System size for 1.3kWh/day

gactrx

Hi, this is a bit backwards as I have 1730w panel system and a 24v inverter. I'm getting a 230v fridge and freezer which use 477kWh/year in total. My batteries are 440ah 24volt bank but I'm getting new batteries soon. What would be a good size 24v battery bank for this. The rest of the draw off will be 24v dc lights, water pump which goes every second day for 20 min (4.5a) and radio.The invertor is a 2kw Outback and Midnite 150 controller.
What would be the ah draw from the batteries for the fridge and freezer? (Just calculated this at 6AH. Would this be correct?)
Thanks,

zoneblue

Are you able tomake a spreadsheet totalling your loads in Wh/day? Thats the best palce to start.

petertearai

Hi very similar to my system . Although 12 volts .... Mine works very well . My fridge freezer is a samsung inverter one from memory 317 kw per year.

BB.

Thank

gactrx wrote: »

Hi, this is a bit backwards as I have 1730w panel system and a 24v inverter. I'm getting a 230v fridge and freezer which use 477kWh/year in total. My batteries are 440ah 24volt bank but I'm getting new batteries soon. What would be a good size 24v battery bank for this. The rest of the draw off will be 24v dc lights, water pump which goes every second day for 20 min (4.5a) and radio.The inverter is a 2kw Outback and Midnite 150 controller.
What would be the ah draw from the batteries for the fridge and freezer? (Just calculated this at 6AH. Would this be correct?)

I am always worried when I see kWH/WH loads mixed in with with Amp and Amp*Hour numbers...

Watts and Watt*Hours are "complete" units that fully describe power/energy usage... Amps and Amp*Hours are missing the "voltage" part of the energy budget/equation.

For your refrigerator--Say it averages 477 kWH per year... That is:

• 477,000 Watt*Hours per year * 1/365 days per year = 1,307 Watt*Hours per day
• 1,307 WH per day * 1/0.85 AC inverter eff * 1/24 volt DC bus = 64 AH @ 24 volts per day

Assuming that is 477 kWH per year for a single refrigerator+freezer... If it is a separate Refer + Freezer -- Then the kWH per year number may be double.

So--[I messed up calculation--The 6 AH per day is incorrect. But, it really depends if you are asking the AH question at 240 VAC or 24 VDC--For our battery/off grid systems, we generally only work with AH at the Battery Bus/Battery level voltages. -BB]

To calculate the overall system efficiency and how that would affect your solar array (battery losses, solar panel derating, charge controller losses, and AC inverter efficiency, and hours of sun per day):

• 1,307 WH per day * 1/0.52 end to end AC off grid eff * 1/3 hours of sun per day (typical winter day) = 838 Watt array minimum just to run the fridge

For hours of sun per day (including location, panel tilt, and by month), take a look here:

http://solarelectricityhandbook.com/...rradiance.html

And, you also should size the array to the size of your battery bank... 5% to 13% rate of charge, and suggest >10% rate of charge if full time off grid (makes it easier to manage system and loads, without driving yourself nuts):

• 440 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 829 Watt array minimum
• 440 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,657 Watt array nominal
• 440 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,154 Watt array "cost effective" maximum

Looking at your present system (note that sun is highly variable--If you have lots of marine layer/cloudy weather in your area, solar power will be badly affected):

• 1,730 Watt array * 0.52 system eff * 3 hours of winter sun = 2,699 Watt*Hours typical winter day
• 1,730 Watt array * 0.52 system eff * 4.5 hours of more summery sun = 4,048 Watt*Hours typical summery day

And from the battery bank (your typical no sun/over night loads)

• 440 AH * 24 volts * 0.85 AC inverter eff * 1/2 days of storage * 0.50 max battery discharge (for longer life) = 2,244 WH of stored power per day

So, overall your system seems to be pretty well balanced (depending on your other loads).

-Bill

BB.

Thank you Animatt--I have fixed the calculator crash and burn in the above post (64 AH @ 24 VDC, not 6 AH).

-Bill

gactrx

Hi, thanks for you answers which will take a while to digest. I'll make up a spreadsheet although all I'll be running on the inverter is the fridge and freezer and occasional kitchen appliance and hand tool. I have a 5kVA generator for bigger tools and vacuum cleaner.
The freezer I'm getting is the Elcold 31XLE 255kWh/year and 86w operational and the Gram KS3286 222kWh/year and 45w running. Both are 230volt.
I have 12 volt lighting and radio at the moment. I'll either change these to 24vdc or 230v and rin off the inverter as well but prefer to keep them DC.
Thanks,

BB.

