# Is a Watt a Watt or What?

Registered Users Posts: 21
In figureing how many Watts of panels I need can't I just take the number of Watts the appliance uses and go with that number? Example if my refer
uses 1000 watts per day @ 110 v will 1000 watts of panels @ 24v run it? I know there are losses but just for simplicity will that work? I have seen the formula's that convert from 12 volt to 110 volt , seems complex. I guess my question ,is a watt at 12 v, 24 v 110v the same? I know the amps are different and I will have losses in the system.

• Banned Posts: 17,615 ✭✭
Re: Is a Watt a Watt or What?

No. It is not as simple as 'X' Watts of panel runs 'X' Watts of load.

What you plug in to the wall does not run off the panels, it runs off the batteries. The panels are there to recharge the batteries.

Otherwise a Watt is a Watt as in Volts * Amps.
Then someone will point out that under certain conditions (like inductive AC loads) you have Power Factor to consider, which makes a Watt a Volt Amp because things with a poor power factor actually use more power and inverter output is really measured as Volt Amps even though they say Watt.

Along those lines there s a conversion efficiency from DC to AC Watts based on the particular inverter used, and the inverter itself consumes power which must not be left out when sizing components.

Yes, it is confusing.

And before anybody says it, I already know ozarkgem is talking about an off-grid system.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Is a Watt a Watt or What?
ozarkgem wrote: »
In figureing how many Watts of panels I need can't I just take the number of Watts the appliance uses and go with that number? Example if my refer
uses 1000 watts per day @ 110 v will 1000 watts of panels @ 24v run it? I know there are losses but just for simplicity will that work? I have seen the formula's that convert from 12 volt to 110 volt , seems complex. I guess my question ,is a watt at 12 v, 24 v 110v the same? I know the amps are different and I will have losses in the system.
A Watt is indeed a Watt, but what you need to be thinking about is Watt-hours. Your refrigerator does not use 1000 Watts per day, it uses a certain number of Watt-hours per day. A Watt is a unit of power, which is a rate of energy flow, not a quantity of energy. What you are saying is like answering the question of how far you went today with "I went 100mph." For how long? Hence, Watt-hours.

If your refrigerator were to use 1000 Watts all the time (it doesn't), it would use 24,000 Watt-hours (24 kilowatt hours) per day. It would most definitely be time to shop for a new fridge.
• Banned Posts: 17,615 ✭✭
Re: Is a Watt a Watt or What?

We'd better also mention that a "100 Watt" panel does not produce 100 Watts all the time. It produces 100 Watts when conditions are right (i.e. in a lab for a moment). Out in the real world over what are known as "hours of equivalent good sun" it will produce an average power which is likely 20 to 25 percent less than the "nameplate" rating.

This also means panels do not necessarily put out power at the same time as loads use it, hence the need for batteries in between.

So as ggunn said you have the Watt hours per day which are the quantity of power used but which may be drawn at varying amounts at different times. The batteries are a constantly available power source to supply this on demand. The solar panels then are used to recharge the whole daily amount within the limited time the sunlight is available.

A refrigerator example:

Draws 120 Watts. Runs for 20 minutes of every hour. Total daily power consumption: 120 Watts * 8 hours per day run time = 960 Watt hours (not 100% accurate as it does not include start-up demand, light bulb coming on when the door is open, or defrost cycle if so equipped).

960 Watt hours AC through a typical inverter becomes 1067 Watt hours DC due to conversion inefficiency. Inverter itself uses 20 Watts (for example - they are not all the same) over 24 hours for an additional 480 Watt hours. Total: 1547 Watt hours.

1547 Watt hours on 24 VDC = 64 Amp hours. At a maximum depth of discharge of 50% this equates to a minimum 128 Amp hour 24 Volt battery. This too is not exact as the actual current draw varies with the Voltage (which has an operating range) and as it does so the real Amp hour capacity of the battery changes (Peukart effect).

To "put back the used Amp hours" (another inaccurate phrase) the panels have to be able to produce the equivalent power plus losses due to inefficiencies in recharging over the amount of equivalent good sun time available. Since the Voltage here varies as well as the current, it is not as simple as panels put out 'X' Amps so multiply by 'Y' hours to return the Amp hours.

