Autonomous Photovoltaic System Calculation – Step-by-Step Guide

Calculating an Off-Grid Photovoltaic System Step-by-Step Guide by Hellenic Energy. This guide presents a simple method for calculating power consumption from batteries and sizing a photovoltaic system. Each appliance has a small label indicating its electrical consumption. For example, a 21-inch television might show 220 volts and 0.5 Amps (A). This means it can consume 220 x 0.5 = 110 Watts. Some devices might only show 220 volts and not Amps. In this case, however, they will directly state the wattage. In the previous example, we would see 220 volts and 110 watts.
This means that the above electrical appliance will consume 110 watts for every hour it operates at full capacity. In practice, it might consume less, for example, if it operates with low brightness and low sound volume.

Step 1: Energy Saving.
An example is electric light bulbs. A 60-watt incandescent bulb consumes 60 watts for every hour of its operation. This means that if we have 5 such bulbs operating on average 6 hours a day each, their consumption will be 5 x 6 x 60 = 1,800 Wh per day. Compared to 15-watt economy bulbs, we have 5 x 6 x 15 = 450 Wh, which means a saving of 1,350 watts per 24 hours. With LED bulbs, we can achieve even greater savings.
When designing a photovoltaic system, the most basic and first thing we need to start with is to consider opportunities for energy saving.

Step 2: Consumption Calculation.
Multiply the Watts of each appliance by the number of hours it will operate. The sum of all these products will be our total daily consumption in Wh. Because there are losses in our system and also hidden consumptions, we multiply the previous sum by 1.5.

Step 3: Calculate the size of the accumulators (batteries)
Accumulators (batteries) indicate their capacity in Ah (Amp-hours). Thus, a 12-volt and 100 Ah accumulator provides 12 x 100 = 1,200 watts of direct current (DC) for 1 hour. The general rule is to not allow discharge above approximately 50% during normal use. Therefore, when purchasing accumulators for the photovoltaic system, we choose at least double the capacity we calculated. See, for example, the Pytes V5 LiFePO4 5kWh, a reliable choice for off-grid systems.
If we calculated that we need 600 Wh per day, we choose 12 volts and at least 100Ah for one day's autonomy. We usually also plan for 3 to 5 days without sunshine: 100Ah X 5 = 500Ah at 12 volts.
When using a Victron inverter 12 volts to 220 volts to power devices from a battery, the consumption from the battery is significantly greater due to voltage conversion. Also, the use of an inverter entails losses of 10% to 20%.

Step 4: Calculate the size of the solar panels.
If we have determined the size of the accumulators, we still need to calculate the size of the solar panels. For Greece, we calculate based on 3 hours of sunshine per day in winter (December) or 6 hours for summer use.
For a consumption of 600Wh/24h: we need 600/3=200Wp for winter-summer or 600/6=100Wp only for summer. For hybrid solutions with grid-tie capability, see the DEYE SUN-10K Hybrid Inverter 10kW.

Victron simulation program
Victron Energy offers a free online tool for designing ESS systems. Click & Design supports the new MultiPlus-II 4k5, 6k5 and MultiPlus 20k, with the ability to simulate systems up to 240 kW.

https://click-and-design.victronenergy.com/dashboard/sc