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.

Solar power for homes

Power shortage and blackouts in various parts on India in 2012 made me think about solar power generation at home. I had various questions in my mind: How to produce solar power?, What is the cost and return on investment? etc.

I will try to answer the first question in this article.

Introduction

Electricity can be generated from sunlight using photovoltaic cells[1]. The amount of power generated depends on various factors like solar radiation received, efficiency of the solar cells, temperature etc. 

Photovoltaic cells produce DC power. Most of the home appliances run on AC power. So a DC to AC inverter is required to use the solar power. 

As solar power is not available during night, batteries are required if backup is desired.

Various options for using solar power at home

Solar power system can be of different types: off-grid(not connected to external electricity supply) or on-grid, with or without battery backup. I've considered the following:

1. Off-grid + battery backup
2. On-grid - battery backup
3. On/Off grid + battery backup

Off-grid + battery backup

The power generated by solar panel is used charge the batteries. A charge controller is required to cut-off the power when the batteries are fully charged. The output of the batteries is fed into a inverter which converts DC(e.g: 12V,48V etc) to 230V, 50HZ AC. This can be fed to the home appliances.

On-grid without battery backup

This system makes use of an on-grid inverter. This works in 3 different ways:

1. If power from panel == load, no power is drawn from the grid.
2. If power from panel > load, remaining power is supplied to the grid.
3. If power form panel < load, balance power is drawn from the grid.

Power can be supplied to the grid only if the local power company permits you to do so. This is called net-metering and is available in some countries(now aware of such policy in India).

On/Off Grid with battery backup

As the name suggests, this is a mix of the first two options. The difference from option 2 is that this charges the batteries first and supplies to the grid only after the batteries are fully charged. The power from batteries can be used when both solar power and grid power are unavailable.

Comparison of various options

I've compared the various options on the following parameters:

1. Investment cost
2. Utilization (whether 100% solar power received can be used)
3. Maintenance cost
4. Limitations

	Installation Cost	Utilization	Maintenance Cost	Limitations
Off-grid	Very High (due to batteries)	< 100%	High (due to batteries)	Requires very high capacity to replace grid (4KW for every 1KW load)
On-grid	Medium	100%	Low	Net-metering is not available in India
On/Off-grid	Medium to high (depends on batteries)	Close to 100%	High (due to batteries)	Net-metering is not available in India.

Conclusion
An off-grid system is too expensive and seems to impractical for home use. On-grid system is the least expensive and utilizes all the power generated but requires a government policy to buy the power back. If backup is desired an On/Off-grid system with limited backup (to keep the cost under control) would be recommended.