Can a Solar System Be Too Big? Annual True-Up Explained
An oversized solar system can produce kWh your utility never credits. See where the waste shows up in sizing tables and how to pick a smarter size.
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In this article · 9 sections
Short answer
Yes, a solar system can be too big. When your panels make more power over a year than your home buys from the grid, the extra kilowatt-hours (kWh) may earn no credit at all. In our example, the best size is below 100% offset. Past that point, each added panel costs money but saves nothing.
This depends on your utility and your state's rules. Always check how your utility handles extra production before you sign a quote.
What an annual true-up means
Many net metering programs track your credits all year. Sunny months build up credits. Dark months use them. At the end of the year, the utility does a "true-up," which is a yearly settlement of what you sent to the grid and what you bought.
In our model, export credits are capped at a year of grid purchases. That means you can wipe out your annual purchases, but you cannot earn more than that. Any production beyond the cap is uncredited. Some utilities pay a low rate for leftover kWh. Others pay nothing. The model assumes no extra value, so you can see the downside clearly.
For a full comparison of crediting styles, see our guide on net metering vs. net billing payback. If kWh is a new term, start with what is a kilowatt-hour.
The example home
Every number below uses the same example home. Here are its assumptions:
- 10500 kWh/yr of electricity use
- 40% of use happens while the sun is up
- Installed cost of $3000 per kW, with no incentives
- $100/yr maintenance
- Electricity price rising 2.5%/yr
- Panel degradation of 0.5%/yr
- A 25-year horizon
- Export credits capped at a year of grid purchases
Production comes from PVWatts v8 for a south-facing roof at 20° pitch with 14% losses, using one reference city per state. Prices are EIA July 2026 average residential. PVWatts is a free solar production estimator from the U.S. Department of Energy's National Laboratory of the Rockies (formerly NREL). Our methodology page explains the full model.
Where the uncredited kWh show up in the tables
Look at the year-1 savings column. In Texas, savings rise from $677 at 3 kW to $1129 at 5 kW to $1580 at 7 kW. Then they reach $1667 at 9 kW and stay at $1667 at 12 kW. The panels keep getting more expensive, but the savings stop growing. That flat line is the cap at work. Everything produced above it is uncredited.
The same pattern appears in every state we modeled:
| State | 7 kW year-1 savings | 9 kW year-1 savings | 12 kW year-1 savings |
|---|---|---|---|
| Texas | $1580 | $1667 | $1667 |
| California | $3529 | $3529 | $3529 |
| Florida | $1578 | $1578 | $1578 |
| Arizona | $1615 | $1615 | $1615 |
In California, Florida and Arizona, savings are already capped at 7 kW. Going to 9 kW or 12 kW adds cost and no savings in year 1. In Texas, 7 kW is slightly short of the cap, and 9 kW just reaches it.
How the cap changes payback and 25-year gain
Savings that stop rising while cost keeps climbing push payback out. Payback is the time it takes for savings to repay the system cost. Our payback period guide covers how that is calculated.
| State | 7 kW payback | 9 kW payback | 12 kW payback |
|---|---|---|---|
| Texas | 12.5 years | 14.4 years | 18.2 years |
| California | 5.8 years | 7.3 years | 9.4 years |
| Florida | 12.3 years | 15.1 years | 19 years |
| Arizona | 12 years | 14.8 years | 18.7 years |
The 25-year gain tells the same story. In Florida, the 25-year gain is $28296 at 7 kW, $24406 at 9 kW and $15406 at 12 kW. In Arizona it is $31661, $25661 and $16661. In California it is $96966, $91044 and $82044. In Texas, 9 kW edges ahead at $27455 versus $27028 at 7 kW, then 12 kW falls to $18455.
So in these runs, the biggest gain usually sits at or near the size that just covers annual use. A bigger system gives back less, not more.
Why the best size is below 100% offset
Offset means the share of your yearly use that solar covers. You might expect 100% to be the sweet spot. The tables suggest otherwise, for three reasons.
- Model degradation. Panels lose 0.5%/yr, so production shrinks over time. But a system sized to the edge of the cap wastes little early on.
- The cap is a hard wall. Production above annual purchases earns nothing in this model, so the last panels you add have no payoff.
- Your usage may change. If you add an EV or a heat pump, a slightly larger system can make sense. Our EV charging cost guide shows how much that load can matter. If you plan that change, size for it only when it is firm.
Treat the exact best size as an estimate. It moves with your rate plan, your roof and your utility's rules.
How big is "just right" in your state
The kW needed to produce 10500 kWh/yr varies a lot by location:
| State | kW needed |
|---|---|
| Arizona | 6 kW |
| California | 6.3 kW |
| Florida | 6.8 kW |
| Texas | 7.4 kW |
| Illinois | 8 kW |
| New York | 8.1 kW |
| Massachusetts | 8.1 kW |
| Washington | 9.7 kW |
Sunny states need fewer panels for the same output. Washington needs the most of the states listed. These figures are the size where production roughly equals use. Sizing at or a little under these numbers keeps you from paying for uncredited kWh. For help estimating panel count, see how many solar panels do I need.
What to ask your installer and utility
- Does my utility credit extra kWh at the end of the year? At what rate?
- Is my plan net metering or net billing? Does the credit change by time of day?
- Is there a size limit tied to my past usage?
- Will my usage change in the next few years?
You can check state programs in the DSIRE database at https://programs.dsireusa.org/, which tracks current net metering and incentive programs. Rules vary by state and utility. The federal 30% Residential Clean Energy Credit is not available for property placed in service after December 31, 2025, so do not assume it lowers your cost. Our guide on solar without the tax credit explains the effect.
Bottom line
A bigger system is not always a better one. In our example, once production passes your annual grid purchases, extra panels add cost but no savings, and payback gets longer. Aim for a system near your yearly use, then adjust for known changes like an EV. Use the solar calculator to test sizes with your own numbers, and check electricity prices for your state. Then ask your utility how it treats extra production before you buy.
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