IEEE Electrification Magazine - June 2016 - 14

Solar Panel
dc

=
ac

Inverter

ac Load

ac
~

Grid

dc
Battery

Figure 4. A solar-ac system with an inverter for an ac home.

125-Wp solar panel was assumed to be producing
540 Wh of solar energy every day. Solar power is
available from 6 a.m. to 6 p.m., as shown in Figure 3.
b) Table 1 presents the loads assumed in ac and dc
homes along with their power consumption. It also
gives the number of appliances used in each home
along with their operational hours.
c) Let us now take a deeper look at the solar-cum battery
systems that an ac home (using ac appliances) and a dc
home will use. Battery backup systems called inverters

are widely used in Indian ac homes today. Rooftop solar
PVs can be added to such systems, and we will call
them solar-ac systems. These systems, shown in Figure 4, would have an ac-grid input, a solar-PV input, a
two-way connection to the battery for charging and
drawing out power, and an ac output to drive the home
load. Table 2 presents the losses associated with such
systems assuming the power used is less than 500 W.
Such a system has zero GtoL loss when the grid directly
drives the load. However, when solar power directly
drives the load (without going through the battery), the
StoL loss is 15% primarily due to an ac- to dc-converter,
which also ensures that solar panel operates at MPP
and is synchronized to the ac-grid power. When the
grid charges the battery, the GtoB is 15%, and when the
solar panel charges the battery, the StoB loss is 30%
(due to two converters). Finally, the BtoL loss is about
25% with a 10% loss being contributed by the lead-acid
battery. Measurements were carried out on several
available commercial systems with these features at
the Indian Institute of Technology Madras (IITM)
(Center for Decentralized Power Systems), and the losses, given in Table 2 for the solar-ac system, are the best
results obtained from any of these systems.
A dc home, on the other hand, will use a system,
such as inverterless, shown in Figure 5. The system has
been designed at IITM and is being commercialized by
Cygni Energy Private Limited (see http://www.cygni.com/
products-2/ for product-related information). Special care
has been taken to minimize the losses and yet keep the

TABLE 1. Device power consumption, the typical number of appliances in a small home, and usage.
Wattage (W)
Appliance

AC-Powered Appliances
in ac Homes

DC-Powered Appliances
in dc Homes

Numbers

Number of Operational
Hours/Day

Fan1

Bulb

Tubelight

Phone

6.5

1ac-powered induction motor fan at an average speed was used in an ac home, and a dc-powered BLdc fan was used in a dc home.

TABLE 2. Power losses within the system.
GtoB2
(%)

BtoL3
(%)

StoL4
(%)

GtoL5
(%)

ac system 30

dc system 5

System

StoB1
(%)

AC Home
Efficiency
(%)

DC Home
Efficiency
(%)

GtoL

100

GtoL via a battery

63.7

76.5

3Battery-to-load (BtoL) loss.

StoL

5Grid-to-load (GtoL) loss.

StoL via a battery

52.5

85.5

1Solar-to-battery (StoB) loss.
2Grid-to-battery (GtoB) loss.
4Solar-to-load (StoL) loss.

TABLE 3. Efficiencies in delivering power
from the source to the load at homes.

I E E E E l e c t r i f i c ati o n M agaz ine / j un E 2016

http://www.cygni.com/

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2016