ah or kwh
t00ls
Solar Expert Posts: 250 ✭✭✭
battery companies have recently been listing their batteries in kwh instead of ah
it has some new to solar confused ...they think their amps go up as a battery is used and the voltage goes down...instead of what really happens , the amps remain the same as voltage goes down
I have been told by some of these confused people that it is better to know the kwh than the ah because they can know how long they can run....I tried to explain that it is not a good way to know how long they can run because as you use a battery the voltage goes down which means means available watts also decrease...which means the amount of time goes down...all while you can still be using the same amount of watts
I believe the battery companies are doing what the generator companies are doing by list a bigger number to entice the customer...generator companies have been listing their generators surge watts capacity as the size instead of the constant watts as size...IE: 6250 surge 5000 watts constant
I believe it misleads a lot of people especially when they find out they cant run 5500 watts ...the generator breakers will flip when a surge happens
____________________________________________________________________________________________________________
I ask some of the other experts why the battery companies are doing this...any thoughts
it has some new to solar confused ...they think their amps go up as a battery is used and the voltage goes down...instead of what really happens , the amps remain the same as voltage goes down
I have been told by some of these confused people that it is better to know the kwh than the ah because they can know how long they can run....I tried to explain that it is not a good way to know how long they can run because as you use a battery the voltage goes down which means means available watts also decrease...which means the amount of time goes down...all while you can still be using the same amount of watts
I believe the battery companies are doing what the generator companies are doing by list a bigger number to entice the customer...generator companies have been listing their generators surge watts capacity as the size instead of the constant watts as size...IE: 6250 surge 5000 watts constant
I believe it misleads a lot of people especially when they find out they cant run 5500 watts ...the generator breakers will flip when a surge happens
as with the battery companies listing some as 4.8 kwh in a 48 volt configuration....sounds like a lot , right? nope...it is only 100 ah
____________________________________________________________________________________________________________
I ask some of the other experts why the battery companies are doing this...any thoughts
Comments

as an after thought...most homes use 4  6 kwh ....thats 46 kw in an hour...what do they use in 24 hours...anywhere from 70 100 kw...so the 4.8 kwh battery wont last very long as a backup source

