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diy solar

Needing to know how many batteries and solar panels are needed.

Gythy

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Joined
Jul 15, 2021
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Hi so me and my wife have saved up some money to move off grid, since we are quite fed up with everything going on in our state. We are wanting to move to West Virginia because we have family there, it’s cheap, and I can get jobs in Maryland and Virginia to get some extra cash. But anyway enough about me I just need help and I feel very weird that I am not understanding how to size my solar system. Here is what I intend to implement in our tiny little cabin

4 recess lights, each will use 10 watts each, and lets say we plan to use them 6 hours a day.
240 watts a day

bath+fan light combo. About watts when used and lets say we use it for about 2 hours per day
80 watts in total
phone and laptop charger to about 150 watts about 4 hours a day
600 watts in total
and now the real kicker. We want a fridge. We understand this will be the kicker of our solar but we really want a fridge and don’t want to do propane but if we have to we will. It’s about 365 kWh per year (we may downsize if really needed) so let’s say it’s 1000 watts per day

so in total doing these calculations I have got about 1920 watts daily use. Let’s just round up and say I want to use 2000 watts a day. I plan on getting the Growatt or MPPT all in one inverter. I was thinking of doing the 24V one. So all I need is solar panels and batteries but I don’t know how to size these.
With my pathetically limited understanding I think that buying 10 400W solar panels will be enough for the system, but with batteries I am pretty clueless. I intend to live in WV where we get an average of 3.5 peak sun hours. Let’s be safe and round to 3 peak sun hours. That means I would get 4000 Watts, which would be enough for the batteries to get at least 50% percent. (I may up size it more though)

are my calculations laughable and I obviously don’t know what I am saying? please tell me because we just want to get off grid fast. What batteries do I need. Can I use just two battleborn 12 V batteries or do I need more?
 
Your 10 - 400w panels would make a 4000 watt pv array. That is huge for needing 2000 watts a day. Conservatively that 4K array would make 15K watts a day. You are probably looking at 2-3 of those panels to make a 800-1200 watt array. If you can get 4 solid hours a sun a day you should make ~4000 watts a day. That is 2x your daily requirement. Which is good, because you can use all you daily power and have enough to fully recharge your batteries from the day before.

For example: I have a 4000 watt array to power a 21,000 watt battery bank...

If you need 2K a day, if you have a battery than can store 6K watts, you have 3 full days of electricity stored with no sun. There are lots of LifePo4 batteries in 24v or 48v that a have that much storage. Easy to do that with a single 48v or a pair of 24v. There are even some 24v single batteries with that much storage.
 
Hi so me and my wife have saved up some money to move off grid, since we are quite fed up with everything going on in our state. We are wanting to move to West Virginia because we have family there, it’s cheap, and I can get jobs in Maryland and Virginia to get some extra cash. But anyway enough about me I just need help and I feel very weird that I am not understanding how to size my solar system. Here is what I intend to implement in our tiny little cabin

4 recess lights, each will use 10 watts each, and lets say we plan to use them 6 hours a day.
240 watts a day

bath+fan light combo. About watts when used and lets say we use it for about 2 hours per day
80 watts in total
phone and laptop charger to about 150 watts about 4 hours a day
600 watts in total
and now the real kicker. We want a fridge. We understand this will be the kicker of our solar but we really want a fridge and don’t want to do propane but if we have to we will. It’s about 365 kWh per year (we may downsize if really needed) so let’s say it’s 1000 watts per day

so in total doing these calculations I have got about 1920 watts daily use. Let’s just round up and say I want to use 2000 watts a day. I plan on getting the Growatt or MPPT all in one inverter. I was thinking of doing the 24V one. So all I need is solar panels and batteries but I don’t know how to size these.
With my pathetically limited understanding I think that buying 10 400W solar panels will be enough for the system, but with batteries I am pretty clueless. I intend to live in WV where we get an average of 3.5 peak sun hours. Let’s be safe and round to 3 peak sun hours. That means I would get 4000 Watts, which would be enough for the batteries to get at least 50% percent. (I may up size it more though)

are my calculations laughable and I obviously don’t know what I am saying? please tell me because we just want to get off grid fast. What batteries do I need. Can I use just two battleborn 12 V batteries or do I need more?
May be helpful to use some of the items in the Resources like the audit and sizing spreadsheet. https://diysolarforum.com/resources/system-energy-audit-and-sizing-spread-sheet.12/
 
Make sure to use the term watt HOURS. It does seem that you are calculating watt hours but using the term watts. Very different meaning. It sounds like you realize this too but in terms of batteries what you need to know is the after dark loads. Running a fridge is no big deal, I run two fridges and a chest freezer. If your calculation of needing 2000wH a night is correct, a battery with 3000wH or more would work for LFP. 4000 watts of solar would be more than enough to charge this, you might even be able to get away with less. I use about 3-4kWh a night and have 4.3kW of solar and it works great. Mind you, I have excellent all day sun exposure. Which is another aspect you haven't mentioned, to know if a particular wattage of panels is enough we need to have an idea of your exposure.
 
@Gythy
Punched in your 2000watt daily usage to the audit spreadsheet and this is what you get.
Notice a few items
1. # days of storage capacity is 3 days. I live in central Maryland and would say you should target 3 days of storage.
2. Isolation is 3. Summer will get you about 4 - 4.5 but best to round down to 3 as you stated for Winter.
3. System Voltage is 24v and min Battery capacity = 308.6. Based on that you would be close to capacity if you were to build an 8s LFP with 280ah cells.
4. Min Solar Cell Wattage. You could get by with 8 400watt panels.

Screen Shot 2021-07-16 at 12.24.47 PM.png
 
@Gythy
Punched in your 2000watt daily usage to the audit spreadsheet and this is what you get.
Notice a few items
1. # days of storage capacity is 3 days. I live in central Maryland and would say you should target 3 days of storage.
2. Isolation is 3. Summer will get you about 4 - 4.5 but best to round down to 3 as you stated for Winter.
3. System Voltage is 24v and min Battery capacity = 308.6. Based on that you would be close to capacity if you were to build an 8s LFP with 280ah cells.
4. Min Solar Cell Wattage. You could get by with 8 400watt panels.

View attachment 56482
This is very kind of you sir. So really knowing this I think all I need is 8 400 watt solar panels. I hear what you are saying with batteries, I am thinking of having a 24V system and I was thinking of either having two powerful 12 V battleborn batteries or would having one singular 24 V battery be better?
 
One thing you don't mention is a generator. A 5kW or so would be fine for thins setup. I personally don't have three days worth of battery power and never have in over 20 years off grid. I just charge every day, sun or generator. If my power slurping hog self isn't there the system would go three days. Luckily my bro in law is always there to fire the genny. I'd rather put the money that extra battery storage would cost into panels or generator. I only put 50 hours or so a year on my generator.
 
As @boondox questions, are you going to also have a genny @Gythy ? That can definitely make a difference in what choices you make.

You could get by with 2 BBs in series and that would allow you one day of storage capacity but think about the cost diff in those batteries.
2 BB's are going to run you @$1900 and give you 100ah. If you were to build your own 24v battery, that's @$1000 in (8) 280ah cells and say an Overkill BMS for $150. You are paying $750 more and losing about 2/3rds of your possible capacity with purchasing instead of building. No you won't have a warrantee so you have to weigh the risk/reward on that.

I get that you may not want to attempt a battery build yourself but there are so many resources here and those that can help, that its a bit of a no brainer.

When it is cloudy for a few days is your wife going to be ok schlepping out to fire up the generator because you are running minimal battery capacity? Happy wife, happy life....
 
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