my power needs, and having the darnest time finding the formula to use. I know it's out there, I've seen it before, but where is it?! Can someone direct me to this info please? Thank you so much.

 

[sephson]

The two most basic electrical formulas are:

Volts = Amps x Resistance

&

Watts = Volts x Amps

 

[IGBT]

V=I*R and P=V*I gets you everything (with a little math)

power loss in a wire only depends on the current and the resistance, so you substitute I*R for V in the second formula and get:

P = I*I*R or Power = current squared times resistance.  This is why doubling the current flowing through a wire increases the power loss by quadruple.

 

[marigold]

If I understand it right, I need to figure out how many amps my devices will use and then multiply this by hours used to find total power needs per day?

If I have a 200 ah deep cycle battery, that means I have roughly 100 ah of power to use between recharges (less if I use an inverter?) I'm trying to figure out if a 100 watt solar panel with a 100ah battery will be enough for my needs, or if a 200 watt 200 ah situation would be better.

oof.

 

[anewbiewannabe]

V=I*R and P=V*I gets you everything (with a little math)

power loss in a wire only depends on the current and the resistance, so you substitute I*R for V in the second formula and get:

P = I*I*R or Power = current squared times resistance.  This is why doubling the current flowing through a wire increases the power loss by quadruple.

English?

P is Power? R is resistance? V is voltage? What is I?

Where do you find the resistance number or how is it calculated?

 

[Ken in Anaheim]

Just remember: EASY AS "PIE".
P=power, measured in watts (w)
I= current, measured in amps (a)
E=volts, measured in volts (v)
So. watts=current X volts...or, P=IE

(As an aside, P=power, I= intensity and E=electromotive force)
And....KinA = ME !!

 

[akrvbob]

Le's stay on topic folks. He didn't ask for a class on electrical engineering.

You've got the idea, you figure out how many amps something draws and multiply that by how many hours it will be running. Do that for all your devices and add them up and you'll know your power needs per day.

An example: You have a 12 volt TV that draws 24 watts and you use it 4 hours a day. Watts divided by volts = amps so you divide 24 watts by 12 volts and you know it draws 2 amps per hour. If you use it for 4 hours a day, it will use 8 amps out of your battery.

Do that for all your items and add them together, that's your total amps used per day.

But, I think that exercise is a waste of time because there are too many variables. Lattitude, weather, time of the year and temperature alone will make it all moot. It almost never works in the real world through the whole year.

Instead, buy all the solar you can afford and use power conservation to make it work.

Unless you use almost no power, get the 200 watts. In the summer you may have too much power ( a dumb idea if I ever heard one) but in the winter or in a storm, you'll be glad for every bit of it!
Bob

 

[skyl4rk]

This is a rule of thumb, not a law of physics.

Assuming a 12 Volt system: List all of the appliances or power draws that you expect to use in one day. Find out the amp draw of each one. Estimate the number of hours it will be used per day. For each appliance, amps * hours = amp-hours. Add up all the individual amp-hours for the appliances in one day. That gives you a daily power requirement.

In an average day, you will get about 4 hours of rated solar panel output. Of course this depends on the weather, the angle of your panels, whether you are in Anchorage or Yuma and whether you park under a tree or not. But for sizing a system, Take your daily power requirement in amp-hours, and divide by 4 hours. This gives you amps, and it is the amount of amps that your solar panels should be pushing at their rating.

Solar panels run at about 18 Volts before they are regulated down to the 14 Volts your batteries need to charge. Take the amp calculation above and multiply by 18 Volts to get Watts. This is the Wattage of solar panels that your system needs...BUT WAIT!

There are inefficiencies throughout the system, in particular in charging the batteries. Plus you need to have some extra capacity for cloudy rainy days. Double that number of Watts. Yes, what you just calculated times 2. That is the amount of solar panels that you need to run the list of appliances on a daily basis.

Another rule of thumb is that you need the same amount of battery amp-hours as solar panel Watts.

Or you can start with a 400 Watt system with 400 amp-hours of battery and learn to adjust your power usage so that you don't ever use up more than a quarter of your capacity. Most people can make 400 Watts work, many do well with less.

 

[Canine]

Solar power is great.But only if you have enough power to do what you want.I have to agree with shyl4ark.Double what you think you will need.

 

[jimindenver]

I agree and not just because it's nice to have the excess. When we got solar we wanted it just to run the basics of the trailer, no inverter even. Once we figure how much power it was good for we wanted to do more and more. Now running the trailer is secondary to all the other stuff the solar run. We also have three times as much solar.

 

[LeeRevell]

I disagree on calculating the total power used as 'useless'. It gives a very useful ballpark average of your projected use, to start your system with. But I do agree with getting as much solar/battery as space and finances allow. Too much is fine, too little is very uncomfortable. Usage will obviously every month to month, season to season, just like your electric bill in an S&B home. If possible calculate your absolute worst-case power consumption, then increase by a minimum 50%. 100% is better. Gives you a comfortable margin for error.

 

[marigold]

Thanks everyone, I'm going with the 200 w system

 

[SternWake]

One thing to be aware of when determining how much battery capacity one requires is the Peukert effect which basically say the higher the load on the battery, the less capacity the battery has to give.


So while a 100 AH battery can provide 5 amps for 20 hours before voltage drops to 10.5v which is considered completely dead, a larger load than 5 amps will mean the battery does not have 100AH to give but some number under that.


Conversely loads under 5 amps yield more available capacity, but the additional capacity gained by loads under 5 amps( for a 100 ah battery) is not as rewarding as loads over 5 amps is punishing.


So those who power high consumption devices like Microwaves or electric coffee makers cannot do a simple math of time x amps and say this much was depleted from the battery.  Peukert must be considered.


Just something to be aware of.

 

[akrvbob]

Peukert's law is another reason I don't recommend the elaborate calculations on usage--too many variables!

I followed my own advice, got all the solar I could afford, and now my microwave kicks sand in the face of Mr. Peukert!
Bob

 

[SternWake]

With few exceptions, there is no such thing as too much solar.

People new to the 12v world must know that a 5 amp load for 10 hours is not the same thing as a 10 amp load for 5 hours.

If they think so, then it is mr Peukert who gets to kick the sand in one's face, at some point.

 

[akrvbob]

But if either way it has no impact on your battery bank because it's so large, my microwave (and I) get the last laugh!

The simple answer is still, buy all he solar you can afford and then energy conservation if it isn't enough. If we get a week of rain, then to avoid getting sand kicked in my face, I use my propane stove instead of the microwave. I still get the last laugh!
Bob