Start With an Energy Audit, Not Panels

Most people size their solar system backwards: they pick a number of panels that "sounds right" and hope it covers the bill. The correct order is the opposite: measure what you use first, then size the system to match. A system sized from guesswork either leaves you buying expensive grid power every evening or wastes money on panels whose output you never use.

The unit that matters is the kilowatt-hour (kWh): one kWh is what a 1,000-watt appliance consumes in one hour. Your electricity bill already reports it, but bills average over months, and solar needs your daily number.

Step 1: List Every Appliance

Walk through your home and note each appliance's wattage (on the label or in the manual) and how many hours it runs daily. Typical figures:

  • Air conditioner (1.5-ton inverter): 1,200–1,500W
  • Refrigerator: 120–150W (runs ~8–10h effectively, cycles on/off)
  • Ceiling fan: 60–75W
  • LED TV (43"): 70–90W
  • Washing machine: 400–500W
  • Laptop + charger: 60–90W
  • LED bulbs (each): 7–12W
  • Water pump (0.5 HP): 350–400W

Step 2: Calculate Daily kWh

Daily kWh = Σ (Appliance Watts × Daily Hours) ÷ 1000

Example household:

  • AC: 1,500W × 8h = 12,000 Wh
  • Fridge: 150W × 10h = 1,500 Wh
  • 3 fans: 75W × 8h × 3 = 1,800 Wh
  • TV: 80W × 5h = 400 Wh
  • Lights (10 LEDs): 9W × 6h × 10 = 540 Wh
  • Washing machine: 500W × 1h = 500 Wh

Total: 16,740 Wh = 16.7 kWh/day

Step 3: Account for Real-World Losses

No solar system delivers its nameplate rating. Between inverter efficiency (~95%), wiring losses, dust on panels, and heat derating (panels lose output as they get hot: significant in summer), expect to harvest about 80% of the theoretical maximum. Use an efficiency factor of 0.8 in your maths. In very hot climates, 0.75 is safer.

Step 4: Find Your Peak Sun Hours

Peak sun hours (PSH) is the number of daily hours at a standard 1,000 W/m² irradiance: it's not the same as daylight hours. A place with 12 hours of daylight might have only 5 PSH. Typical yearly averages: about 2.5–3 in the UK and northern Europe (around 4–5 in midsummer, under 1 in December), 4–6 across most of the US, 5–6 in Pakistan, India and the Middle East, and 6+ in deserts. Check a solar irradiance map for your exact location, and size for your worst month if you want year-round coverage: winter PSH is 30–40% lower than summer in South Asia, and far lower than that at UK latitudes.

Step 5: Calculate Required Panel Capacity

Required panel watts = (Daily kWh × 1000) ÷ (PSH × 0.8)

For our 16.7 kWh/day example at 5 PSH:

16,700 ÷ (5 × 0.8) = 16,700 ÷ 4 = 4,175 W of panels

Panel count depends on the wattage you buy:

  • 400W panels: 4,175 ÷ 400 = 11 panels (round up)
  • 550W panels: 4,175 ÷ 550 = 8 panels

Higher-wattage panels cost more each but need less roof space and fewer mounts: worth it on a small roof.

Don't Forget Roof Space and Batteries

A 400W panel is roughly 2 m × 1 m. Eleven of them need about 22 m² of unshaded roof, plus walking gaps. Measure first: roof space kills more solar plans than budgets do.

If you want backup during outages, size batteries separately: add up the kWh you use during the hours you want covered (usually the evening and night), then divide by the battery's usable depth of discharge: double it for lead-acid, which shouldn't drain past 50%, or add about 20% for lithium, which can go to 80–90%. An inverter rated for your peak simultaneous load, usually 3–5 kW for a home, completes the system.

Common Sizing Mistakes

  • Sizing for summer, suffering in winter: check December/January PSH, not July.
  • Ignoring shade: even partial shade on one panel can drag down a whole string. Map shade across the day before fixing panel positions.
  • Forgetting growth: adding an EV or a second AC next year? Leave 20–25% headroom in inverter capacity now; adding panels later is cheap, replacing an inverter isn't.

Size Your System Free

Enter your appliances in our solar panel calculator for the system size, panel count, cost estimate and payback period. Then size backup with the solar battery calculator and pick cable sizes with the solar wire calculator. To sanity-check the investment first, read Is Solar Worth It? Payback by US State and UK Region.