Help with lifepo4 battery mathematics.

Fingers

Hi, I need to buy some lifepo4 batteries, I did think 8x200Ah 12volt was what I needed. But as I have a 48volt system it would be better to have 48volt batteries; so if one goes down I can take it out of the system. So what am I looking for 4x100Ah 48volt?

Is there a on line calculator?

Thanks for your help.

Wheelman55

The math would go like this:

8 batteries x 200 ah = 1600 ah x 12 volts = 19,200 watt hours / 48 volts = 400 ah

That could be 3 Discover 48 volt batteries at 130 amp hours each for a total of 390
ah, or some other combination of Lithium batteries totaling 400 ah or so. 

That’s the math. The experts will want to know how you came up with your “need” for this many amp hours. 

Happy New Year!

Fingers

Ah, thank you.

Looking at my scribbled notes, I think that should have been 4 batteries.

According to my desk top meter, we use around 600watts during the day dropping to around 400watts in the evening. Excluding the washing machine, tumble dryer and dish washer. But with these all on together, the meter reads 4.5kw..

According to my electricity company last January we used 410kwh, so break that down to 14.5kwh per day. Some how I got that to 4x200ah 12volt batteries. I don't know how, I wrote it down but left the maths formulas behind.....where did I put that bit of paper    

We have cloud cover from horizon to horizon during most of December - January, which was probably my thinking (scribbled on paper) to double the battery size.

Thanks for your input.

Wheelman55

If you post what your goals are for your solar system, the experts will help once they get back from the holidays. 

Fingers

Oh, ok. I want to run my house pretty much off grid.

I have 2kw of solar panels, and I have a 48 volt inverter capable of 4.4kw for 30 mins - 5kw for 5mins. But I will mostly only need 600-700watts until laundry time.    

What size batteries is the only question, as it gets dark here in the winter.     

Wheelman55

Are you saying a constant load of 600 to 700 watts 24/7?  And little sun in Dec/Jan?

I believe that you will have more questions as you explore your goal

mike95490

Fingers said:

Oh, ok. I want to run my house pretty much off grid.

    

In December and January, you will be using your generator.  Solar PV panels don't produce much in cloudy weather.

What will generator fuel cost you for 2 months ?   half of what the grid would cost staying connected ?

Fingers

 Solar PV panels don't produce much in cloudy weather.

What will generator fuel cost you for 2 months ?   half of what the grid would cost staying connected ?

Very good point.

I haven't got a generator yet. It's been at t he back of my mind, but something that has taken a back seat to everything else that needs to be done. I haven't looked in to a wind turbine either, but I'd like to get one eventually.  

Fingers

Wheelman55 said:

Are you saying a constant load of 600 to 700 watts 24/7?  And little sun in Dec/Jan?

I believe that you will have more questions as you explore your goal

Yes, for most of the day and les in the evening, then next to nothing when we sleep.

Fingers

Lets say you had a 48volt system, and your buying new lifepo4 batteries; would you keep the volts and amps down and have multiple batteries, & strings, or would you get 48v high amps? Or just get what is cost effective? 

Is lifepo4 so reliable that we do away with strings, and get the bigger batteries? 

Wheelman55

Fingers. List out all of the elements of the solar gear that you have now. Also is your solar array roof mounted?  

You’ll need more than 2kw of panels to charge those batteries, plus solar gear that can work with the lithium batteries. Do you have room for a larger array?

The batteries don’t come cheap. Price out the Discover AES 42-48-6650 batteries to give yourself a reality check. 

BB (Bill) should be back on the forum soon to help you out. 

BB.

I am around... Just not really enough information to make a worthwhile "numbers" post yet.

Fingers, you are on the right track. Measure your loads (both peak power, and kWH per day). And then get on the conservation wagon.

A 5 kWH per day system or ~150 kWH per month is certainly in the range of a very energy efficient home and possible good canidate for off grid solar.

However, 15 kWH per day (410 kWH per month)--That is getting to be a pretty large off grid power system... Not that 410 kWH per month is "horrible" for an average home. In North America, the "average" home uses around 500-1,000 kWH per month (electric heat, A/C, major electric appliances all drive those numbers up).

