Battery Backup Calculations

[zohaib]

Hello,

I want to design 1 to 5 KW solar systems. How we calculate battery backup and inverter which suits for these systems.
This is about general solar system from 1 to 5 KW (residential systems).

Regarding battery backup if we want 5 hours, 8 hours and 10 hours backup for 1 to 5 KW systems. Then how we calculate this thing. Kindly tell me method how we do these calculation and which can also be used for bigger systems.

[SunEagle]

The battery system is not designed around the pv solar wattage but by your daily watt hour usage. If you determine you will need xx # of watt hours in a 10 hour time period then you can calculate your battery system size. Once you have determine the battery system you calculate how many pv watts you need to recharge the battery each day.

To get the most out of a FLA battery system you really should never discharge it more than 25% a day. And to charge it back up you will need an amperage amount about 1/10th the battery Ah rating.

So as an example a 5000 watt pv system can deliver about 104 amps for a 48volt battery system (5000watt / 48volts = 104amps). That would require it to be about 1000Ah.

[zohaib]

Thanks for your response. Let say our daily watt hour requirement is 5000 watt-hr/day and 12/24 volt battery system . So how we calculate battery backup requirement for 5 or 10 hours.

[SunEagle]

You really can't set up a system based on a good day but based on a couple of bad or sunless days in a row.

So to keep from draining a battery more than 25% you calculate one to handle 4 times your daily watt hour needs.

A 5000 daily watt hour usage will require a battery bank ~ 850Ah at 24volt ( 5000w x 4 / 24v = 833 Ah). The will require an 80 Amp MPPT CC and ~ 2000watt of pv.

If you go with a 48volt battery you can get away with a small MPPT CC and keep the wire size down. Both actions will save you some money.

[zohaib]

Thanks for your help again. How you calculate these values 80 Amp MPPT CC and ~ 2000watt of pv.
And please tell me if we want 6 hours battery backup times in a day so how we calculate this using above figures.

[zohaib]

Actually i have just started working in this area. So if you can explain steps in more details then it will be vary helpful for me to understand.
Thanks

[zohaib]

I am sharing an example below it will be very kind if can explain this with this example also . Let say our Total watt hours requirement is 19 kw hour per day.Please correct me if i am wrong somewhere.

Appliances	Watts	Quantity	Total Watts	Hrs/day run	Total Watt- Hrs
Fan Ceiling	100	6	600	9	5400
Energy savers	30	18	540	6	3240
Laptops	60	7	420	9	3780
Printer	500	1	500	1 (Hr max)	500
Refrigerator	500	1	500	12(With Full load)	6000
Total	1190	33	2560	37	18920

Power Consumption:
Total Watts=2600 W.
Total Watt-Hrs= 19 KWHRS per day= 19 units per day
Multiply loss factor = 19000 x 1.3 = 24700 Watt-Hours per day
Lets sayTotal peak hours = 5
Total watts generated = 24700/5 = 4940 W

If we use 250 W solar panels = 4940/250 = 19 Panels required.
Inverter size:

TOTAL WATT- of all appliances = 2600 W
Total Load watts = 2600 x1.3 = 3380 Watts

Lets say we are using 200 AH, 12v batteries with 50% depth of discharge so how we calculate if we require battery backup of 5 and 10 hours. Wait for your kind response.

[Technologist]

Many of the loads posted are probably grossly off. The first thing I would do is run out and buy a killowatt meter and a digital clamp meter and measure all those loads over time. The energy savings you'll get by owning them will pay for them in mo time. You might find your fridge takes half or take double the energy per day yet the printers might only take 50 watt hours instead of 500. Also after cleaning the condenser coils on my fridge it's energy usage dropped to 1/3rd as it was only on 1/3 the time.

[SunEagle]

Actually i have just started working in this area. So if you can explain steps in more details then it will be vary helpful for me to understand.
Thanks

If you go to the section of this forum concerning off grid battery system sizing you will be able to read how to calculate a battery system and pv wattage based on your daily watt hour usage. Below is a link to one of those threads.

https://www.solarpaneltalk.com/forum/off-grid-solar/off-grid-solar-panel-systems/5140-solar-off-grid-battery-design