Assumes 3%/yr rate increases, 0.5%/yr panel degradation and no federal tax credit (the 30% §25D credit ended for systems installed after Dec 31, 2025). How we calculate
Data approachEIA rates · NREL sun hours · 2026 federal policy · methodology
A typical in-law suite adds about 450 kWh of electricity use per month, which takes roughly 4 kW of extra solar, or 10–11 panels of 400 W, at 4.5 peak sun hours. Smaller or more efficient suites can need as few as 7 panels; a large suite with electric heat and water heating can need 20 or more. The right number depends on the suite’s actual usage and on how much sun your location gets, not on its square footage alone.
One 2026 change matters for the budget: the 30% federal tax credit for homeowner-purchased solar ended for installations after December 31, 2025. The costs below are shown before any incentives.
🌎 Electricity Load
How Much Electricity Does an In-Law Suite Add Per Month?
An in-law suite is effectively a second household, so its load looks like a small apartment or small home. There is no single government average for in-law suites, so the figures below are planning scenarios, not measured averages. They are anchored to U.S. Energy Information Administration (EIA) Residential Energy Consumption Survey (RECS) 2020 figures: the average U.S. household used 10,566 kWh a year (about 880 kWh a month), while the average apartment in a building of five or more units in the Northeast used about 4,120 kWh a year (about 343 kWh a month). The average single-family detached home in the South used about 15,819 kWh a year (about 1,318 kWh a month).
Planning scenarios for in-law suite electricity use
Suite scenario
Planning load (kWh/month)
Annual kWh
Studio, efficient, no electric water heating
300
3,600
1-bedroom with kitchenette
450
5,400
Full 1-bedroom with electric water heater
600
7,200
2-bedroom, electric heat and cooling, electric water heater
900
10,800
Four factors move the number most: the suite’s size, your climate, whether the suite has its own electric water heater, and how many people live there. Electric resistance water heating is often the largest single swing. The U.S. Department of Energy notes that heat pump water heaters are two to three times more efficient than conventional electric resistance models, so choosing one for the suite can reduce the solar capacity you need.
For the most accurate starting point, use 12 months of real data: the suite’s own meter or sub-meter if it has one, or the difference in your main bill before and after the suite was occupied. Then plug it into our solar system size calculator.
Data visualization
Extra 400 W panels needed by in-law suite load: 7 to 21 panels at 4.5 peak sun hours. Calculated with the sizing formula in this guide (0.80 system factor); loads are planning scenarios anchored to EIA RECS 2020, not measured averages.
Chart summary: Panel count rises in direct proportion to the suite’s electricity use, from about 7 panels for an efficient studio to about 21 for an all-electric two-bedroom suite. The jump between the 450 and 600 kWh/month bars is mostly an electric water heater, which is why water heating deserves attention before you size the system.
⚡ System Size
How Many Extra Solar Panels Do You Need for an In-Law Suite?
Once you know the suite’s monthly kWh, the calculation is simple:
Extra kW needed = monthly kWh ÷ (peak sun hours per day × 30 days × 0.80)
The 0.80 factor covers typical system losses such as wiring, inverter conversion, heat, and soiling. NREL’s PVWatts calculator uses a default of about 14% system losses, which gives a similar result.
Worked example (450 kWh/month, 400 W panels):
At 4.5 peak sun hours: 450 ÷ (4.5 × 30 × 0.80) = 450 ÷ 108 = 4.17 kW, or 11 panels
At 3.5 peak sun hours: 450 ÷ 84 = 5.36 kW, or 14 panels
At 5.5 peak sun hours: 450 ÷ 132 = 3.41 kW, or 9 panels
Peak sun hours vary widely across the country. As a rough guide, the Pacific Northwest sits at the low end of the range and the Desert Southwest at the high end. Look up your exact ZIP code in NREL’s PVWatts calculator rather than relying on a state average.
Panels needed by suite load and sun hours (400 W panels, 0.80 system factor, rounded up):
Suite load (kWh/month)
3.5 sun hours
4.5 sun hours
5.5 sun hours
300
9 panels (3.6 kW)
7 panels (2.8 kW)
6 panels (2.3 kW)
450
14 panels (5.4 kW)
11 panels (4.2 kW)
9 panels (3.4 kW)
600
18 panels (7.1 kW)
14 panels (5.6 kW)
12 panels (4.6 kW)
900
27 panels (10.7 kW)
21 panels (8.3 kW)
18 panels (6.8 kW)
These counts are calculated from the formula above, not measured results. Roof space, shading, tilt, and orientation can all change the real answer, so treat the table as a starting point for installer quotes.