To a degreeThe change from AH to kWH can be laid to the changes in our loads...
In the "olden days", much of the loads are (more or less) constant Amp loads such as filament lamp loads. And lead acid batteries are (relatively) close to 100% efficient in charging when measuring Amp*Hours. You take out 50 AH, it takes around 50 AH to rechargeWith only the final bit of charging being less efficient (final charging, EQ charging, etc. when gassing).
I first started seeing batteries rated in Watt*Hours for UPS systems. AC inverters are constant power devices. The inverter supplying 300 Watts at 120 VAC constant output, but the input current is also dependent on DC bus voltage: 300 Watts / 12.7 volts charged battery = 23.6 Amps
 300 Watts / 10.5 volts near dead battery = 28.5 Amps
Of course, the higher the current draw (C/20 vs C/5 discharge rateI.e., 20 hour discharge vs 5 hour discharge to dead rate), the lower the apparent capacity of the batterySo you will still have a different capacity in WH rating depending on rate of discharge.
And for nontechnical folks, 300 Watt discharge at 120 VAC vs 300 Watt discharge at 12 VDC is still 300 Watts. And why we here suggest people do all of their load calculations in Watts and Watt*Hours to prevent the confusion when doing Amp and AH calculations at 240 VAC vs 120 VAC vs 48 VDC vs 24 VDC vs 12 VDC. Again a difference when boats/cars/etc. only at native DC loads vs today's usages (cabin, RV) that have 12/24 VDC loads and 120/240 VAC loadsBecause of the AC inverters and DC to DC converters that easily change voltage as needed.
So when talking about a battery bank in WH and kWH storage capacityIt does remove a bit of confusion. We do run into that question frequently hereSomebody tells us they have a 200 AH battery bank for their system, but we always have to ask them what the battery bus voltage is to start the load/charging design questions.
Watts and Watt*Hours are "mathematically complete" descriptions of loads and charging systems. With Amp and Amp*Hours, we are always asking at what voltage (120 VAC and 10 Amps vs 12 VDC and 10 amps10x more power at 120 VAC): Power = Voltage * Current
 12 VDC * 10 amps = 120 Watts
 120 VDC * 10 amps = 1,200 Watts
 Energy (Watt*Hours) = Voltage * Current * Time
 12 volts * 10 amps * 5 hours = 600 Watt*Hours
 120 volts * 10 amps * 5 hours = 6,000 Watt*Hours
 Amp*hours = Current * Time
 AH = 10 amps * 5 hours = 50 AH (at what voltage?)
Regarding home powerWatts is a "moment in time" value... For some examples, say an average energy efficient North American home uses 300 kWH per month: 300 kWH month / 30 days per month = 10 kWH per day
 10 kWH per day / 24 hours per day = 0.417 kWH = 417 Watt constant load over 1 month
 50 Watts average * 24 hours per day = 1,200 WH per day
 1,200 WH per day * 30 days per month = 36,000 WH per month = 36 kWH per month (or about 12% of average home load at 300 kWH per month bill)
 1,200 WH per day * 365 days per year = 438,000 WH = 438 kWH per month on the Energy Star hang tag
 600 Watts fridge starting or defrosting heater
 1,200 Watts microwave
 200 Watts of lighting
 100 Watts TV/radio/cell phone charging
 30 Watts laptop computer
 300 Watts desktop computer
 4,000 Watts well pump
 home heating (central heat vs wall heater)
 cooking (gas/propane vs electric)
 hot water (gas vs electric)
 total = 6,230 Watts "typical worst case home loads"
A reason why we always are asking is that 6,000 Watt (average loads) or 6,000 Watt*Hour (average energy usage per day)...
And why we push conservation... A 300 Watt desktop system vs a 30 Watt laptop1/10th the energy usage for normal house hold usage (vs gaming, scientific, server usage).
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
I think the switch is somewhat reasonable and better with lithium batteries which have a much flatter curve when discharged.
This helps keep comparisons the same when looking at different systems. Number of cells and voltage will vary with slightly different lithium chemistries.
Now if we could get the lead acid batteries people to describe their battery banks correctly or at least in the same ball park. I think for forklift Battery banks, one large seller converting their 6hr rate to 20hr rate at 1.6x the capacity have others describing theirs incorrectly to compete.Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite Epanel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
 Assorted other systems, pieces and to many panels in the closet to not do more projects. 
Offgrid and even grid app's are a small part of the battery business. The different rates help the pro's size their requirements in stationary/mobile power systems.
Anyone think that the guy who makes a purchase order for a fleet of locomotive batteries is having trouble understanding specifications?
It is just due diligence, you have to do your homework or pay the price. Mistakes are often an excellent way learn also....."we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
Email offgridsolar@sti.net 
Is that 'excellent' or 'expensive'...lol
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite Epanel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
 Assorted other systems, pieces and to many panels in the closet to not do more projects. 
There is always the healing power of "and".
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
t00ls said:battery companies have recently been listing their batteries in kwh instead of ah
it has some new to solar confused ...they think their amps go up as a battery is used and the voltage goes down...instead of what really happens , the amps remain the same as voltage goes down.............................................................................................When using an inverter at any given load, what happens to the current draw as the battery voltage falls?
I always have more questions than answers. That's the nature of life. 
It all depends on the type of loads...
 Filament bulbs= tend to be constant current.
 Purely resistive loads such as a heater, current will drop as voltage drops.
 DC motors will generally drop current as voltage fallsHowever motor design and type of loads i.e., fan vs compressor will affect current consumed.
 Electronic loads (LED lighting, motors with electronic controls, AC inverters) are constant power and will generally increase current as voltage falls.
Using math... We can setup a very easy equation (model) for an AC inverter: Power = Voltage * Current (ignoring losses)
 Example: 10 amp load @ 120 VAC load
 Power = Voltage * Current = 120 VAC * 10 Amps = 1,200 Watts
 Assume DC battery bank voltage is from 15.0 volts (charging) to 10.5 volts (near dead):
 Power = Voltage * Current
 Current = Power / Voltage
 DC bus current = 1,200 Watts / 15.0 VDC = 80 Amps (12 volt battery bank charging)
 DC bus current = 1,200 Watts / 10.5 VDC = 114 Amps (12 volt battery bank under heavy load, near dead and/or voltage drop in DC bus wiring)
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Bill,Yes, Sir. My question was directed toward the OP.I always have more questions than answers. That's the nature of life.

Hi Marc,
No problem... I was just not sure I was clear in my earlier postAnd I had some free time to (again) type way too long of answers.
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Marc Kurth said:Bill,Yes, Sir. My question was directed toward the OP.
The guy who considers himself an expert, criticizes others as "confused", and is just plain wrong? Really, if he believes what he says, he should simply change to a lower voltage system and run his stuff with half the energy. 
One of the things I try to do here is avoid "generalities". It is hard to have a "feeling" of what 3.3 kWH per day vs 33 kWH per day is...
I may start with some generalities on how many WH/kWH per day an "average" home/cabin may use and then do some math to show what a generic system could look like (costs/size of system/complexity) because 1,900 Watts of solar panels and 12 "golf cart" batteries starts to make it "real" when trying to power a refrigerator and some lights and an RV water pump.
But then quickly try to figure out what their actual energy usage isOr close too it (from electric bill, KillaWatt meter, location, why/when need for power, etc.).
The problem with generalities is that hard right and wrong are not always clear. As we talked about here, AH @ xx volts vs kWH battery capacity. They both work and are useful. Loads types are specific to an install... When talking about generic systemsI tend to "error" on the conservative side. With off grid solar, one is always over designing the system capacity. We have to because having a system that does what we need when new on a sunny dayHave to allow to supply needed power on a stormy day 5 years down the road instead of leaving the owner in the dark.
BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Marc Kurth said:t00ls said:battery companies have recently been listing their batteries in kwh instead of ah
it has some new to solar confused ...they think their amps go up as a battery is used and the voltage goes down...instead of what really happens , the amps remain the same as voltage goes down.............................................................................................When using an inverter at any given load, what happens to the current draw as the battery voltage falls?
if the voltage fails....the current draw goes to 0
what I am after talking about is this.....48vdc from 650Ah of battery inverted to 240vac drawing say 30 amps...as the 240vac is drawing 30 amps the 48vdc is going down to maybe 47.9vdc....the Wh ...KWh will not remain the same...also the time of 650Ah is counting down
a battery bank will have a certain amount of KW but not KWh.....Ah is always the same......if you have a 10Ah you can always draw 10 amps in 1 hour ...then the battery needs recharged, or should be
thats what this thread is about

