B
Brent Geery
- Jan 1, 1970
- 0
I've been 100% solar for about 8 years now, without any backup
capability, and do just fine. However, when I build my home, I'll
have a much larger system to deal with, so economics are going to have
to get much closer scrutiny, and I will have to finally add some sort
of backup generation capability.
Now my question is, how do you determine the best economic balance
between adding additional battery capacity vs the cost of running and
maintaining a generator? I'm not aware of anyone that actually tries
to calculate the most economic balance.
I did some calculations, and it's no surprise, that adding more
reserve capacity to the battery bank, increases the average lifetime
per kWh cost.
For example, assume a daily average demand of 10 kWh, and you have a
20 kWh battery bank that cost $1,200, and it provides 1.5 days
capacity, and will last 7 years with a normal daily DoD of 50%. This
energy will cost an average of $0.047 per kWh.
If we double the battery bank, to give 3 days capacity, we will
increase the bank life to about 10 years, lower the average DoD to
25%, and raise the cost battery bank cost to $2,400. Now, this energy
will cost an average of $0.066 per kWh. That's an increase of just a
hair under 50%!
If we double the battery bank again, to give 6 days capacity, we will
increase the bank life to about 15 years, lower the average DoD to
12.5%, and raise the cost battery bank cost to $4,800. Now, this
energy will cost an average of $0.088 per kWh. That's almost double
the per kWh costs for one day!
Now to figure the cost per kWh for generator created energy. There is
the cost of the generator itself, then maintenance costs, then fuel
costs. Let's assume we buy the Honda EG3500 for $1000, and that it
will last 10,000 hours, and that maintenance costs are 50% of the
purchase price, over the life of the generator. It's rated at 3000
Watts 125 VAC at full load, and we will assume the AC-to-DC conversion
of the Trace inverters battery charger function is 90% efficient. The
generator uses 0.5 gallons of gas per hour, and we will assume a gas
cost of $2.25 per gallon. Finally, we will assume that when we have
to charge with the generator, we will only give the batteries a bulk
charge (batteries up to 80% charged,) to avoid the long and
inefficient acceptance charge run times. That gives us a grand total
of $0.567 per kWh over the life of the generator.
So, how do we determine the point it's cheaper to run the generator
than increase the battery bank capacity further? To complete the
analysis, I guess, we would need to determine/estimate (some how!) the
number of times a year that there is multiple extended cloudy days.
But how to get this information?
For example, I might discover that there at average 40 times a year,
that there is extensive clouds extending more than one day, ten times
a year with extensive clouds extending more than two days, and 3 times
with extensive clouds extending more than 4 days.
So, using the above weather examples:
40 times yearly of >1 days clouds, if I have a 1.5 day battery bank
capacity, would require 400 kWh a year of generation at a cost of
$226.80, or $1,587.60 over the 7 year life of the 1.5 day capacity
battery bank.
10 times yearly of >2 days clouds, if I have a 1.5 day battery bank
capacity, would require 200 kWh a year of generation at a cost of
$113.40, or $793.80 over the 7 year life of the 1.5 day capacity
battery bank.
4 times yearly of >3 days clouds, if I have a 1.5 day battery bank
capacity, would require 120 kWh a year of generation at a cost of
$68.04, or $476.28 over the 7 year life of the 1.5 day capacity
battery bank.
Adding these up, in our example, give a total generator operation cost
of $2,857.68 over the 7 year life of the battery bank. This is more
expensive (by $457.68) than the cost of doubling the battery bank to 3
days of capacity, so would not be the best economic choice in this
example.
Using the same example, but using a battery bank with 3 days capacity:
40 times yearly of >1 days clouds, $0, as the battery bank carries us
through it.
10 times yearly of >2 days clouds, $0, as the battery bank carries us
through it.
4 times yearly of >3 days clouds, if I have a 3 day battery bank
capacity, would require 120 kWh a year of generation at a cost of
$68.04, or $680.40 over the 10 year life of the 3 day capacity battery
bank.
$476.28 is cheaper than the $1200 it would take to give us another 15.
days of capacity (4.5 total then,) thus would be the most economical
choice in this example.