By the way, the 4.5 hours per day was intetended to be more "summery" weather for your region (guessing). I have updated the post again.

-Bill

gactrx

I have worked out my sunlight per month. My six panels are about 1 meter by 2 meter and although are mono's don't have the little circular gap at each cell corner.
The table shows Cumulative kWh/m2 each month. I have the panels fixed at my latitude of 45 deg South. It's not an overly cloudy area.
Cheers,


Jan
5.5


Feb
4.99


Mar
4.52


Apr
3.4


May
2.56


Jun
2.38


Jul
2.73


Aug
3.2


Sep
4.37


Oct
4.97


Nov
5.33


Dec
5.32

zoneblue

gactrx wrote: »

The freezer I'm getting is the Elcold 31XLE 255kWh/year and 86w operational and the Gram KS3286 222kWh/year and 45w running.

Those are nice models both, and nice specs to go with. Refrigeration isnt a bad thing to pin a design around, as its a relentless load. You can have a suprising number of (even high power) additional intermittent items and in teh scheme of things they make little difference. Same goes for any 24/7 load. Hint dont forget the inverter tare, thats 20W right there...

zoneblue

BTW Surprising number of kiwis on this forum!

gactrx

zoneblue wrote: »

BTW Surprising number of kiwis on this forum!

High quality sunlight in this part of the world I guess.

gactrx

Hi Bill, still getting my head around your calculations. Maths isn’t my strong point.
I see I will be using 64AH from my battery bank and to supply my 477kWhr/year I need a 838w array minimum.
My array should give between 2.6kWh and 4kWh per day. So this is plenty big enough.
With the batteries, and if I don’t want to discharge them more than 50% I have 2.2kWh per day.
Therefore when supplying 1.3kWh/day the batteries would last just under two days.
I think draining them to 50% would be a bit much I’m told. It sounds like I’ll need more battery capacity.
Would that be correct?
Thanks,

westbranch

gactrx wrote: »

I think draining them to 50% would be a bit much

Yes, you do not want to plan that DoD as a regular event. Don't forget about opportunity loads after you are in absorb, any load from the fridge will be supplied by the panels, not necessarily from the battery if your PV is ample enough. I reach float by noon on a summers day right now and 'waste' a lot of potential as I do not have a regular use for ~ a derated (4x140W) 400W after that...

BB.

gactrx wrote: »

Hi Bill, still getting my head around your calculations. Maths isn’t my strong point.

That is OK--Math, apparently, is not my strength either. :p

I see I will be using 64AH from my battery bank and to supply my 477kWhr/year I need a 838w array minimum.
My array should give between 2.6kWh and 4kWh per day. So this is plenty big enough.

Remember your battery "wants" around 5% to 13% rate of charge... And some vendors recommend 10% rate of charge.

And, your refrigerator will be running during the day too... That means less battery discharge during the 6-8 hours of sun (unless a very dark and stormy day). Also, that means that some charging current is consumed by the daytime loads--Which also reducing available charging current to the battery bank.

Your 1,730 Watt array will be put to good use (and reduce generator run time).

With the batteries, and if I don’t want to discharge them more than 50% I have 2.2kWh per day.
Therefore when supplying 1.3kWh/day the batteries would last just under two days.

BB. wrote: »

440 AH * 24 volts * 0.85 AC inverter eff * 1/2 days of storage * 0.50 max battery discharge (for longer life) = 2,244 WH of stored power per day

No--The 2.2 kWH per day of 230 VAC of "useful" AC energy is two days of "no sun" and taking the battery to 50% state of charge--While that is a fairly heavy discharge--Good quality deep cycle lead acid batteries should be up to that job.

Use the batteries... As long as you treat them reasonably well, they should provide you good service.

I think draining them to 50% would be a bit much I’m told. It sounds like I’ll need more battery capacity.

Realistically, most of the time you will probably be recharging the battery bank to >~90% state of charge most days... If you have a stretch of bad weather expected--Certainly you can reduce the optional loads--And/or start up a back genset (if that makes economic sense for you).

More or less, a 2x larger battery bank may last you slightly more than 2x longer... So the out of pocket expenses end up being the same--The 2x longer battery life (larger battery bank) will cost you 2x more to replace--A wash.

And, there is always the risk of something going wrong... New users frequently "murder" their first battery bank or two... And "oh heck" things too (kids/guests run battery bank dead, water pump stays on and kills battery bank, bad battery/batch of batteries, etc.). You have the choice. A smaller battery bank that is cheaper to replace--And you replace the batteries more often. Or a larger battery bank that lasts you many years--But you have a larger cost at replacement time, or to replace if you kill them accidentally.

