# Is there a table for converting ac usage to dc output?

Solar Expert Posts: 1,071 ✭✭✭✭
There is a local member that has called me a couple times that wants a backup power system for short term power outages. He got a kilowatt meter and seems not to understand how to figure his power consumption. He thinks if it tells him how many AC amps and watts it uses that he should have a chart to tell him how many batterys he needs ect. He hasn,t bought any equipment yet. I feel frustrated on how to explain things to him. He wants to run a fridge that is preety new and sounds reasonably energy efficient to me. I told him to monitor his fridge for a 24 hour period and find the amount of kw it uses so he knows how many hours it will run in in 24 hr period. Thinks he don,t need to do that. Just run it for about 4 hrs. and get answer. wants to run 220 v ac submersible pump, furnace and couple lights. So he has the impresion there should be a chart to tell im how big inverter, generator and battery to get. Somebody on here told him all he needs is 2 250 amp 12 volt sealed batterys and one of those 2000i generators. Doesn,t want to go out to start the generator at night . He has a gas well. I told him if all he wants is a backup for power outages he could get a guardian that self starts itself and probably will have less money than buying batterys, inverters and a small generator to charge the batterys. I hope someone here can explain to him how to use the kilowatt meter in simple terms to figure his power requirements. Then tell him in simple terms how to figure how kw the battery stores and how much it can output. How big of inverter and he has his heart set on the small honda. I am not very good at explaining it to him as I am not 100% sure about how much power is in a battery> I think I know what I would need but don,t feel qualified enough to give him a 100% sure answer. S:Dlarvic

Re: Is there a table for converting ac usage to dc output?

You are going in the right direction... For variable on cycle stuff--doing a 24 hour measurement is more accurate.

If you can get Watts Peak, Watts Average, VA, and kWhrs in 24 hours.

Bring the data and his requirements back here (how many hours to run off battery--12 hours, 1 day, 3 days; how often, etc.), and we can put something together for you.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 246 ✭✭
Re: Is there a table for converting ac usage to dc output?

I get phone calls like that lol, "I want a solar system for my house" How much power do you use ? "not much, just a fridge / tv / pump and a few lights"

My usual reply go`s something like,
Yes no problem, It will be somewhere between 15 to 50 thousand Euro,
But what I need you to do before I can give you a more accurate price, Is get a pen and paper and go round the property, write down EVERY appliance, it`s "plated" rating, How many hours per day and days a week it is on, and the time it s in use ( could it be shifted to a more economical time of day?) stress to them it is very important to list every item, as that is what the system is designed around and that number will also be used in the contract, ( safeguards you against "this system does not do what you said " well yes, but you didn`t tell me you run the well pump runs 9 hours a day:roll: )
Then go from there

Have a good one
Tim

ps, on fridges, a "Ball park" way to work it out is they run about 1/3 of the time=8hrs a day

edited to add, if they come back with the numbers as asked, If practical I also put one of these on http://www.diykyoto.com/uk hidden so they can not see the display, I found one customer had failed to list the 300w 24/7 draw off his central heating and circ pumps ,
• Solar Expert Posts: 1,071 ✭✭✭✭
Re: Is there a table for converting ac usage to dc output?

I told this guy to get all the readings with his kilowatt meter and someone would probably help him to make a plan and explain it to him. What is confusing is the water pump which is a submersable. I gave him the figures from mine. Here is the decal for my pump. I don,t understand what it means and hope someone can give me some imput about it. Flotec brand model FP2222-09
3/4HP (.55 kw) 230 v 1ph 60hz
6.8 amps 3450 rpm SF: 1.5
SF MAX AMP 7.4 KVA code J
He thinks his pump is about the same size as mine. I have a trace 2200 transformer for the pump. and it will be the only item needing 220v ac. I plan on connecting transformer output on the pump side of pressure switch. Will 4 400 amp 6 volt batterys at 24 vdc start that pump ok? Or should I buy 8 of the 400 amp batterys? My thinking is 1 set of 4 batterys well be ok. My heating ,clothes dryer, water heater and cook stove will be nat gas. I am not worried about having a backup for more than a day as I have free gas to run my generator. Even though this is a backup system I plan on dedicating some different circuits at different times of the year to make use of some of the power.
Any imput will be apreciated. S:Dlarvic.
• Banned Posts: 17,615 ✭✭✭
Re: Is there a table for converting ac usage to dc output?