I think there is a misunderstanding of the question. The question is "falls" not "fails" (FALL vs FAiL).
And for that answer, the old power equation still works: Power = Voltage * Current
 Current = Power / Voltage
 Current = 1,000 Watts / 15.0 volts (battery charging) = 66.7 Amps
 Current = 1,000 Watts / 10.5 volts (battery near dead) = 95.2 Amps
The Watt*Hour rating is based on a constant power draw (typical inverter is a "constant power" device).
A battery bank rated in Amp*Hours is "less accurate" in today's world of UPS (uninterruptible power suppliesWhere I have seen batteries rated in "native Watt*Hours") vs the typical Amp*Hour rating method (constant current draw).
Here is a glossy that shows one way how to do kWH and AH calcuations for UPS battery banks:
https://www.solidstatecontrolsinc.com/knowledgecenter/~/media/85b8e51754c446bda1f38449f444471c.ashx
The problem is that batteries are discharge rate sensitive... So AH at C/20 (20 Hour discharge rate) is different that C/6 or C/1 discharge rates.
From Trojan (just a random pick):
https://www.trojanbattery.com/products/deepcycleflooded/solarindustriallineflooded/
For example, a 6 volt battery has 610 AH @ 100 hour rate and 3.66 kWH @ 100 hour rate... What is the "average" battery voltage? P=V*I; I=P/V
 3,660 WH / 610 AH = 6.000 volt average voltage
Anyway, just trying to show that a FLA battery has significant "capacity" variation between high discharge rates and low discharge rates... "One magic number" does not really give useful (accurate) answers. kWH capacity will also vary based on rate of discharge.
Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
Marc Kurth said:t00ls said:battery companies have recently been listing their batteries in kwh instead of ah
it has some new to solar confused ...they think their amps go up as a battery is used and the voltage goes down...instead of what really happens , the amps remain the same as voltage goes down.............................................................................................When using an inverter at any given load, what happens to the current draw as the battery voltage falls?
the current draw will go up and down depending on your loads...but unless you are charging with solar or generator, the battery voltage goes down till it gets to the lowest voltage that the inverter will operate
EG: my xw 4548 will operate till it gets to about 41 volts...so
if I am drawing 10 amps.....41v x 10a = 410 w
compared to 48v x 10a = 480w
watt hours will not be consistent on the dc side compared to consistent on the ac side .....because it will always put out the same 120/240 @10 amps...but on the dc side the voltage is always dropping giving you less watts available
therefore I say it is better to use amp hours when dealing with inverters

Just so you don't get the wrong idea about what I was trying to say:With a DC light bulb for example, yes, the wattage will go down with battery voltage. But, inverters are socalled “constant wattage” devices. The amperage draw on the battery bank will increase as the battery bank voltage goes lower, with a constant load on the inverter.
Using a 1,000 watt AC load on the inverter as an example. The inverter will deliver its full wattage all the way down to battery low voltage cutoff.
With a constant 1,000 watt AC load on the inverter / 48v battery = 20.83 amp load on the battery bank
With a constant 1,000 watt AC load on the inverter / 41v battery = 24.39 amp load on the battery bank
The watts are the same.
I always have more questions than answers. That's the nature of life. 
Better not to let the voltage get that low The default XW LBCO is 44 vdc. By lowering it you just are rolling the dice with battery life on a lead acid battery."we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
Email offgridsolar@sti.net 
Yes, Sir. I was just using the OP's number. Doing what I do for a living, I decided a long time ago that some people should not be allowed to own batteriesI always have more questions than answers. That's the nature of life.

He is a good guy and you were right Marc. Just lot's of ways to say this stuff. Nine minutes until we go to the lounge for Happy hour......"we go where power lines don't" Sierra Nevada mountain area
htps://offgridsolar1.com/
Email offgridsolar@sti.net
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