The above solution identifies that a 7.9 kW nameplate rated solar array** would be suitable for completely offsetting 1000 kWh per month average electrical usage under a standard net metering agreement.
Feed in Tarrifs: Sizing battery-less grid tie systems against electrical load demand is done similarly to a net metering application, with one important distinction: The feed in tarrif must be weighed into the calculation of solar system size. The easiest way to do this is to divide your average monthly electrical bill in dollars by the feed in tarrif price. For instance, if we use the Ontario micro-FIT roof mounted solar price of 80.2 cents per kWh we discover that:
($ 250.00 per month bill) / ($ 0.802) = 312
The above solution identifies that a solar system producing 312 kWh per month (on average) would provide an income of $ 250.00 per month (on average). We can now take this number and the average sun hours per day to define your solar system size. For example:
(312 kWh per month / 30 days) / 4.2 hrs*= 2.5
The above solution identifies that a 2.5 kW nameplate rated solar array** will provide an average of 312 kWh per month in electricity. This energy would then be sold to the utility to offset your electrical bill of $250 per month.
* 4.2 hrs based on values obtained from Photovoltaic (PV) potential mean daily global insolation of a south-facing, tilt = latitude fixed array in London Ontario Canada.
**The nameplate rating does not take into consideration any environmental or wiring losses.
Grid Tie with Back-up: A grid-tie system with battery backup feeds excess solar electricity to the grid (possible in areas that allow net metering) and provides backup power when the utility grid is down. With this type of system you sacrifice some power generation efficiency in exchange for having power when there is a utility power failure. The amount of backup power you have depends on the size of the battery and electrical loads that draw on them.
Tip- Grid tie battery banks are best kept small (4 to 12 hrs back-up maximum). By design, these banks are typically kept at float voltage during regular system operation, and are only cycled during periods of power outage. The average length of time of a power outage is less than 4 hours. During this time, your critical loads have been seamlessly maintained by your system. No back-up system can realistically be expected to maintain loads indefinitely. For extended blackouts, a small, good quality back-up generator can be employed to recharge your battery bank and allow the system to continue operating. Most (if not all) battery based grid tie inverters are capable of accepting a good quality generator input to feed power to your loads while charging batteries at the same time. This system design keeps the overall cost of your system low while maximizing the system productivity.
Off Grid: This type of power system is independent of the utility grid. Owners of this type of system often use a gas or diesel generator for backup when the power system does not meet all of the load requirements. A variant on this system is sometimes called parallel co-generation. In this application you allocate a given number of loads to a solar system running "in parallel" with the utility. This type of system can not feed excess solar energy to the utility, but can use the utility for backup power when solar resources are limited.
Sizing these systems requires:
- a complete load evaluation of every piece of electrical equipment that is commonly used by the system,
- a solar array capacity based on matching load against your lowest solar resource months resources, keeping in mind that the lowest resource month is dependant on application. Summer-only cottages can be sized against summer resources, whereas year round loads need winter resource design.
- a battery bank design that provides autonomy to keep your loads up and running even during poor weather, 3 to 5 days autonomy typical for cottages, up to 10 days typical for telecom or other critical applications.