Be careful: water pumps are hugely variable.
The HP of the motor is almost meaningless in application. Design and placement of the pump as well as the start/stop pressures make a big difference in how much they draw on start-up (the killer current!) and run.

For example: 1/3 of an electrical HP should be 248.6 Watts. Call it 250. Yet a 1/3 HP water pump might draw more than 746 Watts running, and certainly will exceed that on start-up. If it has to push against a lot of water or pipe resistance it will slow the motor and draw more current. It is even possible to encounter situations where going to the next size up pump motor will use less power because the motor isn't working as hard.

Maybe what this fellow wants to know is how to convert Watt hours into Amp hours? Maybe he needs a long and expensive course in electrical theory and practice.
• Solar Expert Posts: 246 ✭✭
Re: Is there a table for converting ac usage to dc output?

Just to add on pumps, be extra careful with the long thin borehole type`s , They can take up to 10 times their rated on start ( LRA, locked rotor amps) think of a long and short handle wrench trying to to turn a tight nut, thin pump, short handle wrench, "fatter" motor, longer wrench handle,

Have a good one
Tim
Re: Is there a table for converting ac usage to dc output?
solarvic wrote: »
3/4HP (.55 kw) 230 v 1ph 60hz
6.8 amps 3450 rpm SF: 1.5
SF MAX AMP 7.4 KVA code J
1 HP = 746 watts
• 746 watts * 0.75 = 559.5 watts = 0.56 kWatts
• 6.8 amps * 230 volts = 1,564 VA (not Watts)
• Note: amount of work = 560 watts. There are losses to heat and "out of phase" current due to inductive motor (power factor)
SF Max Amp = 7.4 KVA = 7,400 VA
• SF = "Locked Rotor" VA? = "Stalled/Starting Field current" VA?
• 7,400 VA / 230 volts = 32.2 Amps worst case starting current
• 32.2 amps / 6.8 amps = 4.7x running current = starting current
All the numbers look real (if not ugly--which is what happens when you look at the details of running a large motor).

The 32 amps is a worst case, but hopefully either never happens or only happens for a very short period of time.

The 6.8 Amp running current is probably high--but until it is measured with an Amp/Watt meter, you don't know.

And you need to measure with a Watt / Watt*Hour meter to know the average power used over a day.

Watts / Watt*hours are needed to size the battery bank. And VA/KVA is needed to size the inverter (high current, even if out of phase, is real and wiring/inverter/etc. need to be sized to handle).

For starting loads vs battery size... Normally, we try to run/charge batteries in the C/20 to C/10 range (5% to 10%) of battery's Amp*Hour rating... For heavy surges/starting loads, you probably do not want to plan on drawing more than C/2.5 or C/2 of your bank's capacity.

For your bank 400 Amp*Hour at 24 volt:
• 400 AH * 1/2.5 = 160 Amp surge
• 160 Amps * 48 volt = 7,680 watt surge recommended maximum
You certainly can draw more--but it is not a great idea... Voltage drops, self heating in battery bank, lower battery efficiency at heavy currents, etc.... All can give you less than ideal performance/life/problems.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 9,583 ✭✭✭✭✭
Re: Is there a table for converting ac usage to dc output?
solarvic wrote: »
..... I have a trace 2200 transformer for the pump. and it will be the only item needing 220v ac. I plan on connecting transformer output on the pump side of pressure switch...

Look up what the standby losses on the transformer being powered up 24/7. If it's a 5,000W at 95%, there may be close to 250W just keeping the iron warm ! Switching the primary (120V) side has no standby losses, you just need a beefy switch that can handle the current, and an inverter that can handle the inductive surge of a big transformer coming on-line with a loaded pump behind it.

If i'm wrong, someone tell me, because I'm about to do it that way - switching the primary side of a step-up 240 - 480V transformer, for a long run to my pump, where I step back down to 240V. [ using an XW6048 240V output ]
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

Re: Is there a table for converting ac usage to dc output?

Switching the primary side is a good idea... But I am not sure that the "raw" standby losses are as high as you fear...