This is a choice that only you can make... But if you already have the bank and system as described--I certainly would run with the smaller bank for the next 2-5 years and see how they work out for you. The next bank could always be bigger--If you wish. Also, remember a 2x larger battery bank would put you back at ~5% minimum rate of charge for your present solar array--You might want to add a ~2x larger solar panel array too if you go with a 2x larger battery bank (I tend to recommend keeping the array on the large size vs battery bank capacity--I think it makes it easier to mange the battery bank--You don't have to watch the batteries like a hawk nearly as much).

Depending on your weather--You might be better off even adding more solar panels instead of a larger battery bank--I would certainly entertain that thought at this point (given your present loads). It also gives you more daytime power--Perhaps irrigation/water pumping? And reduction in genset/fuel usage? The array should have a 20+ year life--Batteries are typically in the 3-8 year life range. More batteries are not going to last "forever".

Of course a 2x larger battery bank and array--Gives you a lot of extra power. Perhaps you can think of some more things to plug in.

-Bill

gactrx

Hi, thanks for your reply. Again, I have copied it into a document for bedtime reading. I think it is definitely me worth considering the small battery bank idea.
I have finished my usage information. I decided to keep three separate systems as I have three different types of solar panels. I'll have a main 24v bank to run the inverter and then a couple of batteries for the pump and a couple for lighting and stereo. I'll use my existing bank for the lights and pump and purchase a new set of batteries for the 24v inverter supply. I was incorrect with my panel wattage. The Midenite calculator says 1920w output.
Any ideas on the 24v battery bank I should get would be much appreciated. We would of course drop the TV watching in favor of the fridge and freezer if needed.
Cheers,

6 x JA Solar 320w panels (1920w) and Outback FX2024 Inverter 230v
Fridge 45w 255kWh/year
Freezer 86w 222kWh/year
TV 140w 150kWh/year
Stick blender 70w 36kWh/Year
Sewing machine 100w 6kWh/year
Hair straightener 70w 1kWh/year
Total 867kWh/year

3 x Helios H750 75w panels and 12 volt batteries
Car stereo 12v
9 LED house lights 12v
6 garage LED lights 12v

2 x Solarex MSX-83w panels and batteries
12v water pump 9A- 20 minutes per day.

BB.

gactrx wrote: »

6 x JA Solar 320w panels (1920w) and Outback FX2024 Inverter 230v
Fridge 45w 255kWh/year
Freezer 86w 222kWh/year
TV 140w 150kWh/year
Stick blender 70w 36kWh/Year
Sewing machine 100w 6kWh/year
Hair straightener 70w 1kWh/year
Total 867kWh/year

I really like WH per day--Doing a yearly average vs how much usage can change seasonally might trip you up (i.e., refrigeration usage might be higher in summer/auto--warmer weather, freezing harvest. but more lighting/fan circulating air around wood stove, etc.). But if I take a simple adding your 1 year usage up and divide by 365 days per year, I get:

• 670,000 WH per year / 365 = 1,836 WH per day

Your refrigerator/freezer usage sounds low (in the US, it is very difficult to find units that are under ~300 kWH per year rating). And, your TV seems high--A new LED screen type TV can use 1/2 the power (depending on lots of variables). No Satellite box, washing machine, laptop computer, cell phone, two way radio needs?

1.8 kWH per day is a pretty small load for a full time home+refrigerator+freezer. I would be guessing 2x more power (3.3 kWH per day or more)--But energy usage is a highly personal set of choices.

Anyway--The math. First sizing the battery bank on daily usage:

• 1,836 WH per day * 1/0.85 AC inverter eff * 1/24 volt batter bank * 2 days of storage * 1/0.50 maximum discharge = 360 AH @ 24 volt battery bank

Note, a 369 AH @ 24 volts would suggest a maximum inverter size of ~1,800 Watts (rule of thumb of ~500 Watts of AC inverter--And solar panels--Per 100 AH of 24 volt battery bank). Since you are already looking at a 2 kWatt AC inverter and 1,920 Watts of panels, I would suggest starting with a ~400 AH battery bank (4x 6 volt @ ~200 AH golf cart batteries in series * 2 parallel string = 8 GC batteries total).

I don't know anything about your fridge and freezer--But a 2kWatt AC inverter driving a pair of refrigeration compressors would probably be the absolute minimum I would suggest--If the two can start at the same time (random timing being what it is). In the US, two compressors starting at the same time would usually be a suggestion of ~2.4 to 3.0 kWatt of AC inverter. Standard compressors need a lot of starting current. If these are "Inverter" type compressors (or other technology), you may not need such a large AC inverter.

