Solar power for specific load in a building

Welcome to the forum Siraj,

This is a very difficult question to ask without more context... Why? (save money, save genset fuel, emergency backup power, full time power, go green, etc.).

Second... kWatts (1,000xWatts) is a "rate"--Like Kilometres per Hour... Yes, need to know the maximum power needed (like the speed limit on a road, 30 kph, 100 kph, etc.). But also need to know the quantity of energy needed (like 30 kph * 10 hours per day = 300 km per day).

Also need to know where this will be located... In Northern Germany, not much sun--In the winter, very little sun. In Karachi Pakistan, more sun (I am guessing your location as your IP address is "all over"--Kansas, France, Afghanistan, Pakistan using iplocation.net).

The reality is that off grid (battery powered) solar is very expensive (typically something like 5-10x more expensive than grid power; and equal to, or more expensive than generator fuel). Conservation is usually crucial to getting power needs low enough that solar can make sense.

I will do a quick sizing for you based on some guesses for a first pass estimate/give you an idea/back of the envelope design--That is a typical sizing for a full time off grid use (every day). You can adjust the calculations based on your true needs--not my guesses--It will still be the same math--Just different numbers.

First, 60 kWatts is a big load -- If this is a "rate" (peak loads)--That would be 250 Amps @ 240 VAC. Lets look at something slightly more reasonable (at least for solar). And guess that your requirement is 60 kWH spread over a 12 hour period (actual time in theater).

• 60 kWH per day (amount of energy per day) / 12 hours per day = 5 kW = 5,000 Watt load (rate--average "Watt" load over 10 hours) (lights, AC, medical monitoring equipment, etc.)
• 60 kWH (amount of energy per 10 hour day) * 1/0.85 AC inverter eff * 2 days of battery storage (night, no sun, bad weather) * 1/0.50 max discharge (longer battery life, another day of "emergency" power available) * 1/48 volt battery bank = 5,882 AH @ 48 volt battery bank

A 5,882 AH @ 48 volt battery bank is not small.. You can look for deep cycle batteries in your region for pricing. For example in the USA with good quality battery bank:

https://www.solar-electric.com/rolls-2-ys-62p-flooded-deep-cycle-battery.html

• 24x 2 volt @ 5,054 AH cells in series = 48 volt @ 5,054 AH battery bank (close enough for our quick calculations)
• 24x $1,900 flooded cell lead acid batteries = usd $45,600 worth
• 24x 258.5 kg/battery = 6,204 KG of battery weight

Next, solar panels... Two calculations needed. First is based on charging the battery bank--Larger battery bank, larger array needed.

• 5,054 AH battery bank * 59 volts charging * 1/0.77 solar panel+controller derating * 0.10 rate of charge = 38,725 Watt array nominal
• 5,054 AH battery bank * 59 volts charging * 1/0.77 solar panel+controller derating * 0.13 rate of charge = 50,343 Watt array "typical" cost effective maximum
  

Solar panels typically cost around usd $0.50 to $1.00 per Watt... And you will need to mount these on metal frames (another $1 per Watt???).

And sizing the solar array based on your amount of sun (hours per day, by season) and your loads. Guessing Karachi Pakistan, fixed array facing south:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Karachi
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 65° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.58	6.25	6.41	6.50	6.13	5.74
Jul	Aug	Sep	Oct	Nov	Dec
4.99	4.99	5.80	6.06	5.55	5.38

A very sunny area... Lets look at July with 4.99 hours of sun per day:

• 60,000 WH per day * 1/0.52 off grid AC system eff * 1/4.99 hours per day = 23,123 Watt array for July "break even"

Now typically, to minize genset usage, would suggest a 2x larger solar array to cover occational cloudy days, bad weather, etc:

• 23,123 Watt "break even array" * 2 = 46,246 watt array with "safety factor"

So~46,246 Watt minimum array would be suggested (based on my above guesses).

A 10,000 Watt AC inverter and something like a minimum of 10% rate of charge (5,054*0.1=501 Amps) for battery bank.

Compared with a 25 kWatt diesel genset (USA website) for (roughly) usd $10,000 plus ~48 liters of fuel per day:

http://www.centralmainediesel.com/order/Kubota-25-kW-Diesel-Generator.asp?page=KB2500

At 6,000 Watt load--Less than 1 gph (less than 4 liters per hour)--Or 12 hours per day around 12 gallons or 48 liters per day for diesel...

If this is for full time off grid usage--Maybe solar is better (cannot get fuel) for full time off grid use... If this was a "backup system" (few weeks or month or so of use per year), then a genset is usually more cost effective...

Anyway--My guesses... Your questions?

-Bill