Should I size my solar system for summer or winter output?

Size for winter if you need year-round off-grid power. At three places PVWatts has printed for us (inputs as captured: azimuth 180, tilt 33.45, 39 and 47.61 degrees, array type 0, 1 and 0, 14% system losses), the worst month is December, and it sits well below the annual figure: Phoenix 5.45 peak sun hours against 6.57 for the year (17.0% lower), Kansas City 3.83 against 5.18 (26.1% lower) and Seattle 1.59 against 3.83 (58.5% lower). That gap determines whether your batteries stay charged through the darkest month. If you only use the system seasonally, size for the months you will actually be there. Grid-tied systems can use annual averages since the grid covers short-term deficits. For a deeper look at winter sizing strategy, see our battery bank sizing guide .

What is the right number of autonomy days for an off-grid system?

Rolls prints: "Most systems are designed for 1-2 days of autonomy due to the cost of batteries versus the cost of adding a generator and/or additional renewable energy sources." Renogy prints that in sunny areas, 2-3 days might suffice, while cloudier regions might need 5-7 days. The 2-day default sits inside the 1-2 days Rolls prints for most systems and at the low end of the 2-3 days Renogy prints for sunny areas. For your own site, size from your local weather record and from whether a generator can carry you through a long overcast spell. More autonomy means more batteries and higher cost, so balance reliability against budget.

Why does this calculator recommend more panels than I expected?

The calculator accounts for losses that a quick watts-divided-by-sun-hours estimate leaves out. It divides your usage by the inverter efficiency, then derates the array for the 14% system loss (wiring and other losses, PVWatts' printed default), the 98% charge controller and the cell-temperature factor 302 / 320, and it uses worst-month sun hours rather than annual averages. At the defaults the naive figure is 3,000 / 3.83 = 783W; the chain needs 3,000 / 0.92 / (3.83 x 0.86 x 0.98 x 0.94375) = 1,070W, 37% more, and rounding up to three 400W panels gives 1,200W, 53% more. Our solar panel output guide breaks down each loss factor with real numbers.

Solar Panel & Battery Sizing Calc - Off-Grid

7 min read

Size both your solar array and battery bank at once. This calculator works backward from your daily energy usage to tell you how many panels you need and how large your battery bank should be for a fully independent off-grid system. Not sure about your daily watt-hours? Start with the off-grid load calculator to audit your appliances first.

Solar + Battery Sizing banner: a panel array linked to a battery bank.

The 5-Step Solar Sizing Process

Audit your loads. List every appliance with its wattage and daily hours of use. Multiply watts by hours for each to get watt-hours. A 60W fridge running 8 hours per day (compressor duty cycle) uses 480Wh. A 10W LED running 5 hours uses 50Wh. Add them all up for your total daily usage.
Account for the inverter and the losses. If your loads run on AC, divide your daily usage by the inverter efficiency: at the default 92%, 3,000Wh / 0.92 = 3,261Wh a day for the array and the bank to supply. The array then loses 14% to wiring and other system losses (PVWatts' printed default), 2% in the charge controller (Victron's printed 98% maximum) and the cell-temperature factor 302 / 320: Trina's datasheet for its TSM-DE09.05 series prints 302W at NOCT (800W/m²) in its TSM-400 DE09.05 column, the module rated 400W at STC (1000W/m²), and 400 x 800 / 1000 = 320W. This chain derates the array by 1 - 0.86 x 0.98 x 0.94375 = 20.5%, beside Renogy's x1.2 for a kit it supposes is 80% efficient (a 16.7% derate) and Battle Born's additional 20 to 30 percent solar capacity (a 16.7% to 23.1% derate). Check your results against your product data or a licensed professional before acting.
Calculate solar array size. Divide the inverter-adjusted usage by your worst-month peak sun hours and by the derated fraction: 3,261Wh / (3.83 x 0.86 x 0.98 x 0.94375) = 1,070W of solar panels at the default 3.83 sun hours. Round up to whole panels: three 400W panels give 1,200W, which make 3,656Wh a day against the 3,261Wh need, 12% above it.
Size the battery bank. Multiply daily usage by autonomy days, then divide by the bank's nominal voltage, the depth of discharge and the inverter efficiency. A 12V LiFePO4 bank is 12.8V nominal, so 3,000Wh x 2 days / (12.8 x 0.80 x 0.92) = 637Ah, 8.2kWh of bank energy.
Verify your charge controller. Your charge controller must handle the total panel current. Three 400W panels make 1,200W, which at the 12.8V nominal voltage of a 12V LiFePO4 bank is 1,200 / 12.8 = 94A, inside the rating of a 100A MPPT controller.

