Solar panels, inverters and batteries

Rufinus

Joined Apr 29, 2020
161
Hello.

I´m going to put 4 solar panels (24 V 500w each) in my roof. The installation company only gives me the panels and the inverter, wich is able to connect to batteries. I don´t want to have batteries for the night. In my neighborhood we use to have cuts in the electricity (Spain is a very developed country, I promise hahaha, it is a problem only in my neiborhood). So I want to have a backup for a small fridge, internet connection, a laptop and 1 or 2 led lights. I´m not sure if my calculations are right.

I have estimated 600watts/hour, and I want to run them for 10 hours aprox, so I´ll need 600x10 = 6.000w. I´ll use 2 12V batteries in series, so 6.000/24 = 250 amps. Now, increase that for the inverter cosumption, and safety factor of 20% so, with 2 12V 300Amps batteries would be enough?

Thank you

best regards

BobTPH

Joined Jun 5, 2013
6,513
Your units are all screwed up. Watts/hour is not a unit of anything. I assume you simply mean 600W.

Your usage of 600W for 10 hours is 6000 Watt Hours of energy, not power.

Battery capacity is rated in Amp hours. To get the energy, you take the capacity in Amp hours times the voltage giving you Watt hours. Now we can relate that to your energy usage of 6000 Watt hours. You will need a battery that has 6000 Watt Hours. If the battery is 24V that will be 6000 / 24 or 250 Amp hours.

For batteries in series the voltage doubles but the Amp hours stays the same. So you need 2 12V 250Ah batteries.

Boggart

Joined Jan 31, 2022
73
Your units are all screwed up. Watts/hour is not a unit of anything. I assume you simply mean 600W.

Your usage of 600W for 10 hours is 6000 Watt Hours of energy, not power.

Battery capacity is rated in Amp hours. To get the energy, you take the capacity in Amp hours times the voltage giving you Watt hours. Now we can relate that to your energy usage of 6000 Watt hours. You will need a battery that has 6000 Watt Hours. If the battery is 24V that will be 6000 / 24 or 250 Amp hours.

For batteries in series the voltage doubles but the Amp hours stays the same. So you need 2 12V 250Ah batteries.
Not really as simple as that. Assuming you are talking lead-acid batts, then most manufacturers recommend discharge depth to be well above 0%. In fact it's common to see max DoD (depth of discharge) figures of between 50 and 80%.

So, it depends on the battery, and how many cycles you expect the batts to last. You need to look at the life cycle/discharge figures given by your preferred battery manufacturer and go from there. Most manufacturers put this data in their datasheets.

For lithium batts, it's less of an issue, and they can generally be discharged down to their endpoint, especially LFP batts, which are pretty much fine if you cycle them between 0 and 100%, although they will have a longer lifespan with shallower cycles as well.

So, you need to look at all the data for your proposed batts and compare that to your expectaions of longevity etc and go from there.

Rufinus

Joined Apr 29, 2020
161
Thank you for your answers. Yes, I meant 2 batteris of 12V in series to get 24 volts.

And about tpes of batteries, wich option is better to keep them charged and use them only ocassionally, only in case of cuts in the grid?

bassbindevil

Joined Jan 23, 2014
731
If you mean battery types... according to some tests I watched on Youtube, the best battery will be LFP (Lithium Iron Phosphate); . If LFP is too expensive, the best value in lead-acid is probably the cheapest one. If you have the space, a bunch of used car batteries might be just fine. I suspect many are discarded too soon (I had a 20-year old battery in my truck.) Or, build a power wall from recycled lithium-ion cells. I won't suggest nickel-iron (aka Edison), but they are said to last a very long time, but aren't popular or cheap.

As for how to keep them charged, I would consider using the battery bank as the external battery of a UPS (uninterruptible power supply) that just provides backup power to the fridge and the essential devices. You might find a nice used one at a computer recycler for cheap; small ones use 12V batteries, bigger ones 24V (or more). They're usually discarded because the Sealed Lead Acid battery has died. Conventional electric motors may not run so well from the "modified" sine wave that most UPSes produce, but a modern fridge may not have a conventional motor, and things with switching power supplies won't care. Or you can find one with pure sine wave output. The UPS will plug into the mains voltage and keep the battery topped up from its internal charger, then if the power fails the inverter will take over within milliseconds and your computer and lights will keep running. It might be safer to go with a rather oversized unit since smaller ones may overheat if running for more than a few minutes. If there are more than occasional power failures, it may require an external charger (solar or line-powered) to the keep the batteries charged.

I suggested the UPS solution because it'll do the essential job for far less money than converting your solar setup to a "hybrid" one with grid tie and batteries. But re-reading, it looks like you don't have that grid-tie stuff installed yet, so, maybe it's not too late? Here's one on Aliexpress that does 240V. https://www.aliexpress.com/item/32880153076.html But depending on regulatory approvals and the power company situation where you are, it might be much safer to let the local experts handle it all.

Before spending money on batteries and an inverter or UPS, get a power monitor to measure the actual consumption of your fridge and other stuff. The P3 Kill-A-Watt meter said my mini-fridge used about 100 watts when running, but the duty cycle was low so average consumption must have been well below 50 watts.