Comments please. I my next message, I'd like to debate adding PV
capacity vs running a generator.
capability, and do just fine. However, when I build my home, I'll
have a much larger system to deal with, so economics are going to have
to get much closer scrutiny, and I will have to finally add some sort
of backup generation capability.
Now my question is, how do you determine the best economic balance
between adding additional battery capacity vs the cost of running and
maintaining a generator? I'm not aware of anyone that actually tries
to calculate the most economic balance.
I did some calculations, and it's no surprise, that adding more
reserve capacity to the battery bank, increases the average lifetime
per kWh cost.
For example, assume a daily average demand of 10 kWh, and you have a
20 kWh battery bank that cost $1,200, and it provides 1.5 days
capacity, and will last 7 years with a normal daily DoD of 50%. This
energy will cost an average of $0.047 per kWh.
If we double the battery bank, to give 3 days capacity, we will
increase the bank life to about 10 years, lower the average DoD to
25%, and raise the cost battery bank cost to $2,400. Now, this energy
will cost an average of $0.066 per kWh. That's an increase of just a
hair under 50%!
If we double the battery bank again, to give 6 days capacity, we will
increase the bank life to about 15 years, lower the average DoD to
12.5%, and raise the cost battery bank cost to $4,800. Now, this
energy will cost an average of $0.088 per kWh. That's almost double
the per kWh costs for one day!
Now to figure the cost per kWh for generator created energy. There is
the cost of the generator itself, then maintenance costs, then fuel
costs. Let's assume we buy the Honda EG3500 for $1000, and that it
will last 10,000 hours, and that maintenance costs are 50% of the
purchase price, over the life of the generator. It's rated at 3000
Watts 125 VAC at full load, and we will assume the AC-to-DC conversion
of the Trace inverters battery charger function is 90% efficient. The
generator uses 0.5 gallons of gas per hour, and we will assume a gas
cost of $2.25 per gallon. Finally, we will assume that when we have
to charge with the generator, we will only give the batteries a bulk
charge (batteries up to 80% charged,) to avoid the long and
inefficient acceptance charge run times. That gives us a grand total
of $0.567 per kWh over the life of the generator.
So, how do we determine the point it's cheaper to run the generator
than increase the battery bank capacity further? To complete the
analysis, I guess, we would need to determine/estimate (some how!) the
number of times a year that there is multiple extended cloudy days.
But how to get this information?
For example, I might discover that there at average 40 times a year,
that there is extensive clouds extending more than one day, ten times
a year with extensive clouds extending more than two days, and 3 times
with extensive clouds extending more than 4 days.
So, using the above weather examples:
40 times yearly of >1 days clouds, if I have a 1.5 day battery bank
capacity, would require 400 kWh a year of generation at a cost of
$226.80, or $1,587.60 over the 7 year life of the 1.5 day capacity
battery bank.
10 times yearly of >2 days clouds, if I have a 1.5 day battery bank
capacity, would require 200 kWh a year of generation at a cost of
$113.40, or $793.80 over the 7 year life of the 1.5 day capacity
battery bank.
4 times yearly of >3 days clouds, if I have a 1.5 day battery bank
capacity, would require 120 kWh a year of generation at a cost of
$68.04, or $476.28 over the 7 year life of the 1.5 day capacity
battery bank.
Adding these up, in our example, give a total generator operation cost
of $2,857.68 over the 7 year life of the battery bank. This is more
expensive (by $457.68) than the cost of doubling the battery bank to 3
days of capacity, so would not be the best economic choice in this
example.
Using the same example, but using a battery bank with 3 days capacity:
40 times yearly of >1 days clouds, $0, as the battery bank carries us
through it.
10 times yearly of >2 days clouds, $0, as the battery bank carries us
through it.
4 times yearly of >3 days clouds, if I have a 3 day battery bank
capacity, would require 120 kWh a year of generation at a cost of
$68.04, or $680.40 over the 10 year life of the 3 day capacity battery
bank.
$476.28 is cheaper than the $1200 it would take to give us another 15.
days of capacity (4.5 total then,) thus would be the most economical
choice in this example.
Comments please. I my next message, I'd like to debate adding PV
capacity vs running a generator.