Some help calculating power requirements please


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Warpy55

New member
Joined
Jul 16, 2022
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3
I have a 4KW 24v input pure sine wave inverter. I also have a qty of tested recycled 18650 lithium cells. I have a nickel tab welder. I wish to build a power wall to run 3 fridges in my beer brewing garage of solar panels mounted on the garge roof. I haven't as yet bought the solar panels but keen to start. I have been in electronics my entire working career so know about all (I should say some) of the ins and outs. My intention is to build the battery packs, probably as 3.7/4.2V packs, charge them through a BMS module of which I will need 6pcs of BMS module. The questions I have (as I don't know where to start), what voltage of solar panels, what wattage, what specs for the BMS module, do I need an MPPT controller. This is the first project after that, I will look at the house power supply. I haven't worked out the number of cells I will need but I am getting all the "bits"together. The must haves are the inverter I have, the rest seems open. Any help??
 

Wolf

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Joined
Sep 25, 2018
Messages
1,760
#1 Figure out your daily kWh requirements then we can figure out what you need for solar panel output. This also depends on your location. If mostly cloudy in the northern hemisphere may require additional panels.
#2 Yes a MPPT is needed. That is what controls the charging of the "battery".
Once you have some firm # of kwh usage we can certainly aim the arrow in the right direction.
Wolf
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,125
First thing is to decide if you want off-grid (battery required), grid-tie (no battery but power company approval required) or in-between.

I'll assume for this post that you're thinking off-grid. This means you need a battery but you can use grid-assist (automatically) if the battery is too low such as on a cloudy day.

My house refrigerators are ~2.5kwh/day (as I recall). 3 of these would be 7.5kwh/day - let's round to 8kwh / day as you're working target. But as @Wolf says, you should get a killowatt meter (or some other means) and see what you actually need.

The next question is your location - solar power for Los Angeles is more even thru the year than where I live. For me, I only get 25% in the 4 winter months of what I get in the summer. You can use PVWatts https://pvwatts.nrel.gov/pvwatts.php to get an idea of solar panel array size you need for your location to get 8kwh/day (e.g. 31days * 8kwh = 248kwh/month). For example, in Los Angeles a 2,500w PV array (using standard settings) .....
1659728617699.png

will get you in the ballpark of the following kwh/month. The lowest month = December = 247kwh which is 'just enough' for 248kwh target goal.
1659728667849.png


A 2.5kwh PV array = 2,500w. That would be 10 panels at 250w each as an example.

From here - you can size an MPPT charge controller. At this level, you might want to consider an all-in-one such as an MPP Solar 3048LV - https://watts247.com/product/pip-3048lv-mk/ (e.g. off-grid unit) which can handle up to 4,000w of solar panel input, 48v battery, and 3000w of output. This has grid assist + UPS built-in.

But OK, sticking with you're 24v inverter (and battery by implication), you'll need a MPPT charge controller that can handle at least 2,500w @ 24v. I think you'll find that this is pretty high end for 24v and is why I suggested 48v system above. But here's an example of a stand-alone MPPT Charge controller that can reach the 2,500w PV input @ 24v and is popular - https://www.amazon.com/EPEVER-Controller-1250-5000W-Regulator-Lead-Acid/dp/B07KPD1D5B/ref=sr_1_1 Notice in the description is shows the max Solar Panel input per voltage -
  • 100 amp Charge Controller Max Input Solar Panel Power: 1250W/12V, 2500W/24V, 3750W/36V, 5000W/48V. Max input Voltage: 150V. Real-time energy recording and statistical.
and the 2500w/24v just barely meets the theoretical target of 2500w PV array.


And then finally you'll need some batteries. You'll need around 4~kwh/night on average. More in winter, less in summer. To get 4kwh/night at 80% DOD you'd need a 5kwh battery. This would be ~200ah @ 24v which would be ~ 7s80p of 2500mah cells (e.g. 560cells). For 48v it would be 14s40p - same number of cells, just arranged as 14s instead of 7s.

===================
All the above is ESTIMATION based on an off-grid system to deliver on the order of 8kwh/day in a favorable sun location. The point of this is to give you how I would think it thru and to give you some scale and perhaps help you think about how your situation differs.
 
Last edited:

Warpy55

New member
Joined
Jul 16, 2022
Messages
3
First thing is to decide if you want off-grid (battery required), grid-tie (no battery but power company approval required) or in-between.

I'll assume for this post that you're thinking off-grid. This means you need a battery but you can use grid-assist (automatically) if the battery is too low such as on a cloudy day.

My house refrigerators are ~2.5kwh/day (as I recall). 3 of these would be 7.5kwh/day - let's round to 8kwh / day as you're working target. But as @Wolf says, you should get a killowatt meter (or some other means) and see what you actually need.

The next question is your location - solar power for Los Angeles is more even thru the year than where I live. For me, I only get 25% in the 4 winter months of what I get in the summer. You can use PVWatts https://pvwatts.nrel.gov/pvwatts.php to get an idea of solar panel array size you need for your location to get 8kwh/day (e.g. 31days * 8kwh = 248kwh/month). For example, in Los Angeles a 2,500w PV array (using standard settings) .....
View attachment 27975
will get you in the ballpark of the following kwh/month. The lowest month = December = 247kwh which is 'just enough' for 248kwh target goal.
View attachment 27976

A 2.5kwh PV array = 2,500w. That would be 10 panels at 250w each as an example.

From here - you can size an MPPT charge controller. At this level, you might want to consider an all-in-one such as an MPP Solar 3048LV - https://watts247.com/product/pip-3048lv-mk/ (e.g. off-grid unit) which can handle up to 4,000w of solar panel input, 48v battery, and 3000w of output. This has grid assist + UPS built-in.

