Most people who ask me how to size a solar system have already made one of two mistakes before we even talk. Either they’ve gotten a quote for way more panels than they need, or they’re about to install something that won’t come close to covering their bill. Both are expensive. One wastes your money upfront, the other wastes it every month for the next 25 years.
Here’s the good news: figuring out how to size a solar system isn’t actually complicated. It’s a formula, a couple of numbers off your own electric bill, and a bit of common sense about your roof. That’s it. No advanced math, no engineering degree, no need to just trust whatever number a salesperson hands you.
I’m going to walk you through exactly how I’d size a system if you called me up — the same steps, the same formula, the same reference points I’d pull up on my screen. By the end, you’ll have a real number you can either use yourself or bring into a conversation with an installer, confident enough to ask “why is your number different from mine?”
The Quick Answer, If You’re in a Hurry
Here’s the formula, boxed up so you can screenshot it:
(Daily kWh usage ÷ peak sun hours) × 1.15 = DC solar system size in kW
Multiply that number by 1,000 to get watts. Divide by your panel’s wattage to get roughly how many panels you’ll need. A typical U.S. home uses somewhere around 30 kWh a day, which usually lands you in the 6-8 kW range — but that swings a lot depending on where you live and how much electricity you actually use. Keep reading, because the “roughly” part matters more than most sizing guides admit.
What Actually Decides Your System Size
Before you touch a calculator, you need to know which constraint you’re designing around, because it changes the math. Most homeowners fall into one of three camps:
- Budget first. You have a number in mind — say $15,000 — and want the biggest, most efficient system that number buys.
- Space first. Your roof is small, oddly shaped, or partially shaded, and you need to squeeze the most output out of limited square footage.
- Offset first. You want to cover a specific percentage of your electric bill — 80%, 100%, whatever your comfort level is — and you’re building backward from that target.
I’d say offset-first is the most common starting point, and it’s the one this whole guide is built around. But if you’re space-constrained, jump to the panel-selection section below, because that changes which panels make sense for you, not just how many.
A few other things quietly affect your final number, and they’re worth knowing upfront: how much direct sun your roof actually gets, which way your roof faces, whether you’re planning to add an EV or a pool later, the efficiency rating of the panels you choose, and the fact that every panel loses a small percentage of output every year it ages. None of these change the core formula, but they all nudge the final number up or down.
Step 1: Figure Out Your Daily Energy Usage
Pull out 12 months of electric bills if you can. I know that sounds tedious, but a single month tells you almost nothing — usage spikes hard in summer from air conditioning and again in winter from heating, so one month will either overestimate or underestimate your real average.
Add up your kWh usage across all 12 months, then divide by 12. That gives you your average monthly usage. Divide that by 30, and now you’ve got your average daily kWh — the number the whole formula runs on.
If you don’t have a full year of bills yet, or you just want to sanity-check the number, look at your appliances individually. A window AC unit, a water heater, an EV charger — these things add up fast, and knowing which ones are driving your usage helps you decide whether “offset 100%” is realistic or whether you’re better off pairing solar with a few efficiency upgrades first.
One thing worth checking before you go further: does your utility offer net metering? If they do, any extra power your system generates during sunny months gets banked as a credit you can use later, which changes how conservative or aggressive you want to be with your sizing. Not every utility offers this, and the terms vary a lot, so it’s worth a quick call to confirm before you lock in a number.
Step 2: Find Your Peak Sun Hours
This is the part most sizing guides gloss over, and it’s honestly where a lot of the “why did my neighbor’s system come out so different from mine” confusion comes from.
Peak sun hours aren’t the same as daylight hours. They measure how many hours per day the sun delivers enough intensity to count as “peak” solar production — think of it as the sun operating at full strength, condensed into a single number. You can pull this from a peak sun hours reference map, or from NREL’s solar data, using the city closest to you.
Somewhere sunny like Phoenix might average 6.5+ peak sun hours a day. Seattle is going to be closer to 3.5–4. That gap alone can nearly double how many panels two otherwise identical homes need. If you want to understand why that number varies so much by location — it comes down to solar radiation and insolation patterns, plus your local air mass, which is really just how much atmosphere sunlight has to pass through before it hits your roof.
