Your customer may not understand how efficient your system really is, but it is
important that you do. You may determine the appropriate factors by reviewing all
of the system numbers based on all of the other system factors of load, solar com-
ponent design, and installation quality and multiply them by the system efficiency.
Determine Autonomy in Days
System autonomy is expensive. Efforts to maintain it can result in damage to the
batteries over time, and the requirement to have it will increase the size of the
array or require an alternative for energy backup. It will take time and discussion
to come up with the right combination of budget, system size, and alternative
energy generation or usage reduction to finalize a design.
Determine Total Battery Amp-Hours from Watt-Hours
This is an exercise that will require evaluating system battery voltage, battery
selection, and conversion from watt-hours to ampere-hours. Consult with your
battery supplier and manufacturer to determine if they have a battery sizing tool
that allows you to review a number of combinations instead of hand-calculating
every potential option.
Sizing off-grid and grid-tied battery systems takes time to master. It requires
a solid knowledge of the equipment and environment and how they come together.
Discussions with customers are also important. The process is complex and
difficult to discuss unless you have the answers. Once they understand the cause
and effect of their decisions, they will make better-informed decisions.
If the communications are not clear or thoughtful, the customers may be
unhappy with their decisions and blame the messenger.
Sample Calculations for Sizing an Off-Grid, Standalone PV System:
Step 1: Determine the Demand Load in AC watts (Use Sizing Chart)
Total Energy Usage PV System Loss Factor Total Daily Watt-Hours
(20% loss @ 80% efficiency) Required
Load (Watt-Hours) (1) × Loss Factor (2) = Watt-Hours Per Day
The watt-hours per day are load-budget based and will vary based on daily usage
and season. Understanding the real load is important in this step.
8,500 (1) × 1.2 (2) = 10,200 watt-hrs/day (adjust)
Step 2: Battery Storage Requirement
Autonomy (4) × Daily Load (adjust) = Battery Storage (watt hrs)
No-Sun Days × Watt hours/day = Total watt hours Required
3 days (4) × 10,200 watt-hrs/day = 30,600 watt-hrs
128 ADVANCED PHOTOVOLTAIC INSTALLATIONS
The battery storage in total watt-hours required for three days of autonomy tells
you how many watt-hours are required.
It does not tell you what the total watt-hour or amp-hour requirements are.
Those will be based upon DOD, which will need to be converted into amp-
hours at a specific hourly rating. This fine-tuning will tell you how many batteries
you will need at a specific voltage to meet the required 30,600 watt-hours
rating above.
Calculations of the battery requirements in amp-hours will need to be
matched against the number of batteries at a specific size in amp-hours. It will
require factoring in the nominal voltage required (12 Vdc, 24 Vdc, or 48 Vdc) to
meet both series and parallel requirements for the overall battery array, and it
may take looking at a number of battery options to meet the requirements you
have set for autonomy.
Step 3: PV System Array Sizing
Watt-hours per day × Battery Loss Factor ÷ Insolation = Array in DC watts
10,200 watt-hrs/day × 1.2 ÷ 6 peak sun hours/day (2) = 2,040 DC watts
This step indicates the number of DC watts required to meet the daily load for
battery storage.
It is 20 percent more energy than you need to run the load itself to fill the
batteries once.
It will require looking at the PV panels you are considering and finding an
acceptable size and voltage for string sizing that will come to the next step above:
2,040 DC watts of panels. Oversizing is better than undersizing, especially as you
continue the system sizing exercise.
It will then need to be upsized to bring the batteries to full charge because
you will always use some energy each day to bring the batteries to a 100 per-
cent SOC.
If you use a generator, it can provide the bulk charge to bring the batteries up
to where they need to be to top them off with PV. This must be considered in the
budget process, because although a generator may seem relatively inexpensive to
install, when you add the fuel and O&M over time, your cost for a kilowatt-hour
will be from $1.50 to $2.50. This is more than the cost of the energy from your
solar panels. Consider these numbers carefully, especially as fuel costs go up.
If the autonomy is for three days, and you wish to bring the system to full
charge, you will need to upsize the array by a predetermined percentage. Leaving
batteries discharged for extended periods of time will damage them and shorten
their life.
If you wish to have the batteries recharged in three days, you will need to
upsize the array by a third from 2,040 DC watts to 2,734 DC watts at full sun.
CHAPTER 6 Standalone PV Systems 129
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