You do not need to guess how many solar panels to buy. Your electricity bill and one simple sum will get you close. Here is how it works for a South African home.

Step 1: Find your daily electricity use

Your electricity bill shows how many kilowatt-hours (kWh) you used in the month. Divide that by 30:

Daily use (kWh) = kWh per month ÷ 30

Check a few months, as winter bills are usually higher. On prepaid, add up a month of units, since one unit is one kWh. If the geyser and stove stay on Eskom or a separate circuit, the solar only has to cover the rest, so your real figure is lower than the bill.

Step 2: Allow for sun hours and losses

South Africa gets roughly 4.5–6 peak sun hours a day, depending on where you live and the season. A peak sun hour is one hour of full-strength sunshine, and weaker morning and afternoon light counts for less.

Panels also never deliver their full rating, because heat, dust, wiring and the inverter take a share. Allow about 20% for losses. To be safe, plan with 4.5 sun hours. In mid-winter output can dip lower still, so a panel or two extra is never wasted.

Step 3: Do the sum

Number of panels = daily kWh ÷ (panel size in kW × sun hours × 0.8)

The 0.8 is the 20% allowance for losses. A 580W panel works out at 0.58 × 4.5 × 0.8 = about 2.1kWh a day, which is why we say it makes about 2kWh on an average day.

A worked example

Say the loads your solar system will run use 450kWh a month. That is 450 ÷ 30 = 15kWh a day.

Using 630W panels and 4.5 sun hours, each panel makes 0.63 × 4.5 × 0.8 = about 2.27kWh a day.

So you need 15 ÷ 2.27 = 6.6 panels. Always round up, which gives 7 panels, or 7 × 630W = 4,410W (4.41kW) of solar. They make about 7 × 2.27 = 15.9kWh a day, covering your 15kWh with a little to spare.

Running the house and charging the battery

With a battery, the panels have two jobs: run the house while the sun shines, and refill the battery for the night.

In our example, say 6kWh is used during the day and 9kWh at night. A 10.24kWh lithium battery gives about 9.2kWh of usable energy (10.24 × 0.9 = 9.2), so it covers the night. The panels still need to make 6 + 9 = 15kWh in the day, which is what we sized for.

A common mistake is sizing panels for daytime use only, so the battery never fills and you buy grid power at night. As a rule of thumb, three 580W-class panels refill a 5.12kWh battery from empty on a clear day, plus more for the house.

Match the panels to your inverter

Your inverter limits how many panels you can connect. Check its datasheet for:

• Maximum PV input power: keep the total panel wattage within this rating. Panels beyond it just add cost.
• Maximum PV input voltage: panels wired in series (a string) add their voltages together. Add up the open-circuit voltage (Voc) of every panel in the string and keep the total safely below this limit.
• Maximum input current: strings wired in parallel add their current, and large modern panels produce a lot of it.

Leave a margin on voltage, because a panel's voltage rises as it gets colder. On a cold winter morning, when the first sun hits cold panels, the string voltage can climb above the Voc on the label. If it goes above the inverter's maximum, the inverter can be damaged. The temperature coefficient on the panel datasheet shows by how much. The string must also reach the inverter's minimum operating voltage, so very short strings can be a problem too.

Depending on the inverter, our seven panels might go in one string or across two inputs. Send us your inverter model and chosen panel, and we will check the stringing.

Will they fit on your roof?

A 600W-class panel is about 2.4m × 1.1m, so each one needs roughly 2.6m² of roof. Seven panels need about 7 × 2.6 = 18.2m², plus space for rails and gaps.

North-facing roofs give the most energy, with east and west a little less. Avoid shade from trees and chimneys, because shade on one panel can pull down the whole string. You will also need the right panel mountings for your roof.

Bifacial or monofacial?

Bifacial panels also collect light on their back, reflected from the surface below. That helps on a raised ground frame over light gravel, but flush on a roof the back sees little light, so treat it as a small bonus. Monofacial panels suit roofs just as well. Our range includes:

• Deichmann 630W 16BB bifacial: our own brand, coming soon
• Trina Solar 630W Vertex N: bifacial, dual glass
• JA Solar 610W: N-type TOPCon, bifacial
• Jinko Tiger Neo 585W: N-type, bifacial
• JA Solar 465W: N-type monofacial, a smaller 54-cell panel with a black frame for tight roofs

Next steps

Browse our solar panels, or skip the maths with a solar kit, with the inverter, battery and panels already matched. Our solar system sizing calculator runs these numbers for you, and our guide on what size inverter you need covers the rest. Or contact us with your monthly kWh and inverter model, and we will suggest a layout.

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