The good news is you don't actually have to memorize this stuff to get a system to work. There are short-cut rules-of-thumb which will result in a viable system under most typical conditions without the math-induced migraine.
Re: Is a Watt a Watt or What?

And to make it clear(er):

Watts = Gallons per Hour, Miles per Hour
Watt*Hours = Gallons pumped or Miles driven

Watt*Hours (And Amp*Hours) are usually just written as WH and AH.

The simplest conversion of how much power you use to size of solar array...

Measure your load with a Kill-a-Watt type meter (measures kilo Watt*Hours). Say your fridge uses 1.5 kWH per day. And you get, on average, about 4 hours of sun (full noon time equivalent, fixed array, tilted to latitude, adjusted for weather, 4.0 minimum for ~9 months a year--rest of year use backup genset for cloudy weather, etc.). Then the formula to find solar array wattage would be:

1,500 Watt*Hours per day * 1/0.52 off grid system efficiency * 1/4 hours of sun per day = 721 watt solar array minimum

Remember, an idling AC inverter to run a fridge can burn around 10-20 Watts by itself--just waiting for a load. And the modern frost free fridges usually need electricity 24x7 to keep the timers/electronics working correctly... So you may have to add that "loss" too (fridge may average 50 watt load over the day, a 10-20+ watt inverter "tare" loss can significantly increase your array requirements).

There is also the issue of Battery Bank charging current (we use ~5% to 13% rate of charge) to help ensure the batteries are quickly and fully recharged for longer life--I.e., a "large" battery bank can also push the array requirements/size up too.

-Bill

Note, sometimes you will see people type Watt*Hour as W/H (Watt/Hour) or Watt-Hour... Remember it is Watts (times) Hours when working the math (the difference between Gallons per Hour--A RATE--vs Gallons Pumped an "amount").

So, Watts is the "rate" and Watt*Hours is the "amount".

Note that Amp*Hours is very similar to Watt*Hours--The difference is we are missing "voltage"... That is why when somebody types they used 100 AH (Amp*Hours) last night, we ask what voltage that was.

Many times there is confusion between 100 AH at 12 volts (100AH*12V=1,200WH) and 100 AH at 120 volts (100AH*120V=12,000WH).

People see a "small" 10 amp load at 120 VAC, and then think it is 10 Amp at 12 VDC....

But no! It is a 100 Amp load at 12 VDC (power=voltage*current). If you cut the voltage to 1/10, then the current has to go up 10x for the same amount of power.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 21
Re: Is a Watt a Watt or What?

I see a little better now. I am horrible at math. The only thing I flunked in school. I have a kill a watt meter but until I get the solar system in place I don't have the power
to run anything. Don't want to run the gen for 24 hrs. I guess since I have everything in stock I will just install it and tweek as needed. Thank you for your answer.
• Solar Expert Posts: 5,183 ✭✭✭✭
Re: Is a Watt a Watt or What?

for a lot of small items you just need to see what they are consuming for 5 minutes or so, then on to the next item. Dont forget to make a list of every light and appliance room by room. The detail helps, a lot. You'll be surprised at how many things you have.

HTH

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Is a Watt a Watt or What?

i know the guys tried to explain this, but i'll try to super simplify this that i hope you'll understand it better.

in the case of a load with x watts there are losses before getting to the load which causes a higher need of watts to feed to it to insure the x watts that are needed by the load is there. watts are watts, but transferring or converting causes a loss necessitating more than the required load.
• Solar Expert Posts: 1,973 ✭✭✭
Re: Is a Watt a Watt or What?
niel wrote: »
i know the guys tried to explain this, but i'll try to super simplify this that i hope you'll understand it better.

in the case of a load with x watts there are losses before getting to the load which causes a higher need of watts to feed to it to insure the x watts that are needed by the load is there. watts are watts, but transferring or converting causes a loss necessitating more than the required load.

That's simplified? You skipped the part about the flux capacitor.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Is a Watt a Watt or What?
ggunn wrote: »
That's simplified? You skipped the part about the flux capacitor.

ok, how's this watts load = watts in - watts lost?

i was going to say something on you fluxing it.:p
• Solar Expert Posts: 1,973 ✭✭✭
Re: Is a Watt a Watt or What?
niel wrote: »
ok, how's this watts load = watts in - watts lost?

i was going to say something on you fluxing it.:p
...and the horse you rode in on.