Off-grid solar system showing panels, charge controller, battery bank, and inverter. — Solar DC runs through a charge controller to the battery bank, then through an inverter to AC loads; the diagram also shows DC loads.

Example: Sizing a Weekend Cabin System

Consider a small cabin with these loads: LED lights (40W for 5 hours = 200Wh), a compressor fridge (50W average, runs 10 hours = 500Wh), phone and laptop charging (60W for 3 hours = 180Wh), and a water pump (100W for 0.5 hours = 50Wh). Total daily usage: 930Wh.

Take 4.5 worst-month peak sun hours as this example's input (your own site's figure is on pvwatts.nlr.gov) and the page's other defaults: a 92% inverter, 14% system losses, the 98% charge controller and the 302 / 320 cell-temperature factor. The array supplies 930 / 0.92 = 1,011Wh a day, and one 200W panel makes 200 x 4.5 x 0.86 x 0.98 x 0.94375 = 716Wh, so the calculator sizes two 200W panels (400W), which make 1,432Wh a day, 42% above the need.

For batteries, 2 days of autonomy at 80% DoD on a 12V LiFePO4 bank (12.8V nominal): 930 x 2 / (12.8 x 0.80 x 0.92) = 197Ah. Two 100Ah LiFePO4 batteries (200Ah total) meet this target with a small margin.

Total cost estimate as of early 2026: two 200W panels ($200-300), two 100Ah LiFePO4 batteries ($400-600), an MPPT controller rated for at least 400 / 12.8 = 31.25A, the array's current at the bank's nominal voltage ($100-150), and a 1,000W pure sine inverter ($100-200). Prices and rates on this page are our own estimates as of the date shown; they change often. Enter your own figures for a current result. Complete system: roughly $800-1,250.

Example Off-Grid System Sizes

Array	Daily Usage	Daily Output (3.83 / 5.18 sun hours)	Battery Bank (2 days)	Example Use
1 x 400W, 12V	1,000Wh	1,219 / 1,648Wh	212Ah LiFePO4 (2.7kWh)	Small cabin, shed, or boat
2 x 400W, 12V	2,000Wh	2,437 / 3,296Wh	425Ah LiFePO4 (5.4kWh)	Full-time RV or van life
5 x 400W, 24V	5,000Wh	6,093 / 8,240Wh	531Ah LiFePO4 (13.6kWh)	Off-grid tiny house
10 x 500W, 48V	13,000Wh	15,232 / 20,601Wh	690Ah LiFePO4 (35.3kWh)	Full-size off-grid home

Each row is this calculator at the daily usage, panel wattage and voltage shown, with its other defaults: 3.83 worst-month peak sun hours, 14% losses, a 92% inverter, LiFePO4 at 80% DoD and 2 days of autonomy. The array is the panel count it returns. Daily Output is that array's output at 3.83 sun hours and at 5.18, the annual figure PVWatts prints for Kansas City.

Worked Examples

Off-Grid Shed Workshop in Oregon

Context

A woodworker sets up a shed in rural Oregon with a table saw (1,800W, 1h/day), LED shop lights (80W, 5h), a radio (15W, 6h), and phone charging (10W, 2h). Winter sun hours average 3 PSH. The shop runs on a 24V system with LiFePO4 batteries.