Array sizing based on battery bank capacity (5% to 13%, with 10% or greater suggested for full time off grid loading), assuming 400 AH battery bank:

• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 753 Watt array minimum
• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,506 Watt array nominal
• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,958 Watt array "typical cost effective" maximum

So your 1,920 Watt array would easily support a 400 AH or larger 24 volt battery bank for full time off grid use.

If you assume 1,836 Watt*Hour loads as "average" per day, then your array would need this number of hours of sun per day:

• 1,836 WH per day * 1/0.52 typical AC off grid system eff * 1/1,920 Watt array = 1.84 hours of sun minimum to "meet" daily load requirement.

And well above your June 2.38 hours (long term average) minimum sunlight for your area.

Based on what you have told me--This is what I see about your system.

For below, you have not listed battery bank or daily loads--So I cannot really help much here... I would suggest that, perhaps long term, you look at using your main 24 volt system to supply these other loads. they are not great and (at least in the US), AC powered LED lights are cheaper than 12 VDC LED lights and easier to wire up with 230 VAC wiring vs the heavier gauge typically needed to wire up 12 VDC low voltage wiring (lots more current and very little voltage drop allowed).

Also, Car Stereos, may not be the most energy efficient. A modern AC radio may use less power. Your AC inverters are going to be on 24x7 anyway for the refrigeration, so the AC power is "free" at this point for other small loads.

One thing to remember, when you go to 24x7 for your AC inverter, its tare losses (energy used just being "turned on" when no loads are present) can become a significant load for smaller systems. For example your inverter probably draws around 20 Watts:

• 20 Watts * 24 hours per day = 480 Watt*Hours per day just for the inverter to be "on".

Now your daily loading (worst case, because the 0.85 in theory does contain some of the Tare Losses) would be:

• 1,836 WH daily loads + 480 WH inverter tare = 2,316 Watt*Hour per day loads
• 2,316 WH per day * 1/0.52 system eff * 1/1,920 Watt array = 2.32 Hours minimum of sun per day

So, just that addition of a 20 watt 24x7 load pushes your minimum solar array to virtually meet your 2.38 Hour of sun June isolation... Not so much "head room" now and would increase the chances of needing to run the generator more during winter (all things being equal, which they are not).

The 0.52 also includes some tare losses--So adding 480 WH of tare losses is a bit of double counting--But we are getting to splitting hairs now. More or less, in solar power calculations, anything within 10% is pretty much "identical numbers" and not worth pulling hair out over to model refrigerator run time vs idle time for the inverter.

Quick check on battery sizing:

• 2,316 WH per day * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 454 AH @ 24 volt battery bank

Many golf cart batteries are around 220 AH @ 6 volt--So 440 AH is certainly "close enough" to 454 AH that I would not worry about it (again, within 10% is pretty much "dead on" for solar math).

But you can see, even a "small" 20 watt * 24 hour per day load can really trash your assumptions.

For below, need loads and battery size at a minimum to say much here.

3 x Helios H750 75w panels and 12 volt batteries
Car stereo 12v
9 LED house lights 12v
6 garage LED lights 12v

2 x Solarex MSX-83w panels and batteries
12v water pump 9A- 20 minutes per day.

-Bill

BB.

I should add, you probably want to get a hub and a Mate (1, 2 or 3--I am not sure, Outback has made this part complicated) to program your AC inverter set points. What comes out of the box is usually not what you want.

-Bill

gactrx

Hi Bill, thanks for your very useful information. I can see how you arrive at these figures and can base my system around them. 400AH looks a very realistic size and I'll try for a bit more that that if I can.
Below are some ideas on problems and things you have mentioned.
I will also have a 1kVA wind generator which I'm planning to use to heat water using a 24volt element. We are in a windy area on a hill so this may be a backup in winter when our fire is heating the water.
I do have an Outback Mate. Don't think I need a hub as I have only one thing to connect too.
Cheers,

In regard to the Outback inverter for the very odd time both start, I am hoping the surge capacity will handle this:
AC Overload Capability
Surge 5750 VA
5 Seconds 4800 VA
30 Minutes 3100 VA

You are correct in the 20 watt inverter usage. Hopefully the inverter will be in search mode a lot of the time where it uses 6 watts.
Idle Power Full 20 Watts
Search 6 Watts


These fridge and freezer figures seem quite detailed so should be ok to go by.I'll be isolating the fridge radiator from the room and have a flow of outside air from under the house flowing though it.
Fridge
Energy Consumption @ 32ºC (kWh / YEAR) 222
Energy Consumption @ 32ºC (kWh / DAY) 0.61
Energy Consumption @ 25ºC (kWh / year) 142
Energy Consumption @ 18ºC (kWh / year) 132
Operating Consumption (Watts) 45

Freezer
Energy Consumption @ 32ºC (kWh / YEAR) 255
Energy Consumption @ 32ºC (kWh / DAY) 0.70
Energy Consumption @ 25ºC (kWh / year) 234
Operating Consumption (Watts) 86
Temperature rise to -9deg 64 hours (interesting info in case of no sun. Keep freezer closed)

BB.

