off grid 3kwp system

Re: off grid 3kwp system

mahmud sultan wrote: »

hi everybody.i am new to this site.can anybody help me about how can i calculate the amount of solar panel,batteries,inverter,charge controller for the actual operating load of 3 kwp with 2 hrs back up?please tell me as details as anybody can.

Not a lot of info to go by but for off-grid with 6KWh of battery storage at 40-60% DoD, looks like 6 225-260W 60-cell panels (2 strings of 3), 8 golf cart batteries (1 48V string), a single 150V/60A MPPT charge controller, and a single 48V DC / 120V AC outback inverter can handle (3000 or 3600W).

For a bi-modal system, you'd probably want more PV to fully utilize your inverter and charge controller as you would be able to sell to the grid. A single charge controller and a 3000-3600W inverter can handle 4 strings of PV (12 60-cell panels). 12 PV panels is battery balanced with a string of L16s instead of a string of golf cart batteries (or two strings of golf cart batteries) but if you are grid-tied with ability to sell or just can transfer most of your loads to daytime, the battery upgrade would be less mandatory.

You need to be sure that 3KW x 2hr / 6KWh is your true backup needs and 3KW is your true load demand.

Re: off grid 3kwp system

3kW for 2 hours = 6kWh.

Deep cycle batteries should be discharged about 50%, so you'd need 12kWh battery. At 48V, that's 250Ah at the C2 rate[*]. You won't find a battery rated at the C2 rate, more common is C5 for traction batteries or C10, C20 for renewable energy batteries. It's a bit tricky to convert the size between the different C ratings because it's different for each battery. Just as an example, check this datasheet: http://www.dcbattery.com/rollssurrette_s530.pdf
If you need 250Ah at C2, then it means you need to buy a 400Ah battery at C20.

So now you have the battery size. Next step is to work out how much energy you need to recharge that battery.
You're taking out 6kWh every day and need to make up for the losses in charging, the losses could be (thumb-in-the-air-guesses):
- 20% for charging the battery
- 2% lost in the charge controller
- 2% lost in cabling

Make it a round 30% losses. So you need to generate 7.8kWh every day.
Now you should consult solar maps for your region, which can tell you how much solar PV is required to generate 7.8kWh in the worst month. I.e. the month with the least solar output, this will likely be sometime in the monsoon season. Here's one for EMEA: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php# but it doesn't stretch to Bangladesh.

But since you have the grid available, you could also use this to charge the batteries so you don't need to rely on solar 100%. In fact, if it's just 2 hours backup you need, then you don't need solar at all, you could just charge from the grid.

Since you need 3kW peak, you'd need a 3kW inverter like the Outback 3048, Victron multiplus 3kW, etc. And better to get a combined inverter/charger rather than a plain inverter so that you can charge from the grid when you need to.


EDIT: [*] if we're splitting hairs then a 50% Depth of Discharge for 2 hours, would be the 4 hour rate or C4 not C2.

Re: off grid 3kwp system

mahmud sultan wrote: »

it will be installed in Dhaka ,Bangladesh.
yes i have the grid connection.But as i m looking for battery backup for 2 hrs.it will definitely be a off grid system
someone wrote about the size and quantity i need,more or less i also know.
but what i m asking is how can i calculate by myself or the way of calculation.

thanks everybody.

If you are keeping your grid connection but want a battery backup for it then you are really looking for a bi-modal or grid-tie w/ battery backup system. 2hrs of backup doesn't make sense for a fully off-grid system. Off-grid systems usually target 24 hours of battery reserve with a generator and at least 72 hours without a generator.

Somebody, though it applies more to off-grid mentioned in multiple threads a 5% min (20hrs), 10% typical (10hrs), 12.5% cost-effective maximum (8hrs). grid-tie w/battery backup can get away with smaller battery banks relative to PV because they can sell to the grid. Bi-modal systems can push to 20% (5hrs) or even 25% (4hrs). Outback FX/VFX inverters can operate bi-modal. They just lack 240V split-phase capability and a built-in automatic transfer switch for a generator. If you need/want those features you need a Xantrex XW (4.5 or 6KW) or Outback Radian (8KW) inverter.

At least in America, (Not sure about bangladesh), PV is cheap and getting cheaper while batteries are expensive and getting more expensive, while inverter and charge equipment looks somewhat stable.

Surrette is very poor when it comes to losses due to high charge/discharge rates.


Still would say one 48V string of golf cart batteries and 2 strings (6 panels) of PV on the cheap side and one 48V string of L16 batteries and 4 strings (12 panels) of 60-cell PV on the safe side (especially if you can sell to the grid), using a single 150V 60A charge controller and a Outback 3000-3600W inverter. The cheap option will be very hard on the batteries (shorter life). I think 3rd world nation rolling blackouts often run longer than 2hrs, better off with 12 PV panels and a 48V string of L16s.

60-cell PV panels run 225-260W and are typically 235-245W. 6 240W panels is 1,440W and 12 240W panels is 2,880W. Near 'nominal' with a string of L16 batteries, 'grid-tie aggressive' with a string of golf cart batteries. I copied the below from BB's post in another thread, converted it to 400AH (L16 typical capacity) @ 48V and added the 6hr bracket (grid-tie aggressive) and also did for 225AH (GC2 / T-105 / Golf cart typical capacity).

400 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge (12hrs) = 1,496 Watt Array Minimum
400 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge (10hrs) = 2,992 Watt Array Nominal
400 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.125 rate of charge (8hrs) = 3,740 Watt Array "cost effective" Maximum
400 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.166 rate of charge (6hrs) = 4,987 Watt Array Grid-tie aggressive
400 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.25 rate of charge (4hrs) = 7,480 Watt Array not to exceed

225 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge (20hrs) = 841 Watt Array Minimum
225 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge (10hrs) = 1,683 Watt Array Nominal
225 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.125 rate of charge (8hrs) = 2,103 Watt Array "cost effective" Maximum
225 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.166 rate of charge (6hrs) = 2,805 Watt Array Grid-tie aggressive
225 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.25 rate of charge (4hrs) = 4,208 Watt Array not to exceed

These are the battery charge rate recommendations from various sources. Most of it is 8 or 10 hour charge time/rates for flooded and 4-5hrs for AGM.

Northern arizona Wind & Solar Battery FAQ - Flooded (recommended maximum): C-20/8 (12.5% of AH capacity at 20-hour rate)
Northern arizona Wind & Solar Battery FAQ - AGM (recommended maximum): C-20/4 (25% of AH capacity at 20-hour rate)
Surrete (recommended maximum): C-6/6.67 (15% of AH capacity at 6-hour rate, which translates to about C-20/9 or 11% of AH capacity at 20-hour rate)
Trojan flooded (recommended): C-20/10 to C-20/7.7 (10-13% of AH capacity at 20-hour rate)
Trojan AGM (recommended): C-20/5 (20% of AH capacity at 20-hour rate)