Deleted

Qyburn

On-the-coast said:

I might have missed it as i scanned down this thread, but there appear to be several vital data points missing:
..

Exactly. In my example our freezer has a startup load of 1250W, and a running load of around 75W, both of which could only be determined by measurement or reference to data sheet.  Not by dividing 0.55 by 24.

In addition you also need to know the overload characteristics of the generator or inverter.  What is it's peak output, and how does it behave if this is exceeded (voltage droop, frequency drop, trip out, or blow up).

k_man

Deleted_User said:

QrizB said:

It doesn't work like that.

Today I drove 270 miles ( that's a true statement). How far did I drive between 11am and 12 noon?

if you understand what a KWh is then you would realize it DOES work like that!! LOL

The formula for calculating a KWh is this:

KWh = KW x Hours

In order to calculate KWh you need to know KW (power) generated/consumed per hour and multiply it by the number of hours.

If you have the information to do that, you should be able to reverse engineer and calculate KW generated for each hour of the day by using the formula:

KW = KWh / Hours

So what is a kilowatt?
A kilowatt is a unit of power. It is the amount of energy an item needs to function; the rate at which energy is consumed. One kilowatt is equivalent to electricity flowing at a rate of one thousand joules (unit of energy) per hour. A 1kw generator (solar, wind, diesel) will generate power at a rate of one thousand joules (unit of energy) per hour.

It is accurate to say that when working at peak efficiency a 2kw solar array will generate 2kw of power in an hour. Over a two hour period it will generate a total of 4kw. 

It is also accurate to say that when working at 80% efficiency your system will allow your appliances to consume 80% of that generated power - 3.2 kw.

If you identify that in practice your appliances are only able to consume 3kw of that power you have an efficiency problem that you need to investigate and resolve.

You don't need to use KWh (energy) to make these calculations. When you're comparing POWER IN and POWER OUT you don't need to concern yourself with energy.

Sorry, but you are really starting to embarrass yourself here:

It is accurate to say that when working at peak efficiency a 2kw solar array will generate 2kw of power in an hour. Over a two hour period it will generate a total of 4kw. 

Qyburn

Deleted_User said:

..  My inverter has an idling power consumption of 30w a day.

I can see you still don't get it, which is frustrating because sometimes it seems like you understand what these units mean, then you revert to your strange "watts per hour" type terminology.  Taken literally 30W per day means power is increasing by 30W per day.  Just like miles per hour means the number of miles is increasing every hour.

QrizB

The reason I want to turn off my fridge at night is because it only has a low draw - if it's like yours then for most of the day it's drawing less than 100w of POWER. But in order to run off my battery I would need to keep my inverter switched on all day. My inverter has an idling power consumption of 30w a day. So the overhead to running my fridge via the inverter is 24 x 30w = 720 watts watt-hours total per day. So whereas the actual daily power energy draw of the fridge might be 600 watts of POWER watt-hours of energy, in order for my inverter to run it the total power energy consumed to run my fridge is 1320 watts watt-hours. Highly inefficient.

Fixed that for you.

And yes, it does sound highly inefficient.

Magnitio

Deleted_User said:

Sorry, but you are really starting to embarrass yourself here:

It is accurate to say that when working at peak efficiency a 2kw solar array will generate 2kw of power in an hour. Over a two hour period it will generate a total of 4kw. 

No one should ever feel embarrassed about engaging in debate!!

A KW is a unit of POWER. It is 1000 watts.

A definition of watts:

"The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3.[1][2][3] It is used to quantify the rate of energy transfer. "

https://en.wikipedia.org/wiki/Watt

So a KW (a UNIT of POWER) is equal to 1000 watts,

During optimal conditions, a 1 KW array is capable of generating 1000 watts/1KW of POWER from the sun's energy.

Therefore, in 2 hours it would generate 2 KW of power, in 3 hours it would generate 4 kw of power, etc, etc.

The convention when energy companies are selling electricity from the grid is to work in KWh (Energy). 

A KWh = KW x Hours.

This is to help people work out how long an appliance of given power draw (KW) can run on a unit of energy (a unit is a KWh).

However, unless you have a need / reason to work with energy (which has a time factor) it makes more sense to use POWER in your calculations.

So, to monitor the efficiency of your system you track POWER IN (in KW - so total energy generated) and POWER OUT (in KW - so total power draw of your appliances). The time factor is irrelevant. If you need to use time (for example to calculate how much power a given appliance drew from the grid whilst it was running, then express that time explicitly. 

