Pages

Thursday, November 1, 2012

Solar power for homes: Some calculations

In my previous post, I considered three options for using solar power at home and concluded that an off-grid system is too expensive. Well, why is it expensive? On what basis did I say that? 

How much does it cost to replace a grid connection with off-grid solar power?
I will take the following example
  • Consumption of 3600 Units (kWh) per year
  • 3 days backup
  • CUF (Capacity Utilization factor) of 15%. See this link for CUF in various parts of India. Number of units produced = Installed Capacity (in kW) * 24 * CUF. For e.g, a 1 kW panel with 15% CUF produces 3.6 kWh per day.
  • 70% battery usage (batteries cannot be fully drained)
Calculations

How many solar panels?
1kW panel produces about 3.6 kWh per day (15% CUF). kWh produced in 1 year = 3.6*365 = 1314.
So ~ 3 kW panels are required.

How many batteries?
Average daily requirement = 10 kWh.
A 12V, 200Ah battery holds about 2.4 kWh (Voltage * Ah/1000). Considering a cut-off at 70% this comes down to 1.68 kWh.
So the number of batteries required is
--  6 for 1 day backup
-- 12 for 2 days backup
-- 18 for 3 days backup

What is the rating of the Inverter?
Peak load may be higher than 3kW. So we need at least a 5kW inverter. Inverter also should support high initial surge current (for e.g if you run a water pump).

How many charge controllers?
This depends on how the panels are divided into banks.  I will assume a 48V/30A solar charge controller. ~3 would be required (48 * 30 =  1440 W. So for 3kW, at least 3)

Cost (INR)
Total cost = Cost of (Panels + Batteries + Inverter + Charge controller + Installation + Other(transport etc))
  • Cost of panels = ~60,000 /kW (See this link. I've added 50% for retail price) = 1,80,000
  • Cost of batteries = 11,000 per battery. 1 day backup price = 66,000. 2 days = 1,32,000, 3 days = 1,98,000
  • Cost of inverter = ~50,000 (Couldn't get an exact price online)
  • Cost of charge controllers = 20,700 * 3 = 62,100
  • Other costs: Structure for installing panels, Transport, Cables, Combiners,   Man power etc 
  • Total cost = 1,80,000 + 1,98,000 + 50,000 + 62,100 + other cost (say 10% extra) = ~4,90,000 + 10% = ~ 5,40,000
Even if we consider just 1 day backup, the cost is around 4,00,000.

On an investment of 5,40,000, one can save (if all goes well. There are various other risks that I have not considered) 3600 kWh per year. Assuming a grid price of INR 5/kWh, the annual return is INR 18,000 (3.3%). This, for me is "very expensive" and not a viable option.



1 comment:

Brielle Franklin said...

What a great break down. After the Hurricane I have been looking into commercial solar power. The hurricane took away our power for over a week. With the solar panels we could have saved our selves during this busy busy time of year. Thanks so much for this information.