Looking to reduce your electrical loads (conservation) is a huge first step. For somebody that has never worked on conservation before (LED lighting, energy efficient appliances, using propane/natural gas/etc. for heating/hot water/drying clothes, lots of insulation, weather stripping, and simply turning off unused loads like desktop computers and entertainment systems when not in use)--Seeing a monthly power bill cut by 1/2 is certainly not unusual.

This is my first go-to recommendation. Using a Kill-a-Watt type meter, and possibly a whole home energy meter can really help here:

https://www.amazon.co.uk/s?k=kill-a-watt+meter&crid=2HEG6QAK191RM&sprefix=kill-a-watt+meter,aps,271&ref=nb_sb_noss

You will usually find out that your 3,000 Watt electric kettle that runs 10 minutes per day or your 1,200 Watt microwave at 20 minutes per day is not always your "energy hog"... It can be that desktop computer/laser printer/networking gear/old monitor that takes ~300 Watts 12-24 hours per day... For example:

• Kettle: 3,000 Watts * 1/6 hour (10 minutes) = 500 WH per day = 0.5 kWH per day
• Desktop computer: 300 Watts * 24 hours per day = 7,300 WH per day = 7.3 kWH per day

Replace that with a cheap Chromebook/laptop computer at 10 Watts and run that 5 hours per day:

• Cheap Laptop: 10 Watts * 5 hours per day = 50 WH = 0.05 kWH per day

I live live in California and we have "crazy" energy policies that are pushing our power costs into the range of USD $0.20 to $0.40 per kWH... Don't know what your UK power costs are, but the whole contenenet seems to be going nuts with "renewable energy" which is seeing similar costs, and now in deep winter, possible seeing costs in the Euro 1.00 to 3.00 per kWH (if the articles are correct).

We had these crazy laws forcing utility companies to sell power at $0.xx per kWH and by it on the "open market" at $x.xx per kWH (and in Texas during the last ice storms and some folks on "open market power plans" paying $x.00 per kWH ($9 per kWH wholesale in some cases)... We end up with bankrupt customers and even utilities (been there, done that in California 20 years ago with "deregulation").

A round of applause for your taking control of your energy usage. But don't get ahead of yourself buying solar/off grid hardware just yet. Do more work on paper first (faster, easier, and a whole lot cheaper to have paper designs vs a bunch of hardware in the garage and on the roof).

Just to give you a very rough idea on planning for costs vs conservation... Say your "all in" solar power costs are roughly $2.00 per kWH (in the US, $1.00 to $2.00+ per kWH is the norm for off grid solar in sunny regions). What would you do different (replace old appliances, insulation, "do without some appliances" during emergency, etc.) if your power bill was 10x what it was last year?

And what is it you are looking for from off grid solar... If it is to save money--More than likely utility power will be cheaper (on average) than you could ever generate yourself.

Grid Tied solar (utility interactive solar AC inverters, not battery banks)--Yes, that can save money. Getting below $0.15 per kWH for your costs and no maintenance for 10-20 years--Really nice. What your utility will do (does it allow GT Solar? What is the rate plan--Will you save money)? GT Solar is subsidized from other rate payers and government taxes--And in some places, GT Solar has been made illegal, or "too expensive" to be economically viable.

If you are looking for Off Grid/Hybrid Solar to carry you through power failures and economic "hiccups" (crazy power prices for a few weeks or months of bad weather)--It can make sense. But usually only in the case of minimally sized system--Run your lights, computer, TV/Radio, cell phone charging, water pumping--And not things like electric heating, hot water, and other heavy energy users that you have to go without during power emergencies (closer to camping--and just surviving).

Being in the UK (guessing--The same bad weather that knocks out utility scale solar energy, is the same weather that could knockout your home's solar harvest too. An example of solar harvest for Plymouth UK:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Plymouth
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 40° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
1.35	2.22	3.14	4.49	5.01	4.85
Jul	Aug	Sep	Oct	Nov	Dec
4.85	4.64	3.87	2.52	1.73	1.12

3 hours or more hours of sun per day--Pretty useful solar... Down towards 1 hour per day of sun (this is hours of "noon time equivalent" sun)--Solar is not very good (just when you need the most power during cold/stormy winters).

