Tom Bradley
Most homeowners walk into solar quotes with zero leverage. They hand the salesperson a utility bill, nod along while someone explains “peak sun hours,” and end up with a system somebody else sized for them. I’ve watched this happen dozens of times. The salesperson isn’t necessarily lying, but they have every incentive to round up. A bigger system means a bigger commission. What I want to show you is that sizing a solar system yourself isn’t that complicated, and doing it before you ever talk to a contractor changes the entire conversation.
Here's how the math flows from your utility bill to a specific panel count, using realistic numbers you can swap for your own.
| Step | Calculation | Result |
|---|---|---|
| 1. Annual consumption | Sum of 12 monthly bills | 12,000 kWh |
| 2. Planned load changes | +3,500 kWh for new EV | 15,500 kWh target |
| 3. Daily production needed | 15,500 kWh ÷ 365 days | 42.5 kWh/day |
| 4. DC system size (ideal) | 42.5 kWh ÷ 5 peak sun hours | 8.5 kW DC |
| 5. Derate for real-world losses | 8.5 kW ÷ 0.80 (inverter, wiring, soiling, temp) | 10.6 kW DC |
| 6. Panel count (400W panels) | 10,600 W ÷ 400 W | 27 panels |
| 7. Roof space check | 27 panels × 18 sq ft each | ~486 sq ft needed |
General information for comparison, confirm specifics for your situation.
Why Your Utility Bill Is the Starting Point, Not the Finish Line
Pull your last 12 months of electric bills. Not just last month. Twelve months. This is the single most important thing you can do before sizing anything.
You’re looking for your annual kilowatt-hour (kWh) consumption. Most utilities print a rolling 12-month chart right on the bill. If yours doesn’t, log into your online account and download the data. Add up all 12 months. That’s your baseline.
Let’s say you used 12,000 kWh over the past year. That’s pretty typical for a 2,000 square foot home in a moderate climate. Now, before you size a system to replace all of that, ask yourself two questions. Are you about to add load? An electric vehicle, a heat pump, an electric dryer replacing a gas one? Are you about to remove load? Better insulation, new LED lighting throughout the house, a more efficient refrigerator?
I’ve seen clients size their system against a utility bill that reflected a winter they were traveling, or a summer when their pool pump was broken. One couple sized their system during a period when their college-age kid was living at home with a gaming PC running six hours a day. Once he moved out, they were generating way more power than they needed.
Get the 12-month number, then adjust it intentionally. If you’re planning an EV charge at home, add roughly 3,000 to 4,000 kWh annually for an average 12,000-mile-a-year driver. If you’re switching from gas to electric heat, that number can jump significantly depending on your climate and home size.
Peak Sun Hours: The Number That Actually Drives System Size
| Location | Peak Sun Hours | Annual Production (1 kW system) | Panels Needed (400W) for 12,000 kWh |
|---|---|---|---|
| Phoenix, Arizona | 6.5 | ~910 kWh | ~13 panels |
| Mid-Atlantic (South-facing, 30°) | ~5.0 | ~700 kWh | ~17 panels |
| Seattle, Washington | 3.5 | ~490 kWh | ~24 panels |
Helpful resource: Jackery Explorer 300 Portable Power Station is a top-rated option for this. (As an Amazon Associate this site earns from qualifying purchases.)
Most DIY guides get vague here. They say “check your peak sun hours” and link to a map. What they don’t explain is what to actually do with that number.
Peak sun hours aren’t hours of daylight. They’re a measure of solar irradiance, specifically how many hours per day sunlight intensity averages 1,000 watts per square meter. Phoenix, Arizona sits around 6.5 peak sun hours. Seattle, Washington sits closer to 3.5. That difference nearly doubles the number of panels you’d need for the same annual output.
The National Renewable Energy Laboratory’s PVWatts calculator is the tool I recommend to every homeowner doing their own sizing. It’s free, it pulls actual climate data for your specific zip code, and it accounts for roof tilt, azimuth angle, and system losses. Plug in your address, set your DC system size to 1 kW as a test, and it’ll tell you the estimated annual production. Then you can scale from there.