It's much more meaningful and accurate to work in POWER rather than ENERGY for these purposes. This is because of the many anomalies when working with KWh. For example,

•  if you're specifying 10KWh energy from your solar array you SHOULD be able to calculate KW per hour (KW = KWh / hours) but you can't because you don't have the time element, and also the power wasn't generated at a constant rate.
  
  
•  if you're specifying 800kwh energy used by your fridge per day you SHOULD be able to divide 800 by 24 to calculate KW. But you can't.

So the most consistent thing to do is simply work in TOTAL POWER GENERATED (KW) and TOTAL POWER CONSUMED (KW)


The formula for calculating KWh is KWh = KW x hours. I don't have the number of hours that my POWER was generated over. Nor do I have the number of hours that my appliances drew the power over. So the correct thing to do is to specify it in TOTAL POWER (KW). 

If you want to put an arbitrary h on the end have at it. 

You seem to be lecturing people about power and energy on multiple threads. Lots of people disagree with you, so you post even longer explanations. I know you believe you are correct and everyone else is wrong, and that is up to you, but please can you stop the lectures.

EssexHebridean

I do believe this might just possibly be the first thread where I have ever seen mansplaining in action, by a woman, to an awful lot of long-term regular energy board posters who REALLY know their stuff and understand what they are talking about...

For avoidance of doubt for anyone else reading this and wondering what the issue is  - using incorrect phrasing in areas like energy use can actually fundamentally change what someone is saying - literally mean that people cannot help them because they believe they are asking for/talking about something different to their actual intention. THAT is why it's important to learn and understand the phrasing and terminology - it's not just a bunch of people being pedantic. Also - stating incorrect facts which people then take the time and trouble to correct is not "a debate" - facts are facts, and arguing against fact is contradiction, not debate. 

power consumption of 30w a day. 

If this is in fact the case then the energy used is so negligible it is barely worth worrying about. For context, we are talking literally about a penny or so at today's single rate energy prices. 

On-the-coast

Debate is good.
mixing units is bad.
Watts = Power : Agreed

Energy used is measured in Watts expended over time, so the h is not arbitrary.
no more so than it is in MPH (miles per hour)

I have a Canon Flashlight that outputs about 400WS of energy (Watt Seconds)... but only for about 1/200 of a second... please don't convert that into KWh or MWh unless you’re needing a tiny number… clue 1.4MegaWatts isn't a correct or useful measure in this case.

jrawle

Yet another thread descending into confusion over units. As a professional physicist, I do sympathise with the public getting confused by this, due to the unfortunate use of the weird, non-SI, derived  unit of energy, the kilowatt-hour (kWh) which in SI units is equal to 3.6 x 10^6 joules (or 3.6 MJ). Within the last week, I even heard a radio advert (can't remember what for) where they referred to "kilowatts per hour", so how can the public be expected to get it right?

Actually, in this thread, most people have most things right, there is just the odd detail that is wrong, so I don't want to pick on anyone in particular. However, please do use the correct units when you do calculations; they are not arbitrary, but an important part of the value you quote. So when you do a calculation, include the units throughout. For example:

A device with a power of 1 kW, used for 3 hours, uses:
1 kW x 3 h = 3 kWh of energy.

A device using 5 kWh of energy in total over 10 hours overnight has an average power of:
5 kWh / 10 h = 0.5 kW

It's true that you do not know the power at each point in time overnight, but you can still express an average power.

If you are still confused by units, consider a garden that measures 10 metres by 4 metres. What is the area of the garden? It is:
10 m x 4 m = 40 m^2

You wouldn't say the garden has an area of "40 metres"; everyone knows it's "40 square metres" or "40 metres squared". Similarly, you need to keep the dimensions of your units correct when discussing power and energy.

Apodemus

jrawle said:

If you are still confused by units, consider a garden that measures 10 metres by 4 metres. What is the area of the garden? It is:
10 m x 4 m = 40 m^2

You wouldn't say the garden has an area of "40 metres"; everyone knows it's "40 square metres" or "40 metres squared". Similarly, you need to keep the dimensions of your units correct when discussing power and energy.

Oh no! Don't go there!  40 metres squared, could well be construed as a square 40mx40m which would be 1600m^2!  Which is a whole different rabbit hole!  

EssexHebridean

@jrawle at least one of the companies selling "smart thermostats" is currently using all sorts of odd descriptions in its radio advertising - it may well be that one you've heard as well!