When constructing a stand-alone photovoltaic system, it is important to well-sized it with respect to the consumption needs. It is therefore essential to ask, right away, the right questions:
1. How much energy do I want to have?
This is always the first question when you size a PV system…
Energy is not power: a light bulb with a power of 50 Watt (W) that stays on 6 hours will consume 50 x 6 = 300 Watt-hour (Wh). Therefore you have to know the powers of individual utilities and how many hours per day they work. Size the batteries according to the total amount of energy required and take into account the hypothetical days with bad weather. Without the Sun the system can’t charge the batteries! If we design a system that works even with 3 days of bad weather, we should multiply by 3 the amount of batteries calculated for a single day.
> Can I use all the energy in the battery?
It’s not possible to use the energy stored in the battery up to 100%. For lithium it can reach 90% while for lead it is better not to go beyond 50%.
For example, if we wanted to grant 6 hours of 50 Watt light for 3 consecutive bad weather days, using lead acid batteries, I would need 50 (watts) x 6 (hours) x 3 (days) / 0.5 (maximum% of usable battery) = 1800 Wh.
2. How much power do I need?
If the system is in AC we should install an inverter between the batteries and the rest of the utilities. To calculate the power of the inverter we must add all the powers of the single utilities that we think are lit at the same time. Choose the maximum total power as reference.
If, for example, we have 5 lights of 50 Watt, a 500 Watt fridge and a 60 Watt TV, you will get 810 Watt total. However, medium-sized appliances can absorb large amounts of power for a few moments. For this, it is necessary to over-size the inverter power. In this case, a 2000 Watt inverter would be the suggested choice.
3. How many PV modules do I need to install?
If the system works all year, the worst yield period should be taken as reference: in January (if we are in Italy), to install enough modules to cover the daily discharge of the battery.
If, on the other hand, you use the system during the summer or in very sunny places, you can take other references.
The number of modules and their serial and parallel connection must be in the acceptance range of the charge controller (the device between the modules and the batteries). To manage the charge and optimize the yield of the modules, the voltage and current of the regulator must be suitable for the voltage of the batteries and for the voltage and the current of the modules.