The math is straightforward. You need 12,000 kWh per year. PVWatts tells you a 1 kW system in your location produces about 1,400 kWh per year. Divide 12,000 by 1,400 and you get 8.57. Round up and you need roughly a 9 kW system. That’s your target.
What surprised me when I started doing this regularly was how much roof orientation matters. A south-facing roof at a 30-degree pitch in the mid-Atlantic can outperform an east-facing roof in Arizona. Azimuth angle can swing production by 15 to 20 percent. PVWatts lets you model this, and you absolutely should.
Panel Count and Real-World Roof Constraints
Total Cost Breakdown of our Solar Power System & How Many Years to Pay for Itself. · Country View Acres (Formerly Smalltown442) on YouTube
Once you have your target system size in kilowatts, converting that to panel count is straightforward. You just need to know what wattage panels you’re working with.
Consumer-grade residential panels in 2024 typically run 370 to 440 watts. Premium panels from manufacturers like Maxeon or REC Alpha push past 430W on a standard 60 or 66-cell format. For our 9 kW example:
- Using 400W panels: 9,000W divided by 400W = 22.5, so 23 panels
- Using 430W panels: 9,000W divided by 430W = 20.9, so 21 panels
Now measure your usable roof space. A standard 60-cell panel is roughly 65 by 39 inches, about 18 square feet. Twenty-two of them need around 400 square feet of unshaded, structurally sound roof. That’s often more than people expect.
Roof obstructions matter more than most sizing guides admit. Vents, skylights, chimneys, and HVAC equipment aren’t just aesthetically annoying. They create shade, and shade destroys production disproportionately in string inverter systems. If a single panel in a series string gets shaded, it drags down the output of every other panel on that string. This is the main argument for microinverters or DC optimizers on roofs with any meaningful shading. They let each panel operate independently.
If more than 20 percent of your usable roof area is subject to shading for more than an hour during peak sun hours, you need to have a real conversation about whether solar makes sense for your specific roof, not just your location. I’m not saying don’t do it, but go in with eyes open.
Inverter and Battery Sizing: The Part People Undersize
Most DIY solar conversations focus on panels. The inverter and battery decisions are where I see the biggest mistakes.
Inverter sizing is relatively simple. Your inverter’s AC output rating should match or slightly exceed your system’s DC capacity. A 9 kW array needs at least a 7.6 kW to 10 kW inverter. Most grid-tied systems are sized so the inverter handles the full array output. String inverters from SMA, Fronius, or SolarEdge are reliable workhorses. Enphase microinverters add per-panel optimization at a higher cost per watt but lower installation complexity on complex roofs.
Battery sizing is genuinely complicated, and the research on financial payback is mixed. Batteries make the most sense if you’re in an area with time-of-use rates, frequent outages, or no net metering. If your utility offers full-retail net metering, batteries often don’t pencil out financially for ROI, though they add resilience if the grid goes down.
For basic backup: figure out your critical loads. Refrigerator, lights, a few outlets, maybe a well pump. That’s typically 5 to 10 kWh for an overnight backup. A single Tesla Powerwall 3 at 13.5 kWh usable covers that. If you want whole-home backup or multi-day autonomy, you’re looking at two or more batteries and costs that can exceed $20,000 installed.
A home energy monitor like the Emporia Vue or Sense (affiliate link, the site may earn a commission) is genuinely useful here. Installed on your main panel, it shows you real-time consumption by circuit over weeks, so you can identify actual critical loads versus everything else. I’ve recommended these to probably 50 homeowners, and almost every one of them was surprised by what they found.
Step-by-Step: Size Your Own System Before Calling a Contractor
Here’s the process I walk people through.
Step 1: Get your 12-month kWh total Log into your utility account, download 12 months of usage data, and add it up. Adjust for anticipated load changes.
