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I am planning to setup an off-grid solar panel system.

I've read Off-grid solar setup, but I am looking for clarification on my understanding on how to go about it in the most cost-efficient way.

I live in the Philippine tropics. The main bulk of my electricity cost is an air conditioner that according to my meter uses about 12kWh daily average, so I'm just estimating another 3kWh for other miscellaneous things such as my desktop computer maybe 2kWh and some low power electronics like routers, lightbulbs and chargers for another 1kWh.

I'm planning to do this mostly DIY. From what I have researched so far to power everything for the whole day I would need to have a 15000Wh/12V=1250Ah battery. To charge the battery in a 5 hour peak sunlight time I would need (15000Wh/5h/100W=30) 30x100W or 10x300W solar panels.

Have I understood this correctly so far?

I'm thinking of getting a 2000W inverter to have leeway in the wattage spikes my devices may bring or is getting anything higher just overkill?

For the battery I'm thinking of building it myself using 32650 LiFePO4 3.2 5Ah cells and a BMS of course. I would need 1250Ah/5Ah parallel x 4 series = 1000 cells to get a 1250Ah 12.8V battery. Is it better to put this all together in one box or in separate 1250Ah 3.2V or 312Ah 12V containers? Should I get a different battery system altogether? Is 5 hours of sunlight enough to charge this to full? Should I have calculated this based on 24hour operation or can I just give allowance for 12h or 18h for overnight usage?

Lastly, is this the best way of going about it? Can I reduce any of my component costs in terms of the quantity of items to be bought for this scenario, or accounting for conversion inefficiencies should I get more? Am I missing anything crucial for this?

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    \$\begingroup\$ You probably want to target 48V or higher as your main bus voltage. This is to lessen the voltage loss due to high current. As for you batteries, I’d suggest you buy ready made assemblies. You’re talking a significant investment in the cells, so unless you have the experience to assemble the cells in a safe manner, just buy ready made 19” rack ‘bricks’ that have all the tricky work done. I’m all for diy, but some things are just best left to the experts. \$\endgroup\$
    – Kartman
    Jul 3, 2021 at 3:49
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    \$\begingroup\$ Agree. Go 48 V. Also, I don't think a 2kW inverter will start your AC unit. It may not even run your AC unit. I think a larger inverter will be needed. How much power does your AC use when running? You should check that before choosing your inverter. Also, it will be easier to build a battery pack from larger cells. You can buy LiFePO4 cells in 50 or 100 Ah size. Much less work for you. \$\endgroup\$
    – mkeith
    Jul 3, 2021 at 4:25
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    \$\begingroup\$ @mkeith it's just a 0.5 500w peak AC unit just for my bed room. And yeah, those were the first ones I considered. I thought it would be cheaper to make it myself but I guess it's going to be a lot safer just buying the big ones. Thanks \$\endgroup\$
    – KyleLabs
    Jul 3, 2021 at 4:39
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    \$\begingroup\$ If 500 W is the peak power, then either "peak" means something else to you, or it uses less than its peak power on average, so that your 12 kWh are an overestimate. \$\endgroup\$
    – mmmm
    Jul 3, 2021 at 8:34
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    \$\begingroup\$ If I may ask, how did you determine the 12 kWh per day figure? Did you multiply 500 W * 24 hours? The actual consumption is pretty important. It would be good to measure it accurately before designing your system. \$\endgroup\$
    – mkeith
    Jul 3, 2021 at 8:50

1 Answer 1

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This sort of question gets asked here quite often, and two of the major mistakes people make are overestimating the output from their solar panels and overestimating the usable power from their batteries. The good news is that you've been quite realistic with the first one.

If your calculations and measurements on power requirements are correct, you're looking at using 15kWh a day in total, which makes an average consumption of 625W. From the sound of it, this is pretty much constant, which isn't alway the case and would need to be factored into the calculations if not.

Now, you need to think of your system as operating in two distinct modes:

  • 17 hours without sunlight, where the 625W load has to come from the batteries via the inverter
  • 5 hours of sunlight, where the solar panels have to provide the 625W via the inverter AS WELL AS recharging the batteries, because you can't use the batteries to power the load while you're charging them. And that's the third mistake people regularly make with these sort of battery backup designs

Next, you need to add in the power lost in your inverter: let's go with 90% efficiency, which means your average load is now 694W (or 700W for a nice round number). As per mkeith's comment, although a 2kW inverter seems plenty, you need to be sure it can supply the current needed to start up and run your a/c, the former can be twice the latter or more.

In the first case, 700W multiplied by 17 hours gives you 11.9kWh, which is the usable capacity you need from your batteries. This is not the same as the rated capacity, because you really don't want to be charging them and discharging them completely (in fact you'll never get anywhere near the full rated capacity from them). There's a lot of debate over exactly how fully you should charge them and discharge them, but it's certain that going up to 100% and down to zero will stress them far more and reduce their life drastically, especially in a hot climate such as yours. Tied in with this is the correct way to charge them, which is using a CC/CV algorithm that gets them up to about 80% quickly but takes a lot longer to get to 100% (which is why electric car manufacturers often specify the charge time to 80% as it's a lot more impressive that the time to 100%). You haven't mentioned using a proper charger but that's the fourth mistake commonly made - a BMS is not a charger, just a protection mechanism to stop things going dangerously wrong.

Taking a more realistic estimate, aim to charge to 80% and discharge to 20%, which means you're getting 60% of the rated capacity. That means your rated battery capacity needs to be somewhere around 19.833kWh, so, pretty much, 20kWh.

Now comes the second stage. You have 5 hours of sunlight to get that 12kWh (approx) of charge into your batteries, which works out at an average of 2.4kW. It might be wise to allow for maybe 4 or 4.5 hours to get the 12kWh, meaning you have time to get a little beyond the 80% charge at the slower CV rate. Let's go with 4.5 hours, which means charging at a rate of 2.67kW. Now you need to add on the 700W going to your load via the inverter while the batteries are charging, which makes 3.37kW in total. Finally, allow a bit more for losses in the MPPT/solar charge controller, if we call that 90% as well you end up needing 3.74kW from your solar panels, which is a bit of an increase on what you were originally thinking.

Obviously all the figures can be tweaked to some degree depending on your degree or optimism or pessimism, but they won't be far wrong. And as you note, good insulation can make a huge difference to the power requirements for air conditioning, I've seen loads of buildings with aircon running flat out because they have a metal roof and a suspended ceiling with nothing in between to stop the whole thing turning into a giant radiator.

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  • \$\begingroup\$ Realised I worked out the charging based on rated battery capacity not actual delivered, fixed. \$\endgroup\$
    – Finbarr
    Jul 4, 2021 at 12:59
  • \$\begingroup\$ I find this answer both informative and highly accurate representation of how one should address design of any independent energy system. +1 A smart move would be to leave a method of powering everything from the grid in case of catastrophic failure in one of the key components. \$\endgroup\$ Jul 4, 2021 at 13:30

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