Lets say you can "exist" with 4.5 kWH per day (LED lights, Fridge, clothes washer, some LED TV, laptop, and cell phone charging, etc.). In December using Lead Acid batteries (Li Ion are better, and you may get way with 1/2 the AH rating of an FLA bank--With limitations):

• 4,500 WH per day * 1/0.85 AC inverter eff * 1/48 volt battery bus * 2 days storage * 1/0.50 max discharge = 441 AH @ 48 volt battery bank

Note: Power=Voltage*Current... 440 AH @ 48 volts = 880 AH @ 24 volts -- Same amount of energy storage. This is roughly 8x 6 volt @ 220 AH "golf cart" batteries in series * 2 parallel strings for 16x GC battery bank (FLA).

Two solar panel calculations... One based on rate of charge: 5% minimum (sunny weather/weekend/emergency backup system). 10-13%+ rate of charge for full time off grid.

Second array calculations based on energy used per day and daily sun (location).

Battery charging:

• 441 AH * 58 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 1,661 Watt array minimum
• 441 AH * 58 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 3,322 Watt array nominal
• 441 AH * 58 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 4,318 Watt array "typical" cost effective maximum

And based on the amount of sun per day and loads:

• 4,500 WH per day * 1/0.52 off grid AC system eff * 1/1.12 hours of sun per day (December) = 7,727 Watt array "December break even" array

And you should only plan on using 50% to 65% of your predicted daily harvest for base loads (those loads you need 24x7 such as LED lighting and refrigerator):

• 4,500 WH per day December (7,7727 Watt array) * 0.50 base load fudge factor = 2,250 WH per day for December Base loads

Of course, in the other months with >3 hours of sun per day--You will have lots of energy harvest for other loads (assuming 7,727 Watt array).

Or--You can look at winter being backed up with a genset... Of course you need to have a safe place for fuel storage and can run a genset (smoke, smell, noise). The reality is Solar Power needs Sun (clear sky, no shade from buildings/trees/etc.). Genset can make sense for emergency back up power (a few days to few weeks).... Gensets (more or less) only cost you fuel when running--Other 11+ months of the year, don't cost you anything.

For a generator--It is pretty easy to figure out fuel needs (daily needs, storage, etc.). Does depend on what is availabe/practical for your place (petrol, natural gas, diesel, propane)... Suggest that you get as small as practical genset for your needs. A large genset can be noisy and waste lots of fuel. The modern inverter-generators can be a very nice solution (and some gensets are multi-fuel--petrol/propane/natural gas).

I will do an example calculation here. Assume a Honda eu2200i inverter-generator (eu220i for "Euro" version?). I use the older version for my home backup power: The basics:
https://powerequipment.honda.com/generators/models/EU2200i

• 0.95 gallon fuel tank
• 1,800 Watt running max load
• Per tank of petrol: 3.2hr @ rated load 8.1 hrs @ 1/4 load

Some math (will do in US units):

• 1,800 Watts * 1/4 load (average home loading) = 450 Watts average loads
• 450 Watts * 8.1 hours per tank of fuel = 3,645 WH per tank
• 3,645 WH per tank * 1/0.95 gallons per tank = 3,837 WH per gallon = 3.837 kWH per gallon
• $8.00 per gallon petrol / 3.837 kWH per gallon =  $2.08 per kWH
• 24 hours per day * 1/8.1 hours per tank * 0.95 gallons per tank = 2.81 gallons of petrol per day (average loading)
• 2.81 gpd petrol * $8.00 per gallon = $22.48 per day "genset" average costs
• 5 days of backup power * 2.81 gpd = 14.5 gallons per 5 day outages

So--For emergency power, it is something like 4x the cost of utility power (using my SWAGs)? To run a refrigerator, you can probalby get away with running just 12 hours per day (say 6 hours in morning and 6 hours in evening) and possibly cut your fuel costs by 1/2 (or extend stored fuel by 2x).

