Pull up your last electricity bill. Find the monthly kWh figure — that single number is the engine behind every solar panel system size calculator. Panel count, system size, total cost: all of it flows from that one line. Here's the three-step formula installers use daily, plus a worked example you can run in under five minutes.
- Step 1: Find your monthly kWh from your electricity bill
- Step 2: Divide by (peak sun hours × 30 days)
- Step 3: Divide result by panel wattage (0.4 kW for 400W panels)
- 4 factors push that number up or down — location, shading, efficiency, future load
- Skip the math: TrySolar's simulator does this in 60 seconds
How to Calculate the Right Solar System Size for Your Home
Three steps. Work through them once and you'll never need to guess again.
Step 1: Find your average monthly consumption in kWh — it's on every utility bill. Use a 12-month average if you can. Summer AC and winter heating skew individual months badly.
Step 2: Divide by your location's daily peak sun hours multiplied by 30. Peak sun hours aren't total daylight — they're equivalent hours of full-intensity radiation. Arizona pulls 6–7. The UK gets 2.5–3.5. Most of the continental US sits between 4 and 5.5.
Step 3: Divide by your panel's rated wattage in kW. A 400W panel = 0.4 kW.
Worked example: A home using 900 kWh/month with 5 peak sun hours: 900 ÷ (5 × 30) = 6 kW system. At 400W per panel: 6 ÷ 0.4 = 15 panels. Done.
| Home Size | Avg Monthly kWh | Est. System Size | Approx. Panels (400W) |
|---|---|---|---|
| 1,000 sq ft | 500–650 kWh | 4 kW | 10 panels |
| 1,500 sq ft | 700–900 kWh | 6 kW | 15 panels |
| 2,000 sq ft | 900–1,100 kWh | 8 kW | 20 panels |
| 3,000 sq ft | 1,200–1,500 kWh | 10–12 kW | 25–30 panels |
These are starting estimates, not quotes. The four variables below will move them — sometimes by 30% or more. Before calling an installer, knowing how much solar panels cost alongside your system size gives you a clear benchmark to negotiate from.
Key Factors That Affect Your System Size
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1
Peak sun hours by region. The biggest variable. A 6 kW system in Phoenix generates far more electricity than the same system in London. Use a location-specific figure — not a global average — or your calculation is fiction.
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2
Roof orientation and shading. South-facing in the northern hemisphere = 100% of available solar energy. East or west = roughly 80%. Nearby trees or chimneys shave off another 5–20%. Shade losses compound fast — don't underestimate them.
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3
Panel efficiency and wattage. Premium panels now hit 420–440W. Budget panels sit at 300–330W. More watts per panel means fewer panels for the same output — critical if usable roof space is tight. Our guide to the best solar panels for home in 2026 breaks down efficiency ratings across brands.
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4
Future energy growth. Adding an EV? A heat pump? A pool? These can increase household electricity use by 20–40%. Size for your future self, not today's bill. A system expansion in three years costs far more than getting the sizing right now.
The average US household uses around 886 kWh per month, per the Energy Information Administration. At 4–5 peak sun hours — typical across most US states — that translates to roughly a 6 kW solar system. A useful sanity check against your own bill.
James, a homeowner in Sacramento, ran the formula against his 1,050 kWh/month bill and the city's 5.8 peak sun hours: 1,050 ÷ (5.8 × 30) = 6.03 kW. He rounded up to 6.5 kW to cover the EV he planned to buy the following year. His installer confirmed the sizing was exactly right.
Get Your Exact Size in 60 Seconds — Try the Solar Simulator
The formula works. But if you want a personalised recommendation without the manual maths, plug your address and bill into TrySolar's solar simulator. It applies your location's real sun data, roof orientation, and local utility rates automatically. Then use the Solar Simulator ROI Calculator to see your payback period before you commit to anything.
What to Do After You Know Your System Size
Check your roof space. Each kilowatt needs roughly 65–80 sq ft of usable area. A 6 kW system wants 400–480 sq ft. Tight roof? Higher-efficiency panels close the gap without a second thought.
Get at least three quotes. System size should be consistent across bids. If one installer recommends 10 kW and another recommends 6 kW for the same house, demand a written explanation. Our guide on how to choose a solar installer covers exactly what to scrutinise.
Lock in your 2026 incentives. The US federal solar tax credit covers 30% of installation costs through at least 2032. State and utility rebates can add thousands on top. Check the full solar tax credits and incentives for 2026 breakdown before signing anything.
Confirm total cost. System size drives price — directly. Once you have your kW figure, the solar panels on house cost guide lets you benchmark every quote against real market data.
Solar sizing is arithmetic. Monthly kWh ÷ (peak sun hours × 30) ÷ panel wattage = panel count. A 900 kWh/month home with 5 sun hours needs roughly 15 × 400W panels — a 6 kW system. Add 20–40% if an EV or heat pump is coming. Subtract for shading or a west-facing roof. Get three quotes, claim your 2026 incentives, and a well-sized system pays itself back in under a decade in most markets.
Frequently Asked Questions
How many solar panels do I need for a 2,000 sq ft home?
A 2,000 sq ft home typically uses 900–1,100 kWh per month. At 4–5 peak sun hours, that's a 6–8 kW system — roughly 15–20 panels at 400W. Shading, roof orientation, and local sun data will shift the final count.
Can I use a solar calculator without knowing my exact electricity usage?
Yes. TrySolar's simulator estimates usage from your home's size, location, and number of occupants. It's less precise than actual kWh data, but close enough to get you in the right ballpark immediately.
Is a bigger solar system always better?
No. Oversizing costs more upfront and can hit net metering caps — some utilities limit how much excess power they'll buy back. Size for 90–100% of current usage, then add capacity only if an EV or heat pump is genuinely imminent.
What if my roof is too small for the panels I need?
Two options: switch to high-efficiency panels (420W+ cuts the count) or accept partial solar coverage and supplement with grid power at peak. A good installer designs around your actual roof — don't let a one-size-fits-all quote go unchallenged.
Skip the spreadsheet. Enter your address and monthly bill into TrySolar's simulator and get a personalised system size recommendation — panel count, estimated cost, and payback period included.
Try the Solar Simulator Free →- A solar calculator gives you a realistic system size in minutes — no sales call required.
- Your monthly kWh bill and roof orientation are the two inputs that matter most.
- Size for your current usage, not a wishlist — oversizing wastes money and may hit net metering limits.
- Always cross-check the simulator output against a local installer quote before signing anything.
- Payback periods of 7–10 years are realistic for most Australian and US markets with current incentives.