We will assume here you are going to remain grid connected and want to take advantage of a net metering arrangement.Your electric bill probably goes up and down monthly or seasonally, therefore it’s important to consider the details of your net metering rules. If your net metering rules only allow carry forward of your electricity production on a monthly basis, then you will need a system big enough to offset your biggest monthly bill if you want to entirely offset your bill. Take your total kWh usage for the month divide by the total hours in the month (31 x 24 = 744) to get the kW’s you use on an hourly basis on average for the month. This is not the size of system you need unless you know the system will run at full capacity 100% of the time. This depends upon the flows experienced at the time. However, to get a system to run 100% of the time, depending upon your flow, may require the system to be smaller, such that it will not produce the amount of kWh’s to offset your bill.
It may be necessary to install a bigger system that will produce more kWh’s but not run 100% of the time. As you increase the size of the system the capacity utilization will continue to decline. To determine the size of system you need to install you have to estimate the capacity utilization rate of the larger system. If we assume a utilization of 65% overall, then take the kW’s you use on an hourly basis and divide by .65 to get the size of system you need. Keep in mind this is just an estimate. The accurate way to determine the size of system you need is to look at the estimated flows and heads for your site.
If your net metering rules allow carry forward and banking of excess production on an annual basis then your system likely doesn’t have to be as big to entirely offset your bill. Ideally, we would look at your total kWh use for the year, and then based on your water source daily heads and flows figure out the size of system that can take advantage of those heads and flows and offset your bill on an annual basis. However, without knowing those heads and flows which vary, all we can do is look at your total kWh use annually, divide by the total number of hours in the year (365 x 24 = 8760 hours) to get the kW’s you use on an hourly basis on average. This is not the size of system you need unless you know that your flows are high enough to allow this system to run at full capacity 100% of the time. This is where we have to start doing some flow measurements to see if this is true or not. If it’s not, then we have to make some sort of assumption as to the overall capacity utilization a system would experience as we did before. If we assume 65% as we did before, then divide the kW’s you use on an hourly basis on average for the year as determined above by .65 to get the size of system you would need to offset your bill.