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RichardW

batteries & inverters

Hi All,
I am after some advice about battery banks & inverters. I have done a search on here & read all the info plus done lots of googling.

I understand how to calculate the needed battery bank amp hour size to give the needed storage / power for the load over a given time. I can calc the inverter size needed for a given load. What i am strugling with is how to calc the real world load for a whole house. i have got the 24 hour comsumption so know the kwh used in 24 hours but to size the inverter I need the peak load plus the start up surges. Apart from buying an expensive whole house meter to snoop & see whats happening is there a simpler way?

I am also strugling with choosing a battery charger that will replace the used daily load in a fixed number of hours from the mains. I know I will need a min of 70 amps per hour over 7 hours so assume that a 100 amp ish one will be needed due to the tapering up at the start of the charge the taper down at the end.

I know my idea below sounds mad but I have done the numbers on it a few times & it seems to be very cost effective in the medium term & long. Even if you needed to replace all the kit every 4 years you would still be on a winner.

my total kwh per 24 hours is 35
my day time kwh per hour are 1.64 (over 17 hours) = 28
my night time kwh per hour are 1 (over 7 hours) = 7

To provide 28kwh over the 17 hours using a 48v inverter would need a battery store capable of giving out 460 amp hours

the charger would then need to replace that amount over 7 hours (or less). the night time 7 hours power would still come from the mains.

I am looking at a 800 amp hour battery (8 x 100 amp hour 12 v wired in 2 banks of 4) pack so that it has plenty in reserve. Will stacking them that way create a 48v 800amp pack?

You are all now asking why?

well 28 kwh per day = 10220kwh per year which costs over 1200.
If i use a charger during the economy 7 period to charge the battery pack (not sure of this bit as charger consumption is only quoted in max draw) it should use less than 10amps (2.3kw) so should over a year use 5876kwh's & cost 225 (plus the existing night time usage but I am paying that now any way).

What I dont get is how can a double conversion of power / voltage create more power out (10220kwh) than in (5876kwh)? I must have made a mistake but cant see it.

Yes it will cost about 2k to use all new kit but if the numbers are right then it would pay for itself in just over 2 years.

Your thoughts please

Oh yes also what items HAVE to have a pure sine wave inverter?

I think microwaves do
pc's are supposedly reported as needing one but most ups's are not pure sine wave.
what eles MUST have one?

Justme
RichardW

Been doing some more digging & the specs for the chargers just seems wrong. 10amps at 230 / 240 volts is 2.3 /2.4kw. Convert that to 48v & you get 50amps not 100amps (even with 100% efficiency). So consumption would be nearer to 20amps at 230/240v ac. Thats does double the running cost but its still a less than 3 year pay back on investment. It still seems to easy. But at least the input kwh are higher than the outputs now & we are relying on the differing price between standard rate & economy 7 rate. I guess its a bit like the electro mountain power station that pumps water up the hill at night so that it can use it to make power during the day. They use 4 units of electricity or each one they make.

Justme
dpack

bigger than stuff i know to work ,can thecat answer the questions ?
vegplot

If you're using 28Kwh per days you'll to feed in 28 * 1.2 * 1.2 to take into account battery and interver/charger losses. So I'm not sure where you get a 23KWh input. Plus you'll also need to add some additional capacity for when you have power outages.

Also, you'll need to consider the optimum capacity for your system. Batteries have a ideal % discharge (depth of discharge - dod) per day othewise you'll reduce battery life considerably. If your batteries can cope with a dod of 20% then you need 5 of them to supply your daily needs. If your daily usage is 28KWh then you should aim for 3-5 times this in battery capacity for maxiumum battery life. However, this depends on the make and type of battery you use. Never use car or lorry batteries by the way they can't take the depth of discharge your costs will soar if you do as their life is minimal for this type of application.

Also, do you know what you peak load will be? Your inverter and cabling should be sized to take this load.

If you want I can do some further research for you, but not before the end of Jan as I'm still writing up my MSc. thesis (Advanced Environmental Energy Studies).

Take a look at www.windandsun.co.uk for further information.

P.S. the duck we had from you at Christmas was delicious.
RichardW

vegplot wrote:
If you're using 28Kwh per days you'll to feed in 28 * 1.2 * 1.2 to take into account battery and interver/charger losses. So I'm not sure where you get a 23KWh input. Plus you'll also need to add some additional capacity for when you have power outages.



Dont see where you have got the 23kwh from? I did say that the charger looks like it would need 2.3kw max draw & be on for upto 7 hours. So you think that I would need to have a charger that will draw 37kwhs per night? If so thats a running cost of 540 which will still be lower than our existing charges but will take longer to pay back.