Other notes:

• For petrol and diesel, use fuel preservative. Change at least once per year (I put in my car)
• Store fuel safely. Rotate fuel for aging (propane is great--If you have on site)
• Run genset outside (carbon monoxides, fire).
• Install transfer switch/using extension cords as needed, safely
• Standard "Gensets" are fuel efficient at >~50% electrical loading. Below 50%, more or less same fuel flow (gallons per hour)--So at 25% loading, standard genset closer to ~2x more fuel usage vs  inverter genset at 25% loading for inverter-genset...
• Standard gensets are (in general) quite noisy. Not great next to your home--May be unacceptable for neighbors.
• I used Honda eu2x00i genset because very easy to make fuel siphon from 5 gallon gas can (has internal fuel pump and vented/sealed fuel cap):

https://www.amazon.com/s?k=honda+eu2200i+extended+run+kit&sprefix=honda+eu2200i+exten,aps,251

I, personally, chose the Honda because inverter-generator (very fuel efficient compared to standard gensets with small home loads--And few inverter-generators brands 12+ years ago). There are lots more inverter-generator choices out there these days (not all can use "vent siphon for external tank--Other options such as adding fuel pump kit, etc.).

Do the paper designs first. Li Ion vs Lead Acid--"Fix" your home loads first (conservation), Measure the loads you need for "emergencies" and how many days of backup (genset). Then adjust for FLA vs Li Ion power needs, and cost the various options (along with any issues).

Li Ion batteries, cannot charge below ~40F unless they are insulated or have heating. In "poor sun" regions, genset usually needed to help solar during winter.

Make chances... 3 kWatt tea kettle--How about a camp stove and standard kettle?

On your backup power... I have natural gas at my home--But I was planning for Earthquakes--And assumed that Natural Gas service may be lost during quakes (and no fuel stations have electrical backup power in my area). Chose petrol for my backup fuel--Plus our car is petrol, so easy to recycle once a year, and nice backup for car to evacuate if needed.

Lots of questions. Need to look at what your needs are (energy is a highly personal set of choices--What works for me may not work for you).

Your thoughts?

-Bill

Fingers

Hello Bill. First of all I'd like to say thank you for taking the time to write so much for me.

I have to be quick as it's almost time for my son to use my PC.

As a kid, my Dad always moaned at me for leaving lights on etc, so now it just comes naturally to turn anything of that is no longer needed.

My kit:

Solar panels: 2kw due south 40degs, on garage roof. A further 1 or 2kw possible due east 90degs for the morning sun (only in the thinking stage so far).

Inverter: SMA Sunny Island S14.4M-11 inverter charger 48v. My back up inverter is a 6kw (3 maybe 4 kw continuous) Chinese not so power hungry type.

Batteries: Lifepo4. I'm buying them this week. My idea was to have a large bank as to have some juice in reserve.

MPPT controller 50amp.

We long ago changed every bulb in the house to led. We had extra insulation in our loft, and it's warmer up stairs, it will be better when I tackle the double glazing seals, and replace the back door.

I have a Kill-a-watt type meter and have most of the results, none of them are horrendous as all electrical items are pretty good on power consumption. Everything on in the house= 4.5kw, but it's never that bad, as I only use the washing machine, dishwasher and tumble dryer, one at a time. Not all on at once.

I have got used to doing this with the grid tie that I installed three, or four years ago. I'm certain it has paid for it's self by now; I'lI take a look at the meter I put in and add it up tomorrow

We have a gas cooker (and kettle with a whistle), and gas central heating. We run the hot water to the cold fill of the washing machine so it doesn't heat the water and comes out warm; which helps with evaporation

I'm desperately trying to get my log burning stove installed, I having a hard time of that, but I will do it by hook or by crook.

EU citizens have seen their electric bills go up by six times in six months. My bill has doubled in one month and due to go up again soon.

I haven't declared my off grid solar to the electric company, for a very good reason; take a look what happened in Spain. Everyone with solar will now pay solar tax. Most folks took them down. I'm not risking all my hard work for it all to come to nought in the future by some political villains.    

Darn I have to go.

Fingers.

BB.

Sounds like you are off to a good start... I missed that 4.5 kWatts was "peak" power usage. I read it as 4.5 kWH per day for your "monitored" loads as that is a "reasonable" starting point for full off grid energy efficient home with fridge and other loads (lights, laptop, washing machine, etc.) (using natural gas, wood, whatever for cooking/hot water/heating).

In decades past--The whole power market (wholesale and essencially retail/home power) was based on long term contracts between the "buyer" (utility) and supplier (generally fuel for utility owned generators, distribution of natural gas and some storage, etc.). That made for stable long term pricing (and predictable utility returns--I.e., the investment for widows and orphans).

