Eyeballing a solar system size from your monthly electric bill is reasonable, but it's worth doing the math once before committing thousands of dollars. This guide walks through a practical sizing approach — what numbers to gather, what assumptions to make, and how to deal with the gaps reality will leave.
Step 1: find your annual kWh
Your electric bill shows usage in kWh per month or per billing cycle. Pull the last 12 months — solar production varies seasonally, so a single month's number misleads. Add the 12 values; that's your annual kWh. Convert to daily by dividing by 365.
A typical US household uses about 30 kWh per day, but the spread is wide — 15 kWh/day for an efficient small home, 50+ kWh/day for a large home with electric heating or an EV.
Step 2: find your peak sun hours
Peak sun hours (PSH) is the number of hours per day, on average, that your location receives sunlight strong enough to produce a panel's full rated output. It's not 'hours of daylight'; it's an integrated measure of solar irradiance.
Use our solar calculator which has PSH for every US state. For ballpark purposes:
- Northern US (Washington, Vermont, Maine): ~3.5 to 4 PSH
- Central US (Kansas, Missouri, Virginia): ~4.5 PSH
- Southern US (Texas, Florida, Arizona): ~5 to 6 PSH
Use the annual average for production sizing; use the December (worst month) number if you want a system that produces enough in the worst month without grid backup.
Step 3: derate for real-world losses
A solar panel's nameplate rating is its output under Standard Test Conditions. Real-world output is lower due to:
- Inverter losses (about 3 to 5 percent)
- Wiring and connection losses (about 2 percent)
- Temperature derating (about 5 to 10 percent — panels lose output as they get hotter)
- Dust and soiling (about 2 to 5 percent)
- Orientation / tilt not perfectly south at latitude tilt (about 5 percent if off)
The combined derate factor is roughly 0.75 to 0.85. A 0.80 derate is a reasonable planning number for a residential rooftop system.
Step 4: do the math
Daily system output (kWh) = panel wattage × PSH × derate / 1,000.
For example, a 6,000 W (6 kW) system in a region with 5 PSH, derated at 0.80:
6,000 × 5 × 0.80 / 1,000 = 24 kWh per day
If your daily household usage is 30 kWh, that system covers 80 percent of your annual needs. To cover 100 percent, you'd need roughly 7,500 W of panels — but you would oversize or undersize depending on:
- Net metering rules (can you bank summer surplus for winter?)
- Roof area constraints
- Budget
- Whether you also plan to charge an EV in future
Step 5: round and over-spec slightly
Real installs produce 10 to 15 percent less than the simple calculation in the real world, after shading, soiling, equipment aging, and seasons. If your math says you need 7 kW, specifying 7.5 to 8 kW of panels handles the real-world haircut and still gets you close to your target.
That said — buying 20 percent more than you need to 'future-proof' usually isn't worth it unless you're planning a specific expansion (EV, heat pump, etc.). Solar panel efficiency improves year over year and panels get cheaper; the math for adding more later usually works out fine.
Renting or Can't Install Rooftop Solar? Here's What Works
Not everyone can put panels on a roof — renters, apartment dwellers, and anyone with an HOA or landlord restriction still have real options. Plug-in and balcony solar kits let you generate real power without any permits or electrician involved.
Best for balconies: Anker SOLIX Balcony Solar Panel Kit A genuinely plug-and-play system — two 410W panels (820W total) feed solar power straight into a standard outlet, no electrician or installation required. App-controlled and designed specifically for balcony/patio setups where rooftop isn't an option. Price: $899 — Buy on Amazon
Best starter kit: PluggedSolar 800W Plug-In Solar Kit A 4-panel, 200W-per-panel kit built for balcony or roof mounting, with a UL 1741-certified microinverter so it's safe to plug directly into a standard outlet. Expandable up to 1,200W if you want more capacity down the line. Complete kit includes WiFi power monitor and a 50ft plug-in cord. Price: $699 (sale, regular ~$945) — Buy on Amazon
Best for maximizing output: PluggedSolar 1200W Plug-In Solar Kit The expanded 6-panel version of the kit above, for renters or homeowners who want to offset more of their electricity use without a full rooftop install. The maximum capacity you can run on a single standard household outlet — best value per watt if you have the railing or roof space. Price: ~$900–$1,100 (varies by Amazon variant) — Buy on Amazon
Best all-in-one with built-in storage: OSCAL Power Storage 2000 The kits above push solar straight to the grid or an outlet — they don't keep the power you generate. The Power Storage 2000 puts a battery in the same plug-and-play unit: 1,920Wh LiFePO4 (expandable to 9.6kWh), 2,400W solar input across four MPPT trackers, an 800W bidirectional microinverter built in, and IP65 weatherproofing. Runs on-grid or off-grid, needs no electrician, and sits on a balcony the same way the Anker kit does — but your solar actually stays yours. If your net metering terms are poor or you want to use your own solar at night, this is the balcony option to beat. Price: $1,145.60 — Buy on Awin
Quick guidance: If you're mainly looking to offset a modest amount of your monthly usage with minimal setup, start with the Anker SOLIX Balcony Kit. If you want more raw wattage and don't mind a slightly more involved (but still no-electrician) setup, the PluggedSolar kits scale further. And if you want to store and actually use your own solar rather than just offsetting daytime grid draw, the OSCAL Power Storage 2000 is the only balcony kit here with a built-in battery.
Portable panels for charging a power station
The kits above offset household grid use. If you instead want to keep a portable power station charged during an outage or off-grid trip, a folding portable panel is the tool — and a panel that charges an OSCAL PowerMax or BLUETTI station will work on any station with a compatible input. Match the panel wattage to your station's solar input rating (a 400W-input station won't use a panel bigger than ~400W at full power).
Best budget portable panel: OSCAL PM200 200W A 200W folding panel at up to 23% conversion efficiency — light and cheap, and enough to keep a mid-size station (1–2kWh) topped up through a day of off-grid use. Pairs cleanly with the PowerMax 1800 SE and 2400. Price: $259.99 — Buy on Awin
Best for larger stations: OSCAL PM400 Pro 400W Doubles the output of the PM200 for faster solar recharging of bigger batteries like the PowerMax 3600 SE and 6000. Same 23% efficiency, foldable, weather-resistant. The right panel if your station is 3kWh+ and you want it refilled in a day rather than two. Price: $439.99 — Buy on Awin
What about batteries?
Batteries don't change production sizing but they change how much of your production you actually use. Without batteries, surplus production goes back to the grid (where net metering is available) and you draw back at night. With batteries, you store your own surplus and don't draw at night, which matters most where:
- Net metering terms are unfavorable (low credit for exports)
- Time-of-use rates make nighttime grid power expensive
- Outages are common and you want backup
Adding a battery typically costs $5,000 to $15,000 depending on capacity. Read our guide on how to choose a home battery system for the sizing and use-case questions.
On using the calculator
The solar calculator on our home page does this math for you, given your state, panel wattage, and battery capacity. Use it to test scenarios — a single 400 W balcony panel vs. a 6 kW rooftop system — before talking to installers or buying hardware.
Two pieces of advice for the calculator:
- Run winter numbers separately. Annual average production overstates what you'll get in December by about 50 percent in many regions. If you want reliable year-round production, plan to your worst month.
- Be honest about shading. Even partial shade on one panel can disproportionately affect a whole string (depending on inverter type). If your roof has unavoidable midday shade, derate an additional 10 to 20 percent.