Not to worried about power outages as we will be on battery for 17 hours per day any way so could do without for one night. Outages here tend to be in the 20 mins to 4 hours type & about 3 times per year. Even so we would not be any worse of than just relying on the mains if it was off for longer.



vegplot wrote:

Also, you'll need to consider the optimum capacity for your system. Batteries have a ideal % discharge (depth of discharge - dod) per day othewise you'll reduce battery life considerably. If your batteries can cope with a dod of 20% then you need 5 of them to supply your daily needs. If your daily usage is 28KWh then you should aim for 3-5 times this in battery capacity for maxiumum battery life. However, this depends on the make and type of battery you use. Never use car or lorry batteries by the way they can't take the depth of discharge your costs will soar if you do as their life is minimal for this type of application.


Yeh I know not to use car type batts. Been looking at traction batts / cells & similar. EEk 3 to 5 times the battery bank size now that would hit the costs big time. Is that the calc for batts in gerneral or for wind / solar charged ones that need a larger reserve?

vegplot wrote:

Also, do you know what you peak load will be? Your inverter and cabling should be sized to take this load.


still trying to work that out. If we did do it some kit would be timed so that it would not be on at the same time to spread the load.

With the extra batts & running costs its starting to look like its not going to be such a good idea (which to be honest was what I expected).

vegplot wrote:

If you want I can do some further research for you, but not before the end of Jan as I'm still writing up my MSc. thesis (Advanced Environmental Energy Studies).

Take a look at www.windandsun.co.uk for further information.

P.S. the duck we had from you at Christmas was delicious.


Glad you liked it.
vegplot

I've dug out some notes which maybe of use (guidance only):

Sealed gel costs around 10p per KWh with a cycle life of 900 cycles to a depth of discgharge of 80%. With lorry batteries, as an example the cost rockets to 234p per kWh and a cycle life of 30 to 80% DOD. Other types fall somewhere in between.

Typical efficiencies of batteries are 75-85%.

The most cost effective way to discharge a battery is only go down to 50% this will yield a battery life of about 5 years. Discharge down to 80% daily and battery life reduces to 2.5 years. This however, depends on the battery technology but manufacturers will be able to give this information.

So if your requirement is for 35KWh daily you will need 75KWh capacity and expect to replace batteries every 5 years. However, we've not taken into account efficiency losses of around 25% so your required capacity starts to look like 35 x 2 x 1.25 =87.5Kwh, 2.5 times your actual capcity needs. You need more than this to cover inverter/charger and cable losses. 3 times is a reasonably safe assumption.

If your drawing your need over 17 hours that only leaves 7 hours to recharge the batteries which means there charging rates are 17/7 greater than their discharge rates - not good. Most batteries don't like this scenario at all, ideally it should be the other way around or at least on parity. Even pumped water strorage system tend to charge over a longer period than discharge.

You may be better served by offsetting your electricity usage using grid connected wind and/or solar PV. You won't get much for your unused electricity (that needs changing) but going by what you are saying then you won't have much spare to sell back to the grid anyway. The sizing for such a system is reasonably simple though.

I like your economy 7 idea though but I don't think it's going to cost effective for your duty cycle. If you were charging over 17 hours and discharging to 50% over 7 hours then I think it may be economically viable but would need to properly cost it. Sorry to be a dampner on this.
RichardW

Dont worry about the dampper. I sort of thought there would be one hidden some where. Actual 17 hour discharge is the 28kwh as the rest would be supplied direct off the E7 but that dont realy change much does it.

Have been looking at wind power (grid tied to keep costs down) for a while but still dont think that the claims are realistic in any way shape or form. Down side with wind is during the summer I cant see it making much power at all unless its a huge system.

Using the 35kwh over 24h a 5kw turbine seems to fit the bill using a 1/3 of makers claims as a more real world output. Having said that a bigger one seems to cost much less per Kw so perhaps that would be better & selling all the extra produced to cover what you use whilst not producing. I think Npower were offering to buy at a reasonable rate (loads less than selling but better than most).

Would be nice to be able to design the house / system from the ground up to reduce usage first but as most of ours is biss use thats not possible.

Justme
vegplot

Wind may not be as unreliable as you think, some manuafactures such as Proven are pretty honest and accurate about the performance of their products. What is less reliable is the wind itself. Town and cities are pooer areas for turbine due to ground interference unless the turbine is on a really tall mast.

Wind turbine need to get into clear air and that means a tall mast and that menas visual intrusion which some don't like. Forget the B&Q types, bolting a wind turbine on the side or roof of a house is asking for poor performance and short life due to buffeting and give wind turbine technology a bad name. They can also damage the house through vibration (caused by wind buffeting).

You're on the Llyn as should have access to reliable wind but I'd advise getting a wind map of your local area. Failing that buy and install an anemometer, again on a tall mast or aways from obstructions, and measure it's performance over 12-24 months. Log the data as you can always sell it later Wink .

Wind and solar PV together make a good pair often one compensating for the other in lean times. However, my favourite is hydro but how may live close to running water.
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