PV and batttery Calculation

**solar pete** · 02-08-2016, 04:34 AM

Hi kissnaxn and welcome to Solar Panel Talk. This is why you need to have a generator in a off-grid system well if you live in an area where you are likely to get a month of cloudy weather it is a must. Once the batteries are at the pre set depth of discharge you need to charge them via generator, I am assuming you are talking about an off-grid system, cheers.

**kisssnaxn** · 02-08-2016, 08:35 AM

Ya, it is for off grid. So,autonomy days is considered for one time a year or 6 months or ?? So if battery is fully discharged then how many days will it take to full charge consider and how it is calculated as there is no charing current considered in PV calculation.
For example 100A controller with 490ah battery bank 200watt for 24 hour, if 5 days autonomy day considered

**Wy_White_Wolf** · 02-08-2016, 09:41 AM

Your insolation number accounts for cloudy days. That's the main reason you use worst case in calculations for off-grid.

WWW

**Sunking** · 02-08-2016, 04:05 PM

Originally posted by kisssnaxn

I have confusion regarding PV calculation as in battery calculation we multiply by autonomy days in formula which results bigger battery as per days. But in PV calculation we do not calculate charging current to fill that autonomy day back up.

Who told you that?

Batteries are calculated for 5 days. In practice only gives you 2.5 to 3 days to CYA for cloudy days. You never want to use more than 50% of a battery capacity.

Panel wattage required to replace one-day use is for worse case short winter day. In addition any off-grid system requires a generator to perform routing maintance on the batteries and get you through several days of clouds.

As a last check on panel wattage the minimum panel wattage must be able to supply at least a C/12 charge current and no more than a C/8 charge current. If you cannot meet all the requirements you are SOL using solar as your primary power source. In the end going off grid cost lots of money, many times more than a utility will ever charge you for the same power.

**kisssnaxn** · 02-10-2016, 01:23 AM

For sensitive sector like hospitals and telecom, I read somewhere that 5 days autonomy should be considered. Since for solar mostly we are using deep cycle gel battery, 80 percent DOD is OK.
24 hour load is calculated for equipment like in telecom sector and data centres not for worst conditions.
It is true that it will cost more for off grid, but my concern is about solar panel calculation to fulfil that discharged buttery after autonomy day. And for charging current we do not consider current such as c/12 in calculation of solar, just a load current or total calculated PV current for solar controller.

**SunEagle** · 02-10-2016, 10:18 AM

Originally posted by kisssnaxn

For sensitive sector like hospitals and telecom, I read somewhere that 5 days autonomy should be considered. Since for solar mostly we are using deep cycle gel battery, 80 percent DOD is OK.
24 hour load is calculated for equipment like in telecom sector and data centres not for worst conditions.
It is true that it will cost more for off grid, but my concern is about solar panel calculation to fulfil that discharged buttery after autonomy day. And for charging current we do not consider current such as c/12 in calculation of solar, just a load current or total calculated PV current for solar controller.

I am not sure that a GEL deep cycle battery can go to 80% DOD or 20%SOC. What battery are you using?

Next maybe to clear up any confusion. The amount of charging amps coming from the charge controller needs to be divided into the Battery system total Ah. The result should fall between 8 hour and 12 hour.

Example: Battery rated 120Ah. If charging = 15 amps then 120Ah / 15a = 8 hr. If charging = 10 amps then 120Ah / 10amp = 12 hr.

If your load is critical (like a hospital) then an emergency generator is required. That allows you to not oversize your battery system to cover you for multiple sunless days.

**kisssnaxn** · 02-10-2016, 10:54 AM

Battery type is OpzV Gel battery .

So guys autonomy days is considered once a year or month. If there is any calculation which include autonomy days in pv module calculation as un battery.

**SunEagle** · 02-10-2016, 12:02 PM

Originally posted by kisssnaxn

Battery type is OpzV Gel battery .

So guys autonomy days is considered once a year or month. If there is any calculation which include autonomy days in pv module calculation as un battery.

I have just used 5 days as a multiplier to make sure the system is sized to discharge ~ 20% per day. If the sun does not come out for 3 days then a generator would be used to both supply power and recharge the batteries.

Without the generator you would have to use a bigger multiplier than 5 to increase the battery size but then you may be spending a lot more for your system for the "in case" scenario of not having sun for more than 2 days.

IMO it is still cheaper (and wiser) to size for 5 days and get the gen set so you do not to destroy the battery by over discharging.

**Sunking** · 02-10-2016, 12:18 PM

Originally posted by kisssnaxn

For sensitive sector like hospitals and telecom, I read somewhere that 5 days autonomy should be considered. Since for solar mostly we are using deep cycle gel battery, 80 percent DOD is OK.

Who told you that garbage. I build the stuff for Telecom and mission critical systems. For Telecom that are forced to use solar increase to 8 to 10 days and double solar panel wattage with a generator and energy management system. That is what it takes to get 99% availability.

**kisssnaxn** · 02-11-2016, 10:08 PM

Ok that means for telecom u design for 8 days and double panel capacity . If u have any best link for all these calculation please share

PV and batttery Calculation

PV and batttery Calculation

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment