Off-Grid System Sizing Calculator
Below are some guidelines to help you correctly size
your off-grid solar system. The average grid-tied
home uses 20kwh/day while the average off-gird
cabin/home uses 5kwh/day. Obviously, this indicates
that the off-grid systems require a careful selection of
appliances in order to function without degradation.
Basic PV system design:
The first step in sizing a solar system is to determine
the expected daily kwatt-hour usage of the system.
Basically this is the number of watts x the number of
hours that each electrical device uses each day. For
your convenience use the “Load Estimate Worksheet”
to calculate your system kwh/day requirements. Once
you have an estimated kwh/day number, the
calculations are quite straight forward.
Kwatts of PV array required = kwh / daily sun hours /
derating factor
kwh = daily electrical wattage, daily sun hours = the
average seasonal peak (noontime equivalent) sun
hours in your location. Vancouver is about 5hrs. See
the NREL Redbook for the average sun hours at your
location. See also the PVwatts website which includes
the data from NREL and automatically calculates
system solar production.
DC derating factor = a factor which accounts for real
world inefficiencies which affect a battery based PV
system (e.g., heat, humidity, bird droppings, wire
losses, inverter losses, battery losses). Using a
derating factor of .67 to .77 will result in a good safe
estimate of the power that you will obtain from your
system.
Likely, your system output will be higher but it’s better
to estimate a lower system output and be pleasantly
surprised when your system produces a higher than
expected output rather than be disappointed by poor
performance.
PV sizing example (How many solar panels do we
need?):
Now let’s try out the above formula using an example.
Assuming your PV worksheet calculations show that
your system will use 5kwh/day, and your average
daily sun hours for your location is 5, and you plan to
use 190 watt 24 volt panels, let’s do the math:
5kwh/5/.67 = 1.49 kw PV array (1,490 DC watts)
1490/190(watts/panel)=7.8 panels (round up to 8)
4 series strings of 2 panels = 8 modules with each
string producing 48 volts
Battery Bank Sizing:
A battery bank is sized to produce electricity when the
solar modules don’t produce optimum output. A good
rule of thumb is to size the system to provide power
for 3 to 5 days (days of autonomy). Using 3 days is a
good number, while selecting 5 days may result in a
battery bank that is quite expensive.
Continuing with our example:
We first need to convert watt-hours to amp-hours
since that’s how batteries are rated:
5000 watt hours per day/24 (our system voltage) =
208 Amp hours required per day. You may want to
increase this to keep from deeply cycling your
batteries and extend their life.
208 amp-hours x 3 (days of autonomy) = 624 Ah @24
volts
Solar charge controller sizing:
The size of a charge controller is based on the PV
array voltage, desired system battery voltage and the
short circuit current of the solar panels. Make sure
that the solar charge controller that you use has
enough capacity to handle the current from the PV
array.
Power = Volts x Amps
Here we know the power is 1490 watts, the battery
bank is 24 volts, so:
1490 watts/26.6 volts= 56A
In this case we could choose an 60 Amp MPPT
Charge Controller.
Inverter Sizing:
To find the correct inverters size we first determine
the AC watts that will be provided by the Inverter.
Using our example of 1490 DC watts we can convert
back to AC watts by multiplying by the derating factor
that we used earlier. So AC system watts = 1490 x.67
= total system watts = 998 AC watts. Thus, an inverter
capable of handling 1000 watts would work providing
your loads do not exceed its capacity. A great choice
for an off-grid inverter would be a 2000w true sine
wave unit since it would provide some upside
potential if you decide to add more solar panels in the
future.
Hours to Discharge Capacity as percent of 20 hr rating
20 100%
10 84%
5 67%
2 56%
1 47%