Step 3: Run the Formula
Now you plug both numbers in:
(Daily kWh ÷ peak sun hours) × 1.15 = DC system size in kW
That 1.15 isn’t random — it’s a rough efficiency buffer that accounts for the fact that real-world systems lose a bit of output to wiring, inverter conversion, heat, and dust. Skip it, and you’ll consistently undersize.
Let’s run it three different ways, because one example never tells the whole story:
Albuquerque, New Mexico — sunny, roughly 6.1 peak sun hours, daily usage of 33 kWh: (33 ÷ 6.1) × 1.15 = 6.2 kW DC, or about 6,200 watts of panels.
Phoenix, Arizona — even sunnier, 6.5 peak sun hours, same 33 kWh daily usage: (33 ÷ 6.5) × 1.15 = 5.8 kW DC. Notice that — more sun means fewer panels for the same usage.
Seattle, Washington — cloudier, around 3.6 peak sun hours, same 33 kWh usage: (33 ÷ 3.6) × 1.15 = 10.5 kW DC. Almost double the Phoenix system, same household energy needs.
That’s the honest answer to “why does my quote look bigger than my friend’s in another state.” It’s not always upselling — sometimes it’s just your sky.
Once you’ve got your kW number, multiply by 1,000 to convert to watts, then divide by your chosen panel’s wattage to estimate panel count. A 6.2 kW system using 310-watt panels comes out to roughly 20 panels.
How Many Panels for a Typical Home? A Quick Reference
I get asked this constantly, so here’s a rough table based on average usage and moderate sun exposure (around 5 peak sun hours). Treat this as a starting point, not a final answer — your actual bill and location will move these numbers.
| Home Size | Avg. Monthly Usage | Estimated System Size | Approx. Panel Count* |
| 1,200 sq ft | ~600 kWh | 4.6 kW | 15 panels |
| 1,800 sq ft | ~900 kWh | 6.9 kW | 22 panels |
| 2,000 sq ft | ~1,000 kWh | 7.7 kW | 25 panels |
| 3,000 sq ft | ~2,000 kWh | 15.3 kW | 49 panels |
| 4,000 sq ft | ~2,200 kWh | 16.9 kW | 54 panels |
*Based on 310W panels. Higher-wattage panels (370W+) will reduce the count for the same output.
Square footage is a decent starting proxy, but it’s not the real driver — your actual kWh usage is. A 3,000 sq ft home with an efficient heat pump can use less power than a 1,800 sq ft home running a pool pump and central AC around the clock. Use the table to get oriented, then trust your bill over your blueprint.
Fine-Tuning: Roof, Mount, and Panel Choice
The formula gets you a solid estimate, but three physical realities can shift the final design.
Roof mount vs. ground mount. A roof mount is almost always the cheaper, simpler option, and it keeps your panels close to the inverter, which saves on wiring. Pull up your address on satellite view and check whether you’ve got a clean south-facing slope (north-facing if you’re in the Southern Hemisphere — your panels should always point toward the equator). If your roof can’t give you that angle, you’ll either need to oversize slightly to compensate, or look at a ground or pole mount instead, which lets you angle panels however you want regardless of roof geometry. There’s a real tradeoff there in cost versus flexibility — I’ve written more on that comparison in rooftop vs. ground-mounted solar if your roof isn’t cooperating.
Panel size and count. Got a big, unshaded roof? Larger, cheaper-per-watt panels usually make the most financial sense. Got a small or partially shaded roof? Fewer, higher-efficiency panels will get you closer to your target output without needing more space than you have — and you can always add more later if your usage grows.
Tilt and orientation. The angle and direction of your array matters more than most people expect. A poorly angled system can lose a noticeable chunk of output even with great sun exposure. If you want to actually understand what tilt and azimuth are doing to your output (rather than just trusting a default number), it’s worth a look at how solar angles affect production before you finalize anything.
Once you know your rough size, mount type, and orientation, run it through a PVWatts-style calculator to see estimated monthly output. That number is your final sanity check before committing to a design.
Three Mistakes I See Homeowners Make
Sizing for 100% offset without checking net metering terms. It sounds great on paper — cover the whole bill. But if your utility changes net metering rules (and several have in recent years), that “100% offset” system can end up producing more than you get credit for. I’d generally size closer to 85–95% offset unless you’re confident your utility’s policy is stable, or unless you’re planning to add usage soon (EV, pool, home addition).
Ignoring future load growth. If you know you’re getting an EV in the next two years, size for that now. Adding panels later almost always costs more per watt than building it into the original system, because you’re paying for a second mobilization, permit, and labor visit instead of one.
Trusting square footage over actual usage. Square footage is a rough proxy at best. Two identical-looking homes can have wildly different bills depending on insulation, appliances, and how many people actually live there. Your kWh usage is the real input — treat square footage as a gut check, not gospel.
What This Actually Costs
Cost isn’t the focus of this guide, but it’s the question sitting underneath every sizing conversation, so let’s address it directly. A 6 kW system typically runs somewhere in the $12,000–$18,000 range before incentives, depending on your region, equipment choice, and installer. A 10 kW system usually lands between $9,500 and $12,500 per system depending on components — larger systems often have a lower cost per watt, which is part of why “build in room to grow” can make financial sense if you’re confident about future usage.
Imported panels are frequently 10% or so more cost-effective per watt than domestically manufactured ones, though some homeowners prioritize American-made components for other reasons — that’s a personal call, not a right-or-wrong one.
Choosing Your Equipment
Once you know your target size, the equipment conversation gets a lot more concrete. On the grid-tied side, that usually means deciding between a few different combinations of panels and inverters — string inverters with optimizers, microinverters, or a central inverter setup, each with different tradeoffs around cost, shading tolerance, and monitoring. If you’re weighing a fully grid-connected setup against something with battery backup, it’s worth understanding the difference between an on-grid solar system and an off-grid solar system before you finalize your design, since off-grid sizing has to account for battery capacity too, not just panel output.
Most of what’s covered here applies specifically to grid-tied systems. If you’re planning to go off-grid or want battery backup, the sizing math changes — you’ll need to factor in days of autonomy and battery bank capacity, which is a different calculation entirely.
Talk to Someone Before You Commit
The formula above will get you a genuinely solid estimate — good enough to walk into a sales conversation with confidence instead of just nodding along. But a real design should still account for shading you can’t see from a satellite photo, your specific roof structure, local permitting requirements, and your utility’s actual interconnection rules.
If you want a second set of eyes on your numbers, call 1-800-472-1142 and talk it through with a designer, or schedule a free design consultation. Bring your kWh number and your peak sun hours — you’ll already be ahead of most people who walk in with nothing but a guess.
FAQs
How do I calculate what size solar system I need?
Divide your average daily kWh usage by your area’s peak sun hours, then multiply by 1.15 to account for real-world system losses. That gives you your system size in kW. Multiply by 1,000 for watts, then divide by your panel’s wattage to estimate panel count.
How many solar panels do I need for a 2,000 sq ft home?
Most 2,000 sq ft homes use around 1,000 kWh a month and need roughly a 7–8 kW system, which typically works out to 22–25 standard panels — though your actual usage and local sun exposure will shift this number in either direction.
How big is a 10kW solar system, physically?
A 10 kW system usually needs around 24 panels (using 415W panels) and roughly 48 square meters, or about 500 square feet, of usable roof space.
How much does a 10kW solar system cost?
Typically between $9,500 and $12,500 before incentives, depending on your location, equipment choice, and installer.
How do you size an off-grid solar system differently?
Off-grid sizing adds a battery bank calculation on top of panel sizing — you need to account for daily energy use, days of autonomy, and your battery’s depth of discharge, since there’s no grid to fall back on when the sun isn’t out.
Can solar panels run my air conditioner?
Yes, as long as your system is sized to handle the extra load. AC units are one of the biggest single draws on a home’s energy usage, so if you’re planning to run one heavily, make sure it’s factored into your daily kWh number before you finalize your system size.
What’s a peak sun hour, exactly?
It’s not the same as daylight hours. A peak sun hour measures the equivalent of one hour of sunlight at maximum intensity (1,000 watts per square meter). A location might get 12 hours of daylight but only 5 peak sun hours, because sunlight is weaker in the early morning and late afternoon.