What it costs. Marketplace quotes on EnergySage averaged about $2.58 per watt before incentives in early 2026, while Lawrence Berkeley National Laboratory’s installed-price data put the median for cash-purchased systems in 2024 near $3.50 per watt. Small add-on jobs often land toward the higher end because fixed costs are spread over fewer panels. For a 4 kW addition, that range is about $10,300 to $14,000.
Data visualization
A 4 kW solar addition costs $10,320 to $14,000 before incentives. Price points: $2.58/W EnergySage marketplace average (early 2026), $3.00/W midpoint used in this guide’s payback model, $3.50/W LBNL median for 2024 cash purchases. No federal credit applies to 2026 purchases.
Chart summary: A 4 kW addition costs roughly $10,300 to $14,000 before incentives, depending on which price benchmark your installer’s quote resembles. The $3,700 gap between the low and high bars is a good reason to collect at least three quotes.
Run the numbers for your state
The savings calculator loads your state's average electricity rate, sun hours and net-metering rule. Then enter your own bill and installer quote.
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📋 Key Insights
Should You Expand Your Existing System or Install a Separate Array?
There are three realistic paths, and the right one depends on whether you already have solar.
Option 1: Size one system for both from the start. If you have not installed solar yet, this is usually the most economical route. Larger systems tend to cost less per watt because fixed costs such as design, permitting, and installation labor are spread across more panels, and you complete one permit and one utility interconnection application instead of two.
Option 2: Expand your existing system. This works when your roof has unused space and your inverter has capacity for more panels. If your inverter is already near its limit, you may need an additional inverter, which adds cost. Ask your installer to check inverter headroom, panel-level compatibility, and whether the original warranty and monitoring remain intact.
Option 3: Install a separate array for the suite. A garage roof, the suite’s own roof, or a ground mount can host a dedicated array wired to a sub-panel. This offers cleaner tracking of the suite’s generation and can use roof surfaces that face different directions. Separate roof planes can also spread production across more of the day, and microinverters or power optimizers handle mixed orientations better than a single string inverter.
Whichever route you choose, ask each installer to price both the combined and the add-on approach for your property. Also check whether your utility treats the suite as a separate meter, because that affects how credits are applied (see the incentives section below). For a full price breakdown, see our guide to How Much Do Solar Panels Cost in 2026? Complete US.
📈 Payback Period
What Is the Solar Payback Period for a Home With an In-Law Suite?
Here is a transparent model for a 4 kW addition serving a 450 kWh/month suite. Change the inputs to match your own quote.
Assumptions
System cost: $12,000 ($3.00/W), no federal credit
Production: 4.0 kW × 4.5 sun hours × 365 days × 0.80 = about 5,256 kWh in year 1
Electricity price: 18.2¢/kWh, the EIA’s forecast 2026 U.S. residential average
Rate growth: 2% a year; panel output decline: 0.5% a year
Every kWh produced offsets a kWh you would otherwise buy at the full retail rate
Results
Year-1 savings: about $957
Payback: about 12 years (13 years with no rate growth)
25-year cumulative net savings: about $16,700, before inverter replacement, maintenance, and financing costs
How your rate changes the answer (simple payback, no rate growth): at about 30¢/kWh, a rate close to New York’s average in 2026, payback is roughly 7.6 years; at about 12¢/kWh it stretches to about 19 years. High-rate states shorten payback, and low-rate states lengthen it.
Data visualization
Modeled 25-year cash flow for a 4 kW suite addition: break-even in year 12, about $16,700 net by year 25. Assumes $12,000 cost with no federal credit, 5,256 kWh in year 1, 18.2 cents per kWh (EIA 2026 average) rising 2% a year, and 0.5% annual output decline. Excludes financing, maintenance, and inverter replacement.
Chart summary: The blue line crosses the orange break-even line in year 12 and ends near $16,700 in cumulative net savings after 25 years. The model assumes 2% annual rate growth; with flat rates, break-even moves to year 13. A lower electricity rate, lower export credits, or a higher system price pushes break-even later, while a higher rate pulls it earlier.
Incentives in 2026: What Replaced the 30% Federal Tax Credit?
The federal homeowner credit is gone for new installs. The Residential Clean Energy Credit (Section 25D) covered 30% of qualifying solar costs, but the One Big Beautiful Bill Act ended it for expenditures after December 31, 2025. Per the IRS, a system whose installation is completed after that date cannot claim it. Homeowners who finished installations by the end of 2025 can still claim it on their 2025 return using Form 5695 and can carry forward unused credit.
