Step 1: find your annual kWh
Pull 12 months of usage from your utility bill or online account, in kilowatt-hours, not dollars. Most bills include a 13-month usage chart. Dollars are misleading because rates, fixed charges and riders change; kWh is what panels replace.
If you’ve lived in the home less than a year, or recently added an EV, a heat pump or a hot tub, tell us. We’ll adjust the history rather than size to a year that no longer represents your house.
Step 2: production per kW at your address
This is the number most online calculators fudge. We use PVWatts, the solar production model from the national renewable-energy lab in Golden. For Denver, at its default assumptions (a fixed roof mount, standard panel, 14% system losses), a south-facing array at 25° tilt comes out at about 1,626 kWh per kW per year. At 20° tilt it’s about 1,595.
Direction matters more than most people expect. The same Denver array facing due east models at about 1,346 kWh per kW, and due west at about 1,286. That’s roughly 17–21% less than due south. Location matters too: a 20° south-facing array in Fort Collins models at about 1,464. Shade from trees or neighboring homes comes off those numbers, which is why we model your actual roof, not a city average.
Step 3: do the division (worked examples)
A Denver home using 9,000 kWh a year on a south-facing roof: 9,000 ÷ ~1,600 ≈ 5.6 kW. With 420-watt panels, that’s 5,600 ÷ 420 ≈ 13–14 panels. At our installed price of $2.80–$3.25 per watt, a 5.6 kW system runs about $15,700–$18,200 before incentives.
The same home with only east and west roof faces: 9,000 ÷ ~1,315 (averaging the two) ≈ 6.8 kW, or about 16–17 panels, and about $19,000–$22,100. A split east-west array isn’t a bad design. It spreads production across the morning and late afternoon, which can suit time-of-use rates. It just takes more panels to make the same annual energy.
Step 4: check the roof and the rules
A typical 400–450 W panel is roughly 3.5 by 6 feet. Fire-code setbacks from ridges and edges, vents, skylights and chimneys all eat into usable area, so a roof that looks big from the street may fit fewer panels than you’d guess. If the roof can’t fit the full size, a ground mount or a slightly smaller system are both reasonable answers.
Your utility also caps system size relative to your usage. Xcel’s installer materials have described sizing up to 120% of your prior 12 months of use. Fort Collins Utilities caps customer-owned systems at 12 kW or 200% of usage. Other co-ops and municipal utilities set their own limits. We design to your utility’s current rule.
Plan for the loads you’re adding
An EV typically adds a few thousand kWh a year, depending on the vehicle and your miles. A heat pump replacing a gas furnace can add more than that in a cold climate. If either is coming in the next few years, it’s usually cheaper to size for it now than to add panels later, and in some territories, like Longmont, expanding an existing system can change how its exports are credited.
Be realistic, though. Oversizing for a load you may never add can mean exporting power for less than you pay for it. We’d rather size to what’s certain and leave room to expand.
When fewer panels is the right answer
If your utility credits exports below retail, like Longmont’s value-of-solar rate for newer systems or Fort Collins’ time-of-day credits, sizing to cover 100% of annual use may not be the best value. Covering most of your daytime use and pairing it with a battery or smart load timing can pay back faster. That’s a per-utility calculation, and it’s one of the first things we run.
FAQ
Questions we hear
How many kWh does 1 kW of solar produce in Colorado?
In Denver, PVWatts models about 1,600 kWh per kW per year for a south-facing roof at a typical pitch, and roughly 1,300 for east or west faces. Other locations, tilts and shading change it, so we model your address specifically.
How many panels does it take to cover a 1,000 kWh-per-month bill?
12,000 kWh a year ÷ about 1,600 kWh per kW ≈ 7.5 kW on a good south-facing Denver roof, or about 18 panels at 420 W. East-west roofs need closer to 9 kW. Shade and utility size caps can change the answer.
Should I size solar for an EV I plan to buy?
If the EV is coming within a year or two, yes. Adding capacity later costs more per watt. If it’s a maybe, size for today and keep roof space open.
Can I install more solar than I use?
Only up to your utility’s cap, and it’s rarely worth it where exports are credited below retail. Xcel has described a cap of 120% of prior-year usage; municipal utilities and co-ops have their own limits.
Does panel wattage matter?
Higher-wattage panels fit more capacity on a small roof, but price per watt is what matters for cost. On a roof with plenty of space, a standard panel at a good price per watt is usually the better value.
Why is my quote a different size than an online calculator said?
Most calculators use a statewide or city average and ignore your roof direction, shade, setbacks and utility cap. A real design accounts for all four.