Sizing a solar system requires three pieces of information: your annual electricity consumption (from your bills), the average peak sun hours at your location, and the wattage of the panels you're considering. Everything else follows from these three numbers. Let's walk through each one.
Pull out your last 12 months of electricity bills and add up the total kWh consumed. If you only have a few recent bills, multiply your average monthly kWh by 12. A typical home uses 8,000–14,000 kWh per year — though this varies enormously by climate (air conditioning drives usage significantly), home size, number of occupants, and whether you have gas appliances or an EV.
If you've recently added an EV or are planning to, add approximately 3,000–5,000 kWh/year for average EV driving. If you're planning to electrify your gas heating with a heat pump, add another 2,000–4,000 kWh/year depending on climate and home size.
Example: Your bills show 11,000 kWh/year total consumption.
Peak sun hours are not the same as daylight hours. They represent the equivalent number of hours per day of full, direct (1,000 W/m²) sunlight your location receives on average. A location with 5 peak sun hours receives the same total solar energy as 5 hours of ideal noon sun, even if the sun rises at 6am and sets at 8pm.
Approximate peak sun hours by region: Desert Southwest US (Phoenix, Las Vegas): 6.0–7.5. Southern Europe (Spain, Italy, Greece): 5.0–6.5. California, Texas: 5.0–6.0. UK, Germany, Northern Europe: 2.5–3.5. Southeast Asia, India: 4.5–6.0. Most of Latin America: 4.5–6.0. Pacific Northwest, Canada: 3.0–4.0. Use the Global Solar Atlas (globalsolaratlas.info) for a precise figure for your exact location — enter your address and read the "PVOUT" or "GHI" value.
Example: Your location has 4.5 peak sun hours per day.
Formula: System size (kW) = Annual consumption (kWh) ÷ (Peak sun hours × 365 days × System efficiency)
System efficiency accounts for inverter losses, wiring losses, temperature effects, and occasional shading. Use 0.80 (80%) as a conservative real-world efficiency factor.
Example calculation: 11,000 kWh ÷ (4.5 × 365 × 0.80) = 11,000 ÷ 1,314 = 8.4 kW system
This means an 8.4kW solar system would theoretically offset 100% of your electricity consumption. In practice, you might install 7–10kW depending on roof space, budget, and how much offset you want to achieve.
Formula: Number of panels = System size (W) ÷ Panel wattage
Modern residential panels typically range from 380W to 440W each. Using 400W panels as a typical example:
Example: 8,400W ÷ 400W per panel = 21 panels
Check whether your roof can physically accommodate this number. A 400W panel is approximately 1.8m × 1.0m (6ft × 3.3ft). Twenty-one panels require approximately 37–40 square metres of unshaded, accessible roof area. For north-facing roof sections in the southern hemisphere, or south-facing in the northern hemisphere.
If you want to offset 80% of consumption (not 100%): Multiply the system size by 0.80. This reduces upfront cost while still achieving significant savings.
If you want to add battery storage: Oversize the system by 20–30% to generate surplus for battery charging. A larger array charges the battery faster and gives more surplus for self-consumption during evening hours.
If roof space is limited: Use higher-efficiency panels (22%+ efficiency) to generate more power per square metre. SunPower Maxeon or REC Alpha panels let you achieve more output from a smaller roof area.
If your roof has shading: Reduce your expected output by the shading factor and consider microinverters or power optimisers to minimise shading loss. A shading analysis from your installer is worth requesting.
Not all solar generation is worth the same. Power you consume directly from your panels (self-consumption) saves you the full retail electricity rate. Power you export to the grid earns only the feed-in or net metering rate — typically much lower than retail. This means there's a point of diminishing returns when oversizing: the last kilowatt of panels may generate electricity you export at 20% of what you'd pay to buy it.
If your grid's net metering rate is generous (one-for-one credit), oversizing is more valuable. If your export rate is low (6–10 cents per kWh vs 25–40 cents retail), focus on matching the system size to your daytime consumption rather than total consumption.
1–2 person household, no EV, mild climate: 3–5kW system, 8–13 panels.
3–4 person household, no EV, moderate climate: 6–8kW system, 15–20 panels.
3–4 person household with EV, moderate climate: 8–12kW system, 20–30 panels.
Large home with pool, EV, or all-electric heating: 10–15kW+ system, 25–38+ panels.
These are starting points — the calculation above using your actual consumption and local sun hours gives a more accurate result for your specific situation.
Once you have a rough system size in mind, get at least three quotes from different installers. Compare: system size (kW), panel brand and model, inverter brand and model, estimated annual yield (kWh), payback period estimate, and warranty terms. Be sceptical of quotes that estimate yields significantly above the calculation you've done — solar installers sometimes oversell system performance to win business.