Your usage
The biggest lever by far. Double your monthly kWh and you double your panels.
500 kWh/mo11
1,000 kWh/mo21
2,000 kWh/mo42
panels at 5.0 sun hours · 400 W
Home solar · panels, system size & cost
Solar Panel
Calculator
See how many panels your home needs — plus the system size, roof space, and a rough cost — straight from your monthly kWh. No sales calls
For homeowners planning solar — and pros running a fast sizing check.
Erase your bill
live
Erase →
A typical 1,000 kWh/month home needs about 21 panels — an 8.4 kW system on ~441 sq ft — to erase its bill. Add an EV and it climbs to 28. Your sun hours and panel wattage move the number.
Sized from your kWh
not a rooftop guess
NREL sun-hour data
accurate by location
Honest 2026 costs
no expired tax-credit math
Artie has taught physics and electrical theory for over a decade and consulted on real-world electrical design — so every tool here is grounded in both the theory and the field.
Every calculator on this site is checked against the 2023 NEC before it ships — if the math doesn't match the code, it doesn't go live.
University physics lecturer·Consulted on commercial electrical systems·Last reviewed Jul 2026
Solar Panel Calculator
Size a grid-tied home system from your usage and location.
——
kWh / SUN
—panels
—
Enter your usage to size a system.
Your solar system
Panels needed
— × 400 W
rounded up, never under-sized
System size
— kW DC
installed nameplate
System
—
kW DC
Panels
—
× 400 W
Roof
—
sq ft
Annual
—
kWh/yr
Gross cost
—
before incentives
2026: the federal residential solar tax credit (Section 25D) expired Dec 31 2025 — cash/loan buyers get no federal credit. State/utility incentives may still apply.
How it's sized
daily = kWh ÷ 30 · system = daily×offset ÷ (PSH × derate) · panels = ⌈system W ÷ panel W⌉ · roof = panels × 21
—
NEC 705.12 note: on a 200 A panel with a 200 A main, the 120% busbar rule caps solar backfeed at a 40 A breaker → ~7.7 kW AC inverter (40 A × 0.8 × 240 V). Larger systems may need a main-breaker derate — see our Circuit Breaker / Electrical Load tools.
Panels at different offsets
“+EV” adds ~250 kWh/mo (about 3,000 kWh/yr) for a typical electric-car charge on top of a full 100% offset.
How it works
Size your system in four steps
No address, no phone number, no wait. Enter what you use and where you are — the calculator does the rest with NREL sun-hour data.
-
01
Enter your usage
Type your monthly kWh — it's on the bottom of your electric bill. No bill handy? Switch to dollars and enter your monthly cost + rate instead.
-
02
Set your sun hours
Pick your state and we load its NREL peak-sun-hour average — or enter the exact figure for your address from NREL PVWatts.
-
03
Choose panel & goal
Set panel wattage (400 W is typical today) and how much of your bill to cover — 100% by default, or dial it down.
-
04
Read your system
Get your panel count, system size in kW, roof footprint, estimated annual production, and a rough gross cost.
What sets the number
Three things move your panel count
Two identical houses on the same street can need very different systems. It comes down to how much you use, how much sun you get, and how strong your panels are — nothing to do with square footage.
Your sun hours
Location sets how hard each panel works. Sunnier states need fewer panels for the same bill.
Phoenix · 6.5h17
Average · 5.0h21
Chicago · 3.5h30
same 1,000 kWh home · 400 W
Panel wattage
Bigger panels, fewer of them. Higher-wattage modules cover the same system with less roof.
350 W panels24
400 W panels21
450 W panels19
same 1,000 kWh home · 5.0 sun hours
The formula
The math, shown in the open
Every step is here — no black box. Turn your bill into daily energy, divide by how hard your sun works, then split the system into panels.
The method · worked at 1,000 kWh/mo
1
33.3kWh/day
Daily energy
1,000 kWh ÷ 30 days
2
8.33kW
System size
33.3 ÷ (5.0 sun hrs × 0.80)
3
21panels
Panels
8,333 W ÷ 400 W each
Round up — a partial panel is a whole panel. That 21-panel array is an 8.4 kW system.
Why 80%?
Panels never hit their lab rating in the real world. Heat, wiring, the inverter, dust and light shading trim output by about 20% — so we size on 80% of nameplate (a 0.80 derate). It's adjustable in the calculator.
Same as a "production ratio"
Installers fold sun hours and losses into one number — a production ratio of 1.1 to 1.6 across the US. This is the same math, just shown step by step: at 5 sun hours it works out to ~1.46, right in their range.
Worked example
One home, start to finish
An all-electric home uses 1,000 kWh a month and sits near the national average of 5.0 peak sun hours. Here's the whole calculation, filled in.