Today, many states (each state usually has a public utility commission to set "rules and regulations" for utilities--Power, phone, natural gas, etc.). And all of that has been "managed" to go away from coal, and other "carbon based" fuels to renewables (burning wood chips from North East USA to British wood burning power plants--So much better than burning coal/etc.  ).

And moving to "real time" pricing (some locations in US have daily or hourly changes in electricity pricing). And that is what "killed" customers in Texas last year. The utilities offered real time pricing that offered very low rates--Most of the time--And there was no "cap" for pricing. When power went from $0.10 per kWH to as high as $9.00 per kWH--People that have been living with "cheap power", got nailed when power was failing (frozen turbines, power plants, and natural gas pumps--That was pure 50-100 year cold snap and "nobody" was prepared--Plus "renewable wind" was required to power gas compressors--And there was no wind--Which happens). People did not know the pricing (initially) and they needed power to heat their homes or have broken pipes (lots of homes flooded)... Terrible.

Where am I heading with this? More or less, your battery bank can, at best, store 1-3 days of energy--Not nearly enough to "ride through" an Electrical and/or Natural Gas price spike of a week to several months.

And that is where "our betters" are forcing up. Into "Spot Markets" for our energy (unstable pricing out of customer's control)--And taking away our ability to the5 ability to do long term pricing contracts (stable pricing).

If you go "off grid"--The battery bank is used every day and it and the solar panels are sized for your daily power needs... And your genset/utility power is there for "bad days of sun" and winter.

The battery bank itself--Does not store that much energy (compared to petrol as an example). The battery bank is there for "daily use" (charge daytime, use power at night). But as a "longer term storage" device to save money--Not so much.

Say you have a 48 volts @ 200 AH battery bank (9,600 AH @ 48 volt). Lets use Li Ion and assume you can discharge from 90% to 20% State of Charge. Or ~70% of capacity:

• 48 volts * 200 AH * 0.70 bank useful capacity * 0.85 AC Inverter eff = 6,720 Watt*Hours

Compared to an inverter-genset and petrol:

• 6,720 WH of battery power / 3,873 WH per gallon (average usage) = 1.74 gallons of petrol equivalent.

In the end... You will be (hopefully) saving money based daily energy usage... Stored fuel (propane, petrol, diesel, even fuel oil or coal) for "bad winters" is looking better all the time (buy fuel oil, etc. during spring/summer when fuel is cheap, and have enough to ride through winter.

In terms of heating--1 gallon of petrol is ~36 kWH (36,000 WH) of heat--And you can see that 6.72 kWH of electricity is not much in the way of heating fuel per gallon (~$8 per gallon in UK?).

• 1 gallon of petrol = 36 kWH of "heat"
• 1 gallon of petrol = 3.837 kWH of "inverter-genset electricity"
• 7.4 kWH LiFePO4 battery (48 volt) ~ USD $6,000 (Li Ion prices are all over the map--Just an example)

A 2,000 Watt array in Plymouth UK in December==Average (best case) harvest:

• 2,000 Watts * 0.61 end to end Li Ion AC inverter system eff * 1.12 hours of Sun (December average) = 1,366 WH = 1.366 kWH per average December day

What do you current pay (or expect to pay) for electricity (per kWH) these days?

I have time of use charging--And roughly $0.30 per kWH (winter afternoon/evenings):

• 1.366 kWH * $0.30 per kWH = $0.41 per day from a 2,000 Watt off grid solar array December Plymouth UK

I am not trying to tell you that Off Grid is a good or bad idea for your needs... Just showing you how I suggest (at least one way) of putting this is terms of how much this energy such a system will generate for you--Of course this is December (your bill should be comming soon?) and deep in winter is the worst for solar for anyone that is in the farther northern (or southern) hemisphere where there just is not much sun.

And there is always risk... Risk that the battery bank is damaged. Fuel and/or equipment is stolen. Things wear out (new electronics every 10+ years, etc.).

Off grid solar is usually not an "investment" if there is utility power available. For backup power, many times, a genset+fuel cache is a cost effective alternative (with alternate usage--Fuel for heating/cooking/hot water, backup fuel for a vehicle, etc.).

I don't want to see anyone drain their savings and treat solar like an "investment" with a good return.... In sunny areas, GT solar (no batteries) can be--Assuming utility rate plans are favorable (politics--Changing rate plans).

Anyway--Hope the New Year works out well for you and your family, Fingers.

-Bill