How many solar panels do I need for a $200 monthly bill?

At the US average rate of $0.17/kWh, a $200 bill is about 1,176 kWh a month. In a mid-sun location (4.5 peak sun hours) that needs a 10.11 kW system — 26 panels at 400 W each, using roughly 468 sq ft of roof. In a sunnier state the same bill needs about a fifth fewer panels; in a cloudier one, about a fifth more.

What does the 0.85 derate factor mean?

Panels are rated under laboratory conditions. In the real world the inverter loses a few percent, cables lose a little, dust and heat lose more — together, typically about 15%. Multiplying by 0.85 converts the nameplate rating into an honest expected yield. It is the same adjustment installers build into their production estimates, stated openly here.

Is the 30% federal solar credit a discount?

No — and the difference matters. The 30% figure is a US federal tax credit: it reduces the income tax you owe for the year the system is placed in service, which means you pay the installer the gross price up front and recover the credit at tax time. It generally applies to owned systems at a US residence, and if your tax bill is smaller than the credit the unused part may carry forward. Eligibility rules change, so check your own situation before banking on the net cost this calculator shows.

How long do solar panels take to pay for themselves?

For the worked example above — $200 bill, average rates, mid-sun location — about 8.4 years on the net cost after the federal credit. High electricity rates shorten payback far more than high sunshine does: the same system against a $0.35/kWh rate pays back in roughly half the time, because every generated kWh is worth twice as much.

How much roof space does a typical system need?

Plan on about 18 sq ft per panel including access spacing — roughly 180 sq ft per kW. A 6 kW system needs about 325 sq ft of clear, preferably south-facing roof; the 10.11 kW worked example needs about 468 sq ft. Use the Roof Pitch Calculator to check your slope: production holds up well between about 15° and 40°.

Should I size for 100% of my usage?

Usually close to it, but not always above it. Many utilities cap the credit you can earn on exported power, and some limit system size to your historical usage, so oversizing past 100% often earns the worst return per panel. If you expect an EV, a heat pump or an addition, sizing for the usage you are about to have is the reasonable exception.

Solar Panel Calculator — Free, With Formula & Worked Example

About this calculator

Solar sizing starts from the bill, not the roof: how many kWh you use, what each one costs you, and how many your location can generate per installed kW. This calculator turns an average monthly bill into a system size for a 100% offset, counts the panels and roof area, prices the system at a stated cost per watt, applies the federal tax credit, and works out the simple payback and 25-year position — every formula published, every assumption on the page.

What this number means

A solar quote is really three numbers wearing one coat: how much electricity you use, how much a system on your roof would produce, and what installers charge per watt where you live. This calculator separates them, so when a quote lands you can see which of the three it disagrees with — an oversized system, an optimistic production estimate, or a price per watt above your local market.

The result targets a 100% bill offset: a system sized to generate, across a year, the same kWh you consume. That is a sizing convention, not a promise — you will still export power on sunny afternoons and import it at night, and what that export is worth depends on your utility's net-metering terms. The payback figure is deliberately simple (net cost ÷ today's annual savings) so you can check it by hand; the assumptions section states everything it leaves out.

The formula

Monthly electricity use = Monthly bill ÷ electricity rate ($/kWh)
Annual use = Monthly use × 12
Production per kW per year = Peak sun hours × 365 × 0.85 (the 0.85 derate covers inverter, wiring, soiling and temperature losses)
System size (kW) = Annual use ÷ production per kW
Panels = System watts ÷ panel wattage, rounded up · Roof area = Panels × 18 sq ft
Gross cost = System watts × $2.85 per watt (a visible planning assumption — 2026 US installed prices cluster around $2.50–$3.20/W)
Net cost = Gross cost − (Gross × tax credit %)
Payback = Net cost ÷ annual savings · 25-year net savings = (Annual savings × 25) − net cost

Worked example

A $200 monthly bill at $0.17/kWh, 4.5 peak sun hours, 400 W panels, with the 30% credit.

Use: $200 ÷ $0.17 = 1,176 kWh a month → 14,118 kWh a year
Yield: 4.5 × 365 × 0.85 = 1,396 kWh per kW per year → system size = 14,118 ÷ 1,396 = 10.11 kW
Panels: 10,110 W ÷ 400 W = 25.3 → 26 panels, needing about 468 sq ft of clear roof
Cost: 10,112 W × $2.85 = $28,819 gross; the 30% credit is worth $8,646 → $20,173 net
Savings: 14,118 kWh × $0.17 = $2,400 a year → payback = $20,173 ÷ $2,400 = 8.4 years, and $39,827 net over 25 years

Assumptions and limitations

The 0.85 derate is a fixed planning figure for a well-sited, unshaded, roughly south-facing array. Heavy shade, a north-facing roof, or a steep east–west split can cut real production 15–30% below it.
Savings assume every generated kWh displaces a kWh you would otherwise buy at today's rate (full retail net metering). Where export is paid at a lower wholesale rate, or a fixed grid charge stays on the bill, savings and payback stretch accordingly.
No electricity price rises are modelled — historically rates rise, which shortens real paybacks — and panel degradation (about 0.5% of output per year) is not deducted, which lengthens them slightly. The two roughly offset over 25 years.
The $2.85/W price is a national planning midpoint for a standard rooftop install. Complex roofs, ground mounts, battery storage, main-panel upgrades and high-permit-cost cities price above it; large simple systems can price below it.
The federal credit is a US tax credit: it reduces tax you owe, generally requires the system to be owned (not leased) and installed at your US residence, and unused amounts may carry forward. It is not a rebate and not everyone can use the full amount in year one — check your own situation, ideally with a tax professional, before relying on the net figure.