Solar panel calculator: how many panels do I need?
Enter the energy you use in a day, your peak sun hours and a panel size. The calculator adds system losses and battery losses and gives the array size and number of panels.
Enter your measurements
More options: Losses
- Panels
- 3 panels · 200 W
- Installed watts with that many panels
- 600 W
- Expected output per day
- 2,136 Wh
- Expected output per month (30 days)
- 64.1 kWh
Each watt of panel makes 4.5 sun hours × (1 − 14% losses) × 92% battery round trip = 3.56 Wh a day
2,000 Wh ÷ 3.56 = 562 W of panels
562 W ÷ 200 W = 3 panels (600 W)
Quick answers for common sizes
| Example | Answer |
|---|---|
| How many solar panels to charge a 100Ah battery | 3 × 100 W panels (288 W) |
| How many solar panels for a small cabin | 3 × 400 W panels (948 W) for 3 kWh a day |
Array size at other peak sun hours
| Peak sun hours per day | Solar array needed | Panels |
|---|---|---|
| 2 | 1,264 W | 7 panels |
| 3 | 843 W | 5 panels |
| 4 | 632 W | 4 panels |
| 5 | 506 W | 3 panels |
| 6 | 421 W | 3 panels |
Worked examples: how many solar panels to charge a 100Ah battery and how many solar panels for a small cabin. Sizing from your electric bill instead? Use the solar system size calculator.
How the array is sized
Wh per watt of panel per day = peak sun hours × (1 − system losses) × battery round-trip efficiency array watts = Wh per day ÷ Wh per watt of panel panels = array watts ÷ panel watts, rounded up
The panel rating is what it makes in full sun at 25 °C. Peak sun hours turn a day of changing light into equivalent hours of that full sun. System losses — dirt, heat, wiring, mismatch, controller — take a share; NREL’s PVWatts uses 14% as its default. If the energy goes into a battery and back out before you use it, the battery takes its cut too: Victron lists 92% round-trip efficiency for its LiFePO4 batteries, and puts lead-acid at about 75% for full cycles and about 89% in partial-state-of-charge use.
Peak sun hours: the number that matters most
| City | Yearly average | Lowest month | 2 kWh/day needs (LiFePO4) |
|---|---|---|---|
| Phoenix, AZ | 6.57 | 5.45 | 385 W (year) · 464 W (worst month) |
| Los Angeles, CA | 6.09 | 4.61 | 415 W (year) · 548 W (worst month) |
| Denver, CO | 5.74 | 4.56 | 440 W (year) · 554 W (worst month) |
| Miami, FL | 5.57 | 4.7 | 454 W (year) · 538 W (worst month) |
| Dallas, TX | 5.41 | 4.36 | 467 W (year) · 580 W (worst month) |
| Chicago, IL | 4.46 | 2.66 | 567 W (year) · 950 W (worst month) |
| New York, NY | 4.45 | 2.87 | 568 W (year) · 881 W (worst month) |
| Seattle, WA | 3.83 | 1.59 | 660 W (year) · 1,590 W (worst month) |
Notice the gap between the yearly figure and the worst month. A cabin used all winter in Chicago needs well over twice the panels of the same cabin used only in summer. Size on the season you actually need power in — or plan on a generator for the darkest weeks.
Practical tips
- Round up, then add a little. Panels age, and the worst week is always worse than the average.
- Mounting matters. Panels lying flat on a van roof make noticeably less in winter than tilted ones; the best angle for solar panels in winter shows the angle to aim for.
- Check the controller. More panels means more current; the charge controller calculator sizes it, and the series and parallel calculator checks the voltage.
- Shade kills output. A shadow on one panel can pull down a whole series string.
What you’ll need
- 100 W solar panelMid-range price
The building block of most van, boat and RV systems. Check the data-sheet Voc and Isc before you decide how to wire several of them.
See options on Amazon (opens in a new tab) - 400 W residential-size solar panelPremium price
Fewer, larger panels for a cabin, shed or ground mount. Their higher voltage usually means an MPPT controller.
See options on Amazon (opens in a new tab) - MPPT solar charge controllerMid-range price
Converts a higher panel voltage into charging current, so you can wire panels in series and lose less in the cables. Size it on output amps and maximum PV voltage.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many solar panels do I need?
Divide your daily watt-hours by the energy one watt of panel makes in your location (peak sun hours × 0.86 after typical losses, times about 0.92 if it goes through a lithium battery). 2,000 Wh a day at 4.5 sun hours needs about 560 W — three 200 W panels.
How many solar panels to charge a 100Ah battery?
To put back the usable 80% of a 12.8 V 100Ah LiFePO4 battery in one day at 4.5 peak sun hours, about 290 W of panels — three 100 W panels. See how many solar panels to charge a 100Ah battery.
What are peak sun hours?
The day’s sunlight expressed as hours of full 1,000 W/m² sun. NREL’s PVWatts gives a tilted panel about 6.6 a day on average in Phoenix and 3.8 in Seattle. See peak sun hours by city.
Do I need more panels in winter?
Yes — often far more. Chicago’s lowest month averages about 2.7 peak sun hours against a yearly 4.5 on PVWatts, so the same load needs about two-thirds more panel in December.
Related guides
Sources
- National Laboratory of the Rockies (formerly NREL) — PVWatts® Calculator (opens in a new tab) — Look up the solar resource (peak sun hours) and yearly output for your own address; default system losses 14%.
- NLR (formerly NREL) — PVWatts® v8 API, NSRDB PSM V3 TMY data (opens in a new tab) — City figures on this site: fixed open-rack array facing due south, tilted at the site’s latitude, 14% system losses, queried October 2026.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.