Leases and PPAs are different. Companies that own the equipment can still claim the commercial credit (Section 48E) under current law, subject to deadlines and eligibility rules, and some may pass part of the value to you through lower lease or PPA pricing. Compare the total 25-year cost of a lease or PPA against buying, and read any escalator clauses carefully.
State and utility programs. Rebates, property-tax exemptions, and sales-tax exemptions vary by state and utility. Search your location in the DSIRE database and ask installers which programs they handle.
Net metering depends on where you live. Many utilities credit exported solar power at less than the retail rate. California, for example, moved new customers to a net billing tariff in 2023 with lower export credits, and Texas has no statewide net metering requirement, so buyback plans vary by electricity provider. Because most of a suite’s load is used during evenings and mornings, a battery or time-of-use strategy can improve savings where export credits are low. See our state pages for California and Texas, and ask your utility these questions before you sign:
Does it allow a second meter, or does the suite stay on a sub-panel behind one meter?
Are exports credited at retail or at a lower rate?
Are there fees, system-size caps, or special rules for accessory dwelling units?
If you rent the suite to a tenant, tax treatment can differ from owner-occupied use. Consult a tax professional and your local permitting office before you finalize the design.
Use our solar savings calculator to project your 25-year value using your own rate and system size.
Sources and Methodology
All calculated figures in this guide (panel counts, system costs, payback, and cumulative savings) come from the formula and assumptions stated above and are estimates, not quotes or guarantees. Suite loads are planning scenarios.
Direct answers for US homeowners — sized for a $75/month electric bill.
Most in-law suites need about 7 to 21 extra 400 W panels, depending on the suite's electricity use and your local sunshine. A suite using about 450 kWh a month needs roughly 4.2 kW, or 10–11 panels, at 4.5 peak sun hours. The same suite needs about 9 panels at 5.5 sun hours and about 14 at 3.5. Use the suite's actual monthly kWh as your starting point, then model your ZIP code in NREL's PVWatts calculator.
No, not for homeowners who buy a system. Congress ended the Residential Clean Energy Credit (Section 25D) for expenditures after December 31, 2025, so a cash- or loan-funded system installed in 2026 gets no federal credit. Companies that own leased or PPA systems can still claim a commercial credit (Section 48E) under current law and may pass part of that value to you through lower pricing. Confirm current rules with your installer and a tax professional.
Sizing for the suite from the start is usually cheaper per watt. Larger systems tend to cost less per watt because fixed costs such as design, permitting, and crew time are spread across more panels, and a single install means one permit and one utility interconnection application. Adding capacity later can also run into inverter or roof-space limits. Ask installers to quote both options for your home so you can compare real numbers.
A modeled 4 kW add-on costing about $12,000 and producing about 5,250 kWh a year pays back in roughly 12 years at the 2026 U.S. average rate of about 18.2¢/kWh, assuming 2% yearly rate growth and full retail-rate credit for the power. At about 30¢/kWh the simple payback is closer to 8 years; at 12¢/kWh it stretches to about 19 years. Your own quote, rate, and net metering rules decide the real figure.
It depends on your utility and state. Net metering credits typically apply to the meter the solar system is connected to, so a suite on its own meter may not receive credits from a system tied to the main home's meter. Some utilities and states offer multi-meter or virtual net metering programs, and many homeowners keep one meter and a sub-panel for the suite. Ask your utility how it handles additional meters before you finalize the design.
Same usage, bill-based guide
Your in-law suite target maps to roughly a $75/month electric bill nationally.
18.19¢/kWh — US average residential price. Source: EIA Electric Power Monthly, Table 5.6.B (year-to-date through July 2026), residential average retail price (July 2026) (EIA)
Solar production
4.5 peak sun hours/day × 0.82 system derate. Approximate state-average daily solar resource (peak sun hours) based on NREL solar resource data (NSRDB); not location-specific — use NREL PVWatts for an address-level estimate. (NREL PVWatts)
Installed price
$3.00 per watt before incentives. Blended 2026 US residential installed price used by this site; EnergySage marketplace reported about $2.60/W (mid-2026); full-market medians are higher.
Federal tax credit
$0 for homeowner-owned systems installed in 2026 — the 30% §25D credit ended Dec 31, 2025 (IRS)
Net metering
National blend — solar assumed to offset 75% of the bill (87% in full-retail net-metering states, 55–70% elsewhere).
Estimates only — not tax, legal or financial advice. Get at least three installer quotes and confirm incentives with your utility and a tax professional.