How to calculate energy consumption

In order to design a Photovoltaic system, the following parameters must be consider

Note: Before any installation, the inspection for ensures that there is an area available is mandatory. This is for the installation of the PV system and that there are no shading phenomena such as trees, houses, poles, etc. below is the list with all details which are considered.

• The energy required daily in kilowatt-hours (kWh)
• Calculation of the photovoltaic power (Wp = Maximum power)
• Battery capacity in Ah
• Charge controller
• Inverter sizing according to the DC side voltage and the AC side power
• Size of electric cables
• Short circuit and overload protections
• The ratio between the efficiency of the photovoltaic panel and the available surface.

Daily energy requirement

When you want to build an off-grid photovoltaic system, the first parameter to keep in mind is the average energy requirement in kilowatt-hours (kWh) per day.

 First, I clarify what means Power and Energy.

• Power is that value in Watts given by the formula P = V x I.
• Electric energy is the power multiplied by the time and given by the formula E = P x t (kWh).
• The kWh is the unit of electricity consumption measurement, which is the quantity that we usually find in the electricity bill.
• Although less used, we can also use the Watt-hour (Wh) defined as the total energy provided by the power of one watt (W) maintained for one hour (h).

To calculate the Energy consumed daily, I need to know the consumption in Watts of each device that I want to power with the photovoltaic (the power in Watts can be found on the label placed under the device or in the instruction booklet) and multiply each value with time that I intend to use it, then making the sum.

For example, a 70 Watt light bulb kept on for 2 hours will consume 140 Watt-hours (Wh), and a laptop that consumes 30 watts, if I keep it on for 6 hours I will have used 180 Watt-hours.

The total will be daily requirement will be given by the sum of the 140 Wh of the light bulb, plus the 180 Wh of the computer, for a total of 320 Wh.

To make it easier to calculate consumption, we can add the Powers in Watts regardless of the supply voltage.

In summary: we can add the power of a 50 Watt light bulb + one always of 50 Watt + one of 100 Watt, and the result will be a total power of 200 Watts, regardless if powered at 220 V or 12 V. Figure 1 shows exactly this.

Figure 1 - Bulb Lamps

To learn more: Difference between  Power and Energy, and between kilowatts and kilowatt-hours.

SIZING EXAMPLE

For the sizing of the photovoltaic system not connected to the electricity grid, it is a good idea to respect a general rule: the energy produced must be greater than that consumed.

For example, to create a workstation for two people connected to the Internet as smart working, not connected to the electricity grid but powered only by photovoltaics, should be available: 2 Laptops, 1 Router, 1 printer, 1 Switch, 2 led lamps.

To calculate the daily energy needed to power the workstation, we will have to add up the various powers in Watts that I find written on the label under the device or in the instruction book. Then, we will multiply them by the hours of use (see Figure 2 below)

Figure 2 - Typical devices of Installation

Let's calculate the theoretical consumption:

The total simultaneous power for their operation is 375 Watts, and the absolute energy requirement is 1660 Watt-hours which we approximate to 1700 Watt-hours (1.7 kWh).

CALCULATION PHOTOVOLTAIC POWER

Figure 3 - Typical Solar Panels Installation

First, you need to establish the period of use:

• Summer only
• Winter only
• all the year

After that, consult the tables or use the ENEA, PVGIS simulator ( or others) to check the irradiation in kWh / m² year and obtain the equivalent daily sunshine hours (hse / g).