Inputs
Monthly usage1,000 kWh
Peak sun hours5.0 / day
Panel wattage400 W
Goal100% of bill
Result
21 panels
8.4kW system
~441sq ft roof
12,264kWh / year
That's 102% of the home's 12,000 kWh a year — a full offset with a hair of headroom.
What it costs in 2026
At about $3.00 per watt, an 8.4 kW system runs roughly $25,200 gross. Note the change: the 30% federal residential tax credit expired December 31, 2025 — if you buy with cash or a loan in 2026 there's no federal credit, so plan on the full amount. State, utility, and net-metering programs may still help; check your area.
Reference
Sun hours, costs & hookup
The numbers behind the estimate — how much sun each state gets, what solar actually costs in 2026, and the code rule that decides how much you can connect.
Peak sun hours by state · annual average
| State | Sun hrs/day | Solar zone |
|---|---|---|
| New Mexico | 6.8 | Southwest |
| Arizona | 6.5 | Southwest |
| Nevada | 6.4 | Southwest |
| Colorado | 5.5 | Sunbelt |
| California | 5.4 | Sunbelt |
| North Carolina | 5.25 | Sunbelt |
| Texas | 5.0 | Sunbelt |
| Florida | 5.0 | Sunbelt |
| U.S. average | 5.0 | — |
| New York | 3.8 | North |
| Massachusetts | 3.8 | North |
| Ohio | 3.5 | North |
| Washington | 3.5 | Pacific NW |
| Illinois | 3.1 | North |
| Alaska | 3.0 | Far North |
Approx. annual averages — NREL PVWatts v8 / NSRDB. Look up your exact address on NREL PVWatts for a precise figure.
Heads up · 2026
The federal tax credit changed
The 30% federal residential solar tax credit (Section 25D) expired December 31, 2025. If you buy with cash or a loan in 2026, you get no federal credit — a lot of older calculators still assume 30% and quote you numbers that no longer exist.
Still on the table: solar leases/PPAs (the installer claims the commercial §48E credit and may pass savings through), plus state, utility, and net-metering programs. Check dsireusa.org for your area.
Hooking up to your panel
NEC 705.12's 120% rule: your main breaker plus the solar backfeed breaker can't exceed 120% of the busbar rating. On a 200 A panel with a 200 A main, that caps solar near 7.7 kW AC — bigger needs a main-breaker derate or a line-side tap.
Panel wattage today
Standard modules run 400–420 W, high-efficiency 425–450 W, premium 450 W+. Higher wattage means fewer panels and less roof for the same system — at a higher price per panel. We default to 400 W.
Size to the year, not the winter
Net metering banks your summer surplus against winter nights, so you size to annual usage. Chasing your darkest December month oversizes a system 3–4×. Real-world losses (~20%) are already baked into the estimate.
What changes it
The levers — and the common traps
A few things push your panel count up or down. A few more quietly wreck the estimate. Here's both sides.
What drives your size
- +Your electricity usage. The single biggest factor — more kWh, more panels, in direct proportion.
- +Location & sun hours. A sunny state's panels do more work, so you need fewer for the same bill.
- +Panel wattage. Higher-efficiency modules cover the system with fewer panels and less roof.
- +Roof orientation, tilt & shade. South-facing and unshaded produces most; east/west or shaded needs more.
- +Temperature. Panels lose output when hot, which is why the estimate derates ~20%.
What trips people up
- —Sizing to the worst winter month. This oversizes a system 3–4×. Net metering lets you size to the annual average instead.
- —Using square footage, not kWh. Your habits and appliances set usage — a big house can use less than a small one.
- —Ignoring system losses. Nameplate is a lab number; real output is ~20% lower after heat, wiring and inverter.
- —Assuming the 30% tax credit. It expired for cash/loan buyers on Dec 31, 2025 — don't bank on it in 2026.
- —Forgetting the panel limit. Your main panel's 120% interconnection rule can cap how much solar you can connect.
The winter trap
Your roof gets maybe half the sun in December that it gets in June. Size to that dark month and you'll buy a system 3–4× too big that dumps free power all summer. Net metering exists precisely so you can size to your yearly usage and let summer surplus cover winter nights.
This is a planning estimate, not a design. Roof orientation, shading, and your utility's net-metering rules change the real answer — get quotes from licensed installers and check your utility. Start from your real usage with the Electrical Load Calculator.
Quick reference
How many panels for your usage
Tap your monthly usage and your local sun hours for a fast panel count — or scan the table below.
