Designing a solar system involves the matching of site requirements and limitations matched to electrical outcomes.
There are two ways to approach a design – one is to have an export agreement and maximize the site – two is to match the design of the system to the sites usage profile.
Due to current and foreseeable low export rates it makes no sense to design for excessive capacity. The only time this may be required is for future increase – or allowance for later hybrid battery installation.
The easiest way to determine day time usage starts with the power bill. By looking at the number of kWhrs used per day an estimate can then be determined by applying a usage pattern of say 50-70% of the total.
So take the number of kWhrs from the main tariff total – divide it by the number of days billed and then take what is commonly 50-70% of that for a domestic scenario.
It is important to remember that devices such as pool filters and electric hot water systems make excellent dumps for excess solar PV and can easily be placed on a timer relay to ensure that happens.
When looking at a business most power usage will be during the day – generally as high as 90% – however for very large customers it is always important to look at their bulk purchase rate to ensure financial viability.
So lets say a building uses 60kWhr per day and we estimate that 50% of that is daytime usage. To produce an average of 30kWhr per day the building would need 30 divided by 4kWhr per kW installed = 7.5kW
If they were to then put up enough panels to allow for charging of batteries large enough to supply the entire installation they would need roughly 15kW of solar PV feeding into a hybrid unit with 30kWhr of storage.
Remember that its always good to have a little head room!
Once a size is determined one can then match an inverter and panels to the requirements. It is important to ensure the string and array design voltages match the MPPT sweet range of the inverter.