Have you measured them yet? Typically, you may have high(er) than desired current flow--but it is not in-phase with the voltage (lagging inductive current). So, the actual power loss in Watts driven by the inverter can much lower...

However, if the inverter can switch to "search" mode and not need to circulate those currents (all your loads are off at night), you certainly will have have lower losses overall with your plan because the inverter will not need to supply that current.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,071 ✭✭✭✭
Re: Is there a table for converting ac usage to dc output?
mike90045 wrote: »
Look up what the standby losses on the transformer being powered up 24/7. If it's a 5,000W at 95%, there may be close to 250W just keeping the iron warm ! Switching the primary (120V) side has no standby losses, you just need a beefy switch that can handle the current, and an inverter that can handle the inductive surge of a big transformer coming on-line with a loaded pump behind it.
Mike I was going to let the pump pressure switch turn on the transformer when the pump needed to run. It tells in the manual how to connect it. Bill Thanks for explaining my pump statistics. I printed it for reference. Is there a way of printing just one posting to a thread without printing all of them? My person is a 1 person household so I can watch everything as far as the pump goes. Maybe instead of dedicating my offgrid system to pump maybe I shouldn,t. Maybe just use the backup for pump when there is an actual power outage. I think the guy that has called me probably can,t run his pump with just 2 250amp 12 volt batterys with a 24 volt inverter. S:Dlarvic
Re: Is there a table for converting ac usage to dc output?

At the top of each post, there is a number like:
Just click on that number and it will pop-up a window with just that post.

The other way (in many web browsers) it is to do "Print Preview" and just print out that page(s) with the post(s) that you need.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,071 ✭✭✭✭
Re: Is there a table for converting ac usage to dc output?
BB. wrote: »
At the top of each post, there is a number like:
Just click on that number and it will pop-up a window with just that post.

The other way (in many web browsers) it is to do "Print Preview" and just print out that page(s) with the post(s) that you need.

-Bill

Thanks Bill S:Dlarvic
• Solar Expert Posts: 100 ✭✭✭
Re: Is there a table for converting ac usage to dc output?

since he's on grid a current electric bill would be a big help. won't tell how big of a load the inverter would have to start, but it will give a good average daily use. (just divide by 30)
• Solar Expert Posts: 1,071 ✭✭✭✭
Re: Is there a table for converting ac usage to dc output?

He just wants power to run his furnace and refrigerator and a couple lights. Was talking about the water pump also but for all he wants to get he will never run the water pump. He just wants 2 250 amp 12 volt batterys and a honda 2000i to charge the batterys, and a 24 volt inverter. Its a big old farmhouse with a dairy barn that is for sale. Wants to store enough power so he won,t have to go out to start the generator in the middle of the night. Doesn,t want any solar panels and just wants to use the generator to charge batterys. I told him he should forget about batterys, inverters and generators and all the stuff he was going to need and just buy a guardian that selfstarts and he would have his backup. Said he didn,t want outside generator, Eyesore ect. Hope he don,t call me again!!
S:grrlarvic
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Is there a table for converting ac usage to dc output?

maybe if you don't want him to bother you anymore that you should tell him you will be charging him consultation fees. he'll run.
Re: Is there a table for converting ac usage to dc output?

That 2,000 Watt Prosine 2 would be my choice! About as close to automatic as he could get.

Get the 3kW (24 volt bank) if he really wants to crank on some more current.

But that is already getting near \$2,500 it look like... Another \$500, get into the XW 4kW or 4.5kW inverter/charger range... Supports 120/240 VAC without any extra components.

May need a 1/20/240 VAC genset to charge though (not sure).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 246 ✭✭
Re: Is there a table for converting ac usage to dc output?

If he is a farmer, Give him a stupid price, he will never call you again,
On second thoughts,,,,, If he is a farmer, Tell him you expect paying at all, he will never call you again lol:D (works with English farmers)

Have a good one
Tim
• Solar Expert Posts: 1,020 ✭✭✭✭
Re: Is there a table for converting ac usage to dc output?

A friend of mine with a body and paint shop will sometimes give someone an outrageous price just to get them out of the shop...and then they want him to do the job! Dang, should have tripled, not doubled the real price!!

Ralp;h