Monthly usage
kWh/mo
Peak sun hours
hrs/day
You'd need
19 panels
7.6 kW system
~399 sq ft
Panels by usage & sun hours · 400 W panels
| kWh / month | 3.5 hrs | 4.5 hrs | 5.5 hrs | 6.5 hrs |
|---|---|---|---|---|
| 500 | 15 | 12 | 10 | 9 |
| 750 | 23 | 18 | 15 | 13 |
| 1,000 | 30 | 24 | 19 | 17 |
| 1,250 | 38 | 29 | 24 | 21 |
| 1,500 | 45 | 35 | 29 | 25 |
| 2,000 | 60 | 47 | 38 | 33 |
Assumes 400 W panels, a 20% system loss, and covering 100% of your bill. Round up — a partial panel is a whole panel.
| Monthly usage | 3.0 sun hrs | 3.5 sun hrs | 4.0 sun hrs | 4.5 sun hrs | 5.0 sun hrs | 5.5 sun hrs | 6.0 sun hrs | 6.5 sun hrs |
|---|---|---|---|---|---|---|---|---|
| 400 kWh/mo | 14 | 12 | 11 | 10 | 9 | 8 | 7 | 7 |
| 600 kWh/mo | 21 | 18 | 16 | 14 | 13 | 12 | 11 | 10 |
| 800 kWh/mo | 28 | 24 | 21 | 19 | 17 | 16 | 14 | 13 |
| 1,000 kWh/mo | 35 | 30 | 27 | 24 | 21 | 19 | 18 | 17 |
| 1,200 kWh/mo | 42 | 36 | 32 | 28 | 25 | 23 | 21 | 20 |
| 1,500 kWh/mo | 53 | 45 | 40 | 35 | 32 | 29 | 27 | 25 |
| 1,800 kWh/mo | 63 | 54 | 47 | 42 | 38 | 35 | 32 | 29 |
| 2,000 kWh/mo | 70 | 60 | 53 | 47 | 42 | 38 | 35 | 33 |
| 2,500 kWh/mo | 87 | 75 | 66 | 58 | 53 | 48 | 44 | 41 |
| 3,000 kWh/mo | 105 | 90 | 79 | 70 | 63 | 57 | 53 | 49 |
How many solar panels do I need for my house?
Take your monthly kWh, divide by your peak sun hours and a ~20% loss factor to get system size, then divide by panel wattage. A typical 1,000 kWh/month home at average sun needs about 21 panels — an 8.4 kW system. Sunnier states need fewer; cloudier states need more.
How many panels for 1,000 kWh per month?
About 21 panels of 400 W at 5 peak sun hours — an 8.4 kW system on ~441 sq ft of roof. In a sunny state (6.5 hrs) that drops to about 17; in a cloudy one (3.5 hrs) it climbs to about 30.
Is there still a solar tax credit in 2026?
Not for most buyers. The 30% federal residential credit (Section 25D) expired December 31, 2025 — buy with cash or a loan in 2026 and you get no federal credit. Leases/PPAs can still pass through the commercial credit, and many states and utilities offer their own incentives.
How much does a solar system cost?
At roughly $2.50–$3.50 per watt, a typical 8 kW system runs about $20,000–$28,000 gross. With the federal credit gone in 2026, cash and loan buyers pay the full amount — though state, utility, and net-metering programs may lower it.
Do solar panels work in winter or on cloudy days?
Yes — just less. Panels run on daylight, not heat, so they still produce when it's cloudy or cold, at a lower rate. Peak-sun-hour figures are annual averages that already fold in cloudy days, and net metering banks summer surplus for winter.
How much roof space do I need?
Plan on about 21 sq ft per 400 W panel — so a 21-panel system needs roughly 441 sq ft of usable roof. Real installs add ~25% for fire setbacks, walkways, and gaps, so budget closer to 550 sq ft.
How many panels to run a whole house off-grid?
More than a grid-tied system. Off-grid has no utility to lean on, so you size for your worst days plus a battery bank for nights and cloudy stretches — often oversizing 1.5–2×. This calculator sizes grid-tied systems; off-grid needs a battery-autonomy design.
Does my house size determine how many panels I need?
Not really. Your actual electricity use drives the number, not square footage. A small home with electric heat, a pool, and an EV can out-use a big, efficient one. Always size from your kWh, not your floor plan.
Keep going
Related electrical calculators
You’ve got the amps — now put them to work. These pick up from there: convert watts, size the wire for the load, and see the V · I · R · P relationship behind it.
01
Electrical Load Calculator
Size your home's actual service load — the baseline your solar and any standby are built around.
Open
02
Generator Size Calculator
Planning backup too? Size a generator to your loads for when the sun's down and the grid's out.
Open
03
Circuit Breaker Calculator
Check the 120% interconnection limit and the breaker your solar backfeeds into.
Open
Browse all electrical calculators
How we keep this accurate
Sizing uses NREL peak-sun-hour data and a standard 20% system-loss assumption; interconnection follows NEC Article 705. Results are planning estimates — real production varies with roof orientation, shading, temperature, and utility rules, and the federal residential tax credit expired for cash/loan buyers after 2025. Verify with NREL PVWatts, your utility, and a licensed installer.