Solar Production Estimates & Simple Payback: PVWatts-Style Modeling

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Let's get into Solar Production Estimates & Simple Payback. Here's why it's worth your full attention: every savings number in a proposal rests on two estimates — how much energy the system will make and what that energy is worth — and both can be done honestly with free tools and simple math. Learn the basics and you'll catch inflated proposals, including your own.

Educational overview only — not electrical design, legal, tax, or financial advice, and not a substitute for licensed electricians, engineers, tax professionals, or attorneys. Follow the NEC as adopted by your authority having jurisdiction (AHJ), the building and fire codes in force there, the manufacturers' instructions, your utility's interconnection rules and tariffs, and current IRS guidance. Code editions, utility tariffs, and incentive rules change — verify before you rely on anything here.

Production Basics

A PV system's annual production depends on:

• System size (kW DC) — the total nameplate of the modules.
• Location — the solar resource varies a lot by region and climate.
• Orientation and tilt — azimuth and pitch of each roof face.
• Shading — trees, chimneys, neighboring buildings, and the seasons.
• System losses — soiling, wiring, mismatch, inverter efficiency, temperature, availability, and more.

A useful shorthand is specific yield: annual kWh per kW DC installed. It varies significantly by region and orientation — a sunny desert site and a cloudy northern site can differ by a wide margin — so always model the actual site rather than using a national rule of thumb.

PVWatts-Style Modeling

PVWatts is a free calculator from the U.S. Department of Energy's national lab system, and many commercial tools work the same way. You enter:

• Location (address), which pulls typical-year weather data.
• System size in kW DC.
• Module type (standard or premium) and array type (fixed roof mount, open rack, tracker).
• Tilt and azimuth.
• System losses — PVWatts uses a default total loss of about 14 percent, which you should adjust for real shading and conditions.
• DC-to-AC ratio and inverter efficiency.

It returns estimated monthly and annual AC energy in kWh. Because it uses typical weather, real production in any given year can be noticeably above or below the estimate. Professional design software adds detailed shading from site measurements or 3D models, which matters most on partially shaded roofs.

What the Energy Is Worth

Production only becomes savings through the tariff:

• Energy used on site avoids the retail rate (including time-of-use differences).
• Exported energy earns whatever the net metering, net billing, or avoided-cost rules pay.
• Fixed monthly charges and minimum bills don't go away.

That's why two identical systems can have very different value in two utility territories.

Simple Payback

Simple payback (years) = net installed cost ÷ first-year annual savings.

• Net installed cost = price minus any incentives the customer has confirmed they'll receive.
• First-year savings = the value of avoided purchases plus export credits, minus any new costs.

Simple payback ignores financing cost, degradation, utility rate changes, and maintenance — which is why it's "simple." It's a sanity check, not a full financial analysis.

A Worked Example (Made-Up Round Numbers)

Using hypothetical figures purely for practice: an 8 kW DC system at a site with a modeled specific yield of 1,250 kWh/kW/year produces about 10,000 kWh/year. If the customer uses 60 percent on site at an effective rate of 15 cents/kWh and exports 40 percent at a net-billing credit of 6 cents/kWh, savings are 6,000 × 0.15 + 4,000 × 0.06 = 900 + 240 = $1,140/year. At a hypothetical net cost of $20,000, simple payback ≈ 17.5 years. Change the tariff to full retail net metering and savings become 10,000 × 0.15 = $1,500, payback ≈ 13.3 years. Same panels, very different answer. Real prices and rates vary widely by region and over time — these numbers are for the math only.

Going Deeper (Intermediate)

Add realism step by step: degradation (modules lose a small fraction of output each year — check the module warranty for the guaranteed rate), inverter replacement for string inverters partway through the system life, rate escalation (state the assumption and offer a scenario with no escalation), and financing cost. Lifetime metrics such as net present value, internal rate of return, and levelized cost of energy follow from the same inputs.

Advanced / Pro-Level

Pros model hourly: production by hour against the customer's interval data from the utility, priced through the actual tariff periods. That's the only accurate way to estimate savings under time-of-use rates and net billing, and to size batteries. They also validate their models against real production data from past installs, adjusting loss assumptions until estimates match reality, and they present ranges (expected, conservative) rather than a single best-case number.

Practice Challenge

A competing proposal shows a 7 kW system in your region producing far more per kW than your validated model — and claims "100 percent bill offset." What do you check? (Answer: compare their specific yield to a PVWatts run for the same address, tilt, and azimuth; look for unrealistically low losses or ignored shading; check whether they used retail net metering when the utility uses net billing; and whether fixed charges were ignored. Show the customer the side-by-side assumptions.)

In Practice

A designer who runs a quick PVWatts check on every proposal catches bad inputs before they reach the customer. Honest estimates build referrals; inflated ones build complaints when the first true-up bill arrives.

Common Mistakes to Avoid

• Using one national kWh-per-kW number for every site
• Ignoring shading or using default losses on a shaded roof
• Valuing all production at the retail rate under a net billing tariff
• Including unconfirmed incentives in the net cost
• Presenting a single best-case number instead of a range

Takeaway: Honest savings start with a site-specific production model and the real tariff — simple payback (net cost ÷ first-year savings) is a quick sanity check, and every assumption behind it should be written down for the customer.