Page 308 - Applied Photovoltaics

where S is the number of days of storage, L is the average load per day, DOD is the
allowable depth-of-discharge for the batteries and Ș out is the storage-to-demand path
efficiency.

Figure G.1. Number of days of battery storage as a function of loss-of-load
probability for design 2 (©1987 IEEE, Chapman).

For Ș out , since we are using an AC load, we need an inverter. If we allow for a peak
load of 1000 W ac , our inverter should be rated about 20% higher, i.e. 1200 W ac . The
inverter efficiency will vary as a function of load and needs to be determined in
conjunction with the anticipated load profile throughout each day. A typical average
daily inverter efficiency of 0.76 is assumed.
The other contributor to Ș out is the battery controller which, because of parasitic
power drains, will be in the vicinity of 95% efficient. Losses associated with charge
leaving the battery need not be considered here, since the rating of battery capacity is
in terms of charge obtainable from the battery. Therefore, we consider battery
inefficiencies only in terms of charge being stored in the batteries.
Ș out = 0.95 × 0.76 = 0.72
L = 5 kWh/day
S = 5.80 (for design 2)
DOD = 0.8 (using a deep-cycle type battery)

Therefore
CAP = 5.80 × 5 / (0.8 × 0.72)
= 50 kWh.

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