Step 2: Run PVWatts for your specific roof Go to pvwatts.nrel.gov. Enter your address. Set the DC system size to 1 kW. Input your roof’s tilt (slope) and azimuth (compass direction). Note the annual production estimate.
Step 3: Calculate your target system size Divide your annual kWh need by the PVWatts annual production estimate for 1 kW. That gives you your target in kW. Example: 12,000 kWh divided by 1,400 kWh per kW = 8.57 kW, round to 9 kW.
Step 4: Calculate panel count Divide your target wattage by your chosen panel wattage. A 9,000W system with 400W panels = 23 panels.
Step 5: Verify roof capacity Measure usable roof area. Subtract space around obstructions and setbacks required by local code (typically 3 feet from ridges and edges, sometimes more). Confirm you have physical space for your panel count.
Step 6: Size your inverter Match inverter AC output rating to your array size. Check that your chosen inverter is on your utility’s approved equipment list (most have one).
Step 7: Decide on battery storage Only if it makes financial or resilience sense for your situation. Run the numbers honestly.
Permits, Utility Interconnection, and Why You Can’t Skip These
I see a troubling amount of solar content that glosses over permitting as though it’s a minor administrative step.
Every grid-tied solar installation in the US requires a building permit and utility interconnection approval. No exceptions. Some states have streamlined this into a simple online process that takes days. Others involve full plan sets reviewed by a structural engineer. Your local jurisdiction’s building department website is where you start.
Permit requirements typically include a single-line electrical diagram, a site plan showing panel placement, equipment spec sheets, and sometimes a structural assessment of your roof framing. If you’re doing a DIY install, some jurisdictions allow homeowner-pull permits. Many do not. Know your local rules before you commit to DIY labor versus hiring a licensed contractor for the electrical portion.
EnergySage’s market data shows the national average solar installation cost running around $2.95 per watt before incentives. A 9 kW system at that average would run roughly $26,550 before the federal tax credit. The 30 percent federal Investment Tax Credit brings that to about $18,585. That’s your benchmark when getting quotes. If someone comes in 40 percent above that number with no clear justification, push back.
Utility interconnection is separate from permitting. Your utility has to approve your system for grid connection, and they’ll issue a Permission to Operate (PTO) letter before you turn it on. Operating a grid-tied system without PTO is a code violation and can result in being disconnected from the grid.
Do this homework before you request a single quote, and you’ll walk into every conversation knowing what you actually need. You’ll catch upsells, spot undersized equipment, and be able to ask specific questions instead of nodding along. Contractors who know their stuff will respect it. The ones who get defensive or dismissive when you bring your own numbers? That’s information too.
Sources
- Jackery Explorer 300 Portable Power Station
- Emporia Vue or Sense
- Govee WiFi Smart Plug with Energy Monitoring
- Emporia Smart Outlet with Energy Monitoring
- Giant Asparagus
Disclosure: As an Amazon Associate, we earn a small commission from qualifying purchases at no extra cost to you. We only recommend products that genuinely support the topics covered in this article.
- Renogy 200W Solar Starter Kit + 30A Charge Controller (~$169), Complete beginner solar kit, 200W monocrystalline panel, charge controller, and mounting hardware included.
- Renogy 2×100W Monocrystalline Solar Panels (~$99), Expandable 200W panel set from the most trusted DIY solar brand, used widely in off-grid and home backup systems.
- Renogy 200W Solar Kit + 20A MPPT Controller (~$199), 200W panel kit with MPPT charge controller for maximum energy harvest.
Recommended Resources
Disclosure: As an Amazon Associate, we earn a small commission from qualifying purchases at no extra cost to you. We only recommend products that genuinely support the topics covered in this article.
- Renogy 200W Solar Starter Kit + 30A Charge Controller (~$169), Complete beginner solar kit, 200W monocrystalline panel, charge controller, and mounting hardware included.
- Renogy 2×100W Monocrystalline Solar Panels (~$99), Expandable 200W panel set from the most trusted DIY solar brand, used widely in off-grid and home backup systems.
