Home Solar Kit by Consumption: Three Examples Worked Out Step by Step

What we’ve assumed

So every figure can be redone, these are the starting points. If yours are different, change the number and repeat the calculation:

• A home with no grid connection, relying on the kit alone.
• Madrid’s sun in its worst month, December: 1 kWp of panels gives 3 kWh a day, according to the World Bank’s Global Solar Atlas. The table of 24 cities has the figure for yours.
• 450 W panels on a roof: 4% less, as in the Atlas’s own examples.
• Lithium batteries of which 80% is used, as Australia’s national science agency CSIRO recommends so they last longer, and which give back 85% of what goes in, according to the U.S. national energy laboratory.
• One day of battery without sun. The U.S. Department of Energy says to size it for the cloudy spells expected where you live: if they last several days, multiply.
• An inverter with 85% efficiency, the figure our calculator uses.

The three kits

The large home uses about as much as the average home in the European Union: around 300 kWh a month, according to the Odyssee-Mure project. What each home runs at the same time, with wattages from the official table of Argentina’s energy regulator (ENRE):

• Small: fridge-freezer, 4 LED bulbs, a fan, 2 chargers and the TV.
• Medium: the same with 6 bulbs, 2 fans and 3 chargers, plus the microwave and the washing machine.
• Large: the medium home’s list with 8 bulbs and 4 chargers, plus a 2,200-frigorie (about 8,700 BTU) inverter air conditioner and a ½ HP water pump.

The maths, step by step (medium kit)

1. The panels. What 1 kWp gives per day, after losses: 3 × 0.96 (roof) × 0.85 (battery) = 2.45 kWh. The home uses 5 kWh, so you need 5 ÷ 2.45 = 2.04 kWp. With 450 W panels: 2,040 ÷ 450 = 4.5, rounded up, 5 panels. It’s the calculation our solar panel calculator does with the “off-grid” option.

2. The batteries. They must supply 5 kWh through the inverter: 5,000 ÷ 0.85 = 5,882 Wh. Since only 80% is used: 5,882 ÷ 0.8 = 7,353 Wh. To convert to amp-hours, divide by the voltage: 7,353 ÷ 24 = 306 Ah.

3. The controller. The panels’ watts ÷ the battery voltage: 5 × 450 = 2,250 W, and 2,250 ÷ 24 = about 94 A. It’s a rough calculation; the installer fine-tunes it with the panel and controller spec sheets.

4. The inverter. It doesn’t depend on what you use over the day, but on what you run at the same time: 200 + 54 + 180 + 15 + 90 + 800 + 500 = 1,839 W continuous. When it starts, the fridge briefly draws about 600 W more, so it must handle 2,439 W of surge. It’s the method explained in our inverter article.

Why each kit uses a different voltage

The more power, the more amps flow from the batteries to the inverter, and the thicker the cables need to be. With what the large home runs at once, 3,120 W, the inverter would draw from a 12 V battery about 3,120 ÷ 0.85 ÷ 12 = 306 amps; from a 48 V one, about 3,120 ÷ 0.85 ÷ 48 = 76. That’s why large kits run on 48 V. The inverter and controller must match the battery voltage.

If your situation is different

• If you live somewhere sunnier than Madrid in December, like the Caribbean or much of Mexico, you need fewer panels: the city table shows many get over 4 kWh per kWp a day even in their worst month.
• If you get many cloudy days in a row, you need more battery, and more panels to recharge it. The U.S. Department of Energy mentions a backup generator for bad spells; if you use one, first read how to use it without carbon monoxide risk.
• If you’re on the grid and only want to save or ride out outages, the kit can be much smaller: the panels are sized with the yearly average, and the battery just for the outage hours. Use the power outage battery calculator.
• If you don’t know what you use, check your bill or work it out appliance by appliance with the usage calculator.

What each part of the kit is and what’s often missing is explained in what’s in a solar kit. And before you buy, have a qualified installer or electrician check the calculations against the real spec sheets: they handle the installation, protection and connection to the house.