Calculation

Daily usage = (1800×1) + (80×5) + (15×6) + (10×2) = 1,800 + 400 + 90 + 20 = 2,310Wh

Through a 92% inverter: 2,310 / 0.92 = 2,511Wh

Loss-adjusted: 2,511 / (0.80 × 0.98 × 302 / 320) = 3,394Wh (20% losses, a 98% MPPT controller and the cell-temperature factor 302 / 320 = 0.944)

Solar array: 3,394 / 3 PSH = 1,131W → 3 × 400W panels (1,200W)

Battery bank (2 days autonomy): 2,310 × 2 = 4,620Wh ÷ (25.6V LiFePO4 nominal × 0.80 × 0.92) = 245Ah at 24V

Interpretation

Three 400W panels provide a 6% buffer over minimum in Oregon's overcast winters. The 245Ah requirement at 24V means three 100Ah 12V batteries in series pairs (6 batteries total for 24V 300Ah) gives comfortable headroom.

Takeaway

For a workshop with intermittent high-draw tools, make sure your inverter handles the table saw's startup surge. Use our battery size for inverter calculator to confirm the battery bank can deliver peak current without voltage sag.

Grid-Down Emergency Backup in Texas

Context

A suburban homeowner in Austin wants solar backup for grid outages. Essential loads: fridge (150W avg, 10h), modem/router (30W, 24h), LED lights (60W, 6h), and phone charging (20W, 3h). Austin averages 5.5 PSH in summer but only 3.5 in winter.

Calculation

Daily usage = (150×10) + (30×24) + (60×6) + (20×3) = 1,500 + 720 + 360 + 60 = 2,640Wh

Through a 92% inverter: 2,640 / 0.92 = 2,870Wh

Loss-adjusted: 2,870 / (0.78 × 0.98 × 302 / 320) = 3,978Wh

Solar (winter): 3,978 / 3.5 = 1,137W → 3 × 400W panels (1,200W)

Battery (3 days autonomy): 2,640 × 3 = 7,920Wh ÷ (12.8V LiFePO4 nominal × 0.80 × 0.92) = 841Ah at 12V

Interpretation

For 3 days without sun, the battery bank is substantial: nine 100Ah batteries at 12V. That's heavy and expensive. Reducing autonomy to 2 days cuts the bank to 6 batteries. At 3.5 winter sun hours the 1,200W array makes about 3,030Wh a day against the 2,870Wh the loads draw through the inverter, a surplus of about 160Wh a day, so after an outage the bank refills slowly in winter.

Takeaway

An emergency backup system with 3 autonomy days needs serious battery capacity. After sizing, check the solar battery charge time calculator to verify your panels can recharge fully between outage events.

Frequently Asked Questions

Glossary

Autonomy Days

The number of days a battery bank can power your loads without any solar input. Higher autonomy means more batteries and cost, but greater resilience during extended cloudy periods or storms.

System Voltage

The nominal voltage of the battery bank. Victron's RV solar sizing article lists 12V, 24V or 48V. The same power draws half the current at twice the voltage.

Loss-Adjusted Usage

The energy your panels must be rated to make: your daily usage divided by the inverter efficiency and by the array's derate for the system loss, the charge controller and cell temperature. At the defaults that is 1 / (0.92 x 0.86 x 0.98 x 0.94375) = 1.37 times the raw usage: 1.09 for the 92% inverter and 1.26 for the array chain.

Charge Controller

An electronic device between the solar panels and battery bank that regulates charging voltage and current. In Victron's worked example, a 100W panel at 25°C cell temperature charging a 13V battery, with 0.5V lost in the cabling and controller, gives 81W through a PWM controller, 19% less than the 100W through an MPPT controller. At 75°C cell temperature Victron finds the difference negligible.

Check whether your planned battery bank can handle your loads with our battery runtime calculator.

A properly sized off-grid system balances panel output, battery storage, and realistic loss factors. Undersizing any one component creates a bottleneck that drags down the whole system. When in doubt, round up: some spare panel capacity costs far less than running a generator every cloudy week. To convert your panel wattage into the amps your wiring and fuses need to handle, run the numbers through the solar watts to amps calculator.

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Browse all solar calculators: Panel output, battery bank sizing, charge time, and system design for off-grid, RV, and residential setups.

Last updated: October 26, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, wind power, and EV ownership/driving, all from building, maintaining, and operating his own systems.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Methodology reviewed by Doc. dr. sc. Damir Topić, Assistant Professor, FERIT Osijek.