How Many Solar Panels Do I Need? I Did the Math

Every solar quote I got started with a variation of the same question: “What’s your average monthly electric bill?” Then the salesperson would punch some numbers into their tablet and come back with a proposal for 18, 22, maybe 26 panels. I had no idea if that was right.

So I did what any self-respecting mechanical engineer with too much free time does: I built a spreadsheet and figured it out myself.

Why “Average Monthly Bill” Is the Wrong Starting Point

Here’s the problem with using your utility bill as the baseline: it doesn’t tell you when you’re using electricity. A house that runs the dishwasher at midnight looks identical on a bill to one that runs it at 2pm on a sunny July afternoon. But for solar production, timing matters enormously.

The real inputs you need are:

• Annual kWh consumption (your total electricity use for the year)
• Peak sun hours for your location (not daylight hours — actual solar irradiance hours)
• Panel wattage (most residential panels today are 400–430W)
• System efficiency losses (inverter, wiring, temperature, shading — typically 15–25%)

The formula: Annual kWh ÷ (Peak Sun Hours × 365 × Panel Wattage × Efficiency) = number of panels

For my house in central Ohio, that worked out to 19 panels to cover 100% of my annual usage. The salesperson quoted me 22. Turns out the extra three panels were built-in upsell buffer.

Step 1: Find Your Real Annual kWh

Log into your utility account and look at 12 months of actual usage, not estimated bills. (If a shocking bill is what got you here in the first place — like the $412 month that started my solar research — this is the most important number in your analysis.) For most U.S. homes, annual consumption falls between 9,000 and 14,000 kWh per year. Mine was 11,400 kWh.

If you want to go deeper — and I’d recommend it before spending $25,000 on solar — get a Kill-A-Watt meter and spend a week profiling your appliances. You might find, like I did, that your chest freezer in the garage is burning 600 kWh a year.

Step 2: Get Your Peak Sun Hours

This is location-specific. Use the National Renewable Energy Laboratory’s PVWatts tool. Rough benchmarks:

• Ohio, Michigan, Pacific Northwest: 3.5–4.5 peak sun hours/day
• Texas, Colorado, Midwest: 4.5–5.5 hours/day
• California, Arizona, Nevada: 5.5–7 hours/day

My Columbus, Ohio address gets about 4.1 peak sun hours. A Phoenix homeowner with the same electricity usage would need roughly 30% fewer panels.

Step 3: Account for System Losses

A 400W panel doesn’t produce 400W in real conditions. Heat reduces output. Inverter conversion adds 3–5% loss. Shading, wire resistance, soiling — it adds up. I used a 20% loss factor, which is conservative but realistic for Ohio’s climate.

The Math for My House

• Annual consumption: 11,400 kWh
• Peak sun hours: 4.1/day × 365 = 1,496.5 hours/year
• Panel wattage: 410W = 0.41 kW
• Efficiency factor: 0.80
• Output per panel per year: 1,496.5 × 0.41 × 0.80 = 490.9 kWh
• Panels needed: 11,400 ÷ 490.9 = 23.2 panels

The One Thing That Changed My Calculation

About a month into my research, I installed a whole-home energy monitor on my electrical panel. It tracks every circuit in real time and showed me my HVAC was running 40% more than expected (clogged filter) and my water heater was cycling too frequently (failing element).

I fixed both before getting solar quotes. My consumption dropped from 11,400 to 9,800 kWh — taking me from 23 panels to 20. At $200–300 per panel installed, that’s a $600–900 savings from a $150 monitoring device.

The Roof Reality Check

Standard residential panels are about 68″ × 40″. For 20 panels, I needed about 380 square feet of unobstructed south- or west-facing roof. I had it — barely. If your roof can’t fit your ideal panel count: accept partial solar offset, look at higher-efficiency panels (22–23% vs. standard 20%), or consider a ground-mount system.

Don’t Forget Future Load Changes

Size up if you’re planning to add an EV, heat pump, or expand your home. Adding panels later means paying for a second installation trip, new permitting, and possibly a new inverter. I sized for an EV I plan to buy in two years — that added 3 panels, giving me 23 total.

What I’d Tell My Past Self

Don’t hand a salesperson your electric bill and trust the output. Spend two hours with your utility’s 12-month data. Know your annual kWh. Know your peak sun hours. Factor in realistic losses. And before getting quotes, do a proper DIY home energy audit and install an energy monitor — every panel you don’t need is $200–400 back in your pocket. The good news: once the system is in, solar panel maintenance is minimal — but knowing what to watch for matters.