But OK, sticking with you're 24v inverter (and battery by implication), you'll need a MPPT charge controller that can handle at least 2,500w @ 24v. I think you'll find that this is pretty high end for 24v and is why I suggested 48v system above. But here's an example of a stand-alone MPPT Charge controller that can reach the 2,500w PV input @ 24v and is popular - https://www.amazon.com/EPEVER-Controller-1250-5000W-Regulator-Lead-Acid/dp/B07KPD1D5B/ref=sr_1_1 Notice in the description is shows the max Solar Panel input per voltage -
  • 100 amp Charge Controller Max Input Solar Panel Power: 1250W/12V, 2500W/24V, 3750W/36V, 5000W/48V. Max input Voltage: 150V. Real-time energy recording and statistical.
and the 2500w/24v just barely meets the theoretical target of 2500w PV array.


And then finally you'll need some batteries. You'll need around 4~kwh/night on average. More in winter, less in summer. To get 4kwh/night at 80% DOD you'd need a 5kwh battery. This would be ~200ah @ 24v which would be ~ 7s80p of 2500mah cells (e.g. 560cells). For 48v it would be 14s40p - same number of cells, just arranged as 14s instead of 7s.

===================
All the above is ESTIMATION based on an off-grid system to deliver on the order of 8kwh/day in a favorable sun location. The point of this is to give you how I would think it thru and to give you some scale and perhaps help you think about how your situation differs.
Hi Wolf,
Thanks for your time on this. We live in Auckland, New Zealand and so are on 240VAC at 50Hz. I plan to use grid assist. I have a watt meter which says 300W when all fridges are on. But I don't know how long they are on and how many times in 24hours. I have ordered some watt meters off Amazon that will give me the Kwh figures. I am still compiling the batteries and testing but have maybe 300 so far. My garage system would be 24V, the house system sounds like 48V or higher might be a better option. I see solar panels seem to be rated at 18V no load and about 50V no load of varying wattages. I am think 300W plus per panel. Thanks again Wolf.
 

Warpy55

New member
Joined
Jul 16, 2022
Messages
3
First thing is to decide if you want off-grid (battery required), grid-tie (no battery but power company approval required) or in-between.

I'll assume for this post that you're thinking off-grid. This means you need a battery but you can use grid-assist (automatically) if the battery is too low such as on a cloudy day.

My house refrigerators are ~2.5kwh/day (as I recall). 3 of these would be 7.5kwh/day - let's round to 8kwh / day as you're working target. But as @Wolf says, you should get a killowatt meter (or some other means) and see what you actually need.

The next question is your location - solar power for Los Angeles is more even thru the year than where I live. For me, I only get 25% in the 4 winter months of what I get in the summer. You can use PVWatts https://pvwatts.nrel.gov/pvwatts.php to get an idea of solar panel array size you need for your location to get 8kwh/day (e.g. 31days * 8kwh = 248kwh/month). For example, in Los Angeles a 2,500w PV array (using standard settings) .....
View attachment 27975
will get you in the ballpark of the following kwh/month. The lowest month = December = 247kwh which is 'just enough' for 248kwh target goal.
View attachment 27976

A 2.5kwh PV array = 2,500w. That would be 10 panels at 250w each as an example.

From here - you can size an MPPT charge controller. At this level, you might want to consider an all-in-one such as an MPP Solar 3048LV - https://watts247.com/product/pip-3048lv-mk/ (e.g. off-grid unit) which can handle up to 4,000w of solar panel input, 48v battery, and 3000w of output. This has grid assist + UPS built-in.

But OK, sticking with you're 24v inverter (and battery by implication), you'll need a MPPT charge controller that can handle at least 2,500w @ 24v. I think you'll find that this is pretty high end for 24v and is why I suggested 48v system above. But here's an example of a stand-alone MPPT Charge controller that can reach the 2,500w PV input @ 24v and is popular - https://www.amazon.com/EPEVER-Controller-1250-5000W-Regulator-Lead-Acid/dp/B07KPD1D5B/ref=sr_1_1 Notice in the description is shows the max Solar Panel input per voltage -
  • 100 amp Charge Controller Max Input Solar Panel Power: 1250W/12V, 2500W/24V, 3750W/36V, 5000W/48V. Max input Voltage: 150V. Real-time energy recording and statistical.
and the 2500w/24v just barely meets the theoretical target of 2500w PV array.


And then finally you'll need some batteries. You'll need around 4~kwh/night on average. More in winter, less in summer. To get 4kwh/night at 80% DOD you'd need a 5kwh battery. This would be ~200ah @ 24v which would be ~ 7s80p of 2500mah cells (e.g. 560cells). For 48v it would be 14s40p - same number of cells, just arranged as 14s instead of 7s.

===================
All the above is ESTIMATION based on an off-grid system to deliver on the order of 8kwh/day in a favorable sun location. The point of this is to give you how I would think it thru and to give you some scale and perhaps help you think about how your situation differs.
Hi OffGridInTheCity,
Thanks for your time on this. We live in Auckland, New Zealand and so are on 240VAC at 50Hz. I plan to use grid assist. I have a watt meter which says 300W when all fridges are on. But I don't know how long they are on and how many times in 24hours. I have ordered some watt meters off Amazon that will give me the Kwh figures. I am still compiling the batteries and testing but have maybe 300 so far. My garage system would be 24V, the house system sounds like 48V or higher might be a better option. I see solar panels seem to be rated at 18V no load and about 50V no load of varying wattages. I am think 300W plus per panel. Thanks again OffGridInTheCity,.
 
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