If these refers have automatic/self defrost--Search mode inverter operation can confuse the timers for the defrost heaters.

For mechanical timers, if the fridge runs on a 50% duty cycle, the 12 hour cycle time may turn into a 24 hour cycle time. Also, defrost heaters may cycle on/off -- So it is possible that the actual defrost cycle may not work as expected during Search Mode.

For electronic timers (more expensive refrigerators), the timers tend to rest to Defrost every time power is cycled--So the refer may stay "stuck" in defrost mode.

If you have standard (manual defrost), then running the inverter on search mode should be fine.

Not sure if you will have problems or not--But just something to think about.

-Bill

gactrx

Thanks for the info, I'll just check with the suppliers on that. I don't receive the fridge and freezer until the end of August when the shipment arrives.I see the fridge is auto-defrost.
Cheers,

gactrx

Looks like I'm in luck. Reply from supplier:

Both the fridge and freezer have standard thermostats so don’t draw any current between cycles.
With this type of thermostat there is no electronics that requires power to operate.
You wont have any problems.

lkruper

BB. wrote: »

Thank
I am always worried when I see kWH/WH loads mixed in with with Amp and Amp*Hour numbers...

Watts and Watt*Hours are "complete" units that fully describe power/energy usage... Amps and Amp*Hours are missing the "voltage" part of the energy budget/equation.

For your refrigerator--Say it averages 477 kWH per year... That is:

• 477,000 Watt*Hours per year * 1/365 days per year = 1,307 Watt*Hours per day
• 1,307 WH per day * 1/0.85 AC inverter eff * 1/24 volt DC bus = 64 AH @ 24 volts per day

Assuming that is 477 kWH per year for a single refrigerator+freezer... If it is a separate Refer + Freezer -- Then the kWH per year number may be double.

So--[I messed up calculation--The 6 AH per day is incorrect. But, it really depends if you are asking the AH question at 240 VAC or 24 VDC--For our battery/off grid systems, we generally only work with AH at the Battery Bus/Battery level voltages. -BB]

To calculate the overall system efficiency and how that would affect your solar array (battery losses, solar panel derating, charge controller losses, and AC inverter efficiency, and hours of sun per day):

• 1,307 WH per day * 1/0.52 end to end AC off grid eff * 1/3 hours of sun per day (typical winter day) = 838 Watt array minimum just to run the fridge

For hours of sun per day (including location, panel tilt, and by month), take a look here:

http://solarelectricityhandbook.com/...rradiance.html

And, you also should size the array to the size of your battery bank... 5% to 13% rate of charge, and suggest >10% rate of charge if full time off grid (makes it easier to manage system and loads, without driving yourself nuts):

• 440 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 829 Watt array minimum
• 440 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,657 Watt array nominal
• 440 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,154 Watt array "cost effective" maximum

Looking at your present system (note that sun is highly variable--If you have lots of marine layer/cloudy weather in your area, solar power will be badly affected):

• 1,730 Watt array * 0.52 system eff * 3 hours of winter sun = 2,699 Watt*Hours typical winter day
• 1,730 Watt array * 0.52 system eff * 4.5 hours of more summery sun = 4,048 Watt*Hours typical summery day

And from the battery bank (your typical no sun/over night loads)

• 440 AH * 24 volts * 0.85 AC inverter eff * 1/2 days of storage * 0.50 max battery discharge (for longer life) = 2,244 WH of stored power per day

So, overall your system seems to be pretty well balanced (depending on your other loads).

-Bill

On another board there is a different way to calculate the size of the solar panel which gives the same value that yours does when I plug it into my spreadsheet. It is to take the total load in kWh per day divided by the solar insolation value from gaisma.com

I don't understand how these two formulas get the same result!

BB.

I would have to compare the numbers and calculation strategy between what I use and they use... gaisma.com does not seem to take panel tilt (or tracking) into account--So that can matter to those using trackers.

-Bill