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

Size for winter if you need year-round off-grid power. Winter sun hours can be 40-60% lower than summer in northern latitudes, and that gap determines whether your batteries stay charged through December and January. 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?

Most off-grid residential systems use 2-3 autonomy days. That covers a typical stretch of cloudy weather without running a backup generator. Critical systems like medical equipment might need 5+ days. A weekend cabin used only in good weather can get by with 1 day. 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 real-world losses that many quick estimates ignore. System losses of 20% cover charge controller conversion, battery charge/discharge inefficiency, wiring resistance, and inverter overhead. It also uses worst-month sun hours rather than annual averages. These factors combined often mean you need 30-50% more panel wattage than a naive watts-divided-by-sun-hours calculation suggests. Our solar panel output guide breaks down each loss factor with real numbers.

Solar Panel & Battery Sizing Calc

5 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 array feeding charge controller and battery bank through cabled connections.

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 system losses. Multiply your total by 1.2 to 1.25 (for 20-25% losses). These losses come from charge controller conversion, battery charge/discharge inefficiency, wiring resistance, and inverter overhead. A 3,000Wh daily need becomes 3,750Wh after accounting for 25% losses.
Calculate solar array size. Divide loss-adjusted daily usage by your worst-month peak sun hours. 3,750Wh / 4 hours = 937.5W of solar panels. Round up to the next panel — three 400W panels give you 1,200W, which provides a comfortable 28% buffer.
Size the battery bank. Multiply daily usage by autonomy days, then divide by depth of discharge. For 3,000Wh x 2 days / 0.80 DoD = 7,500Wh of battery capacity. At 12V, that is 625Ah.
Verify your charge controller. Your charge controller must handle the total panel current. Three 400W panels on a 12V system produce about 33A each at battery voltage — an MPPT controller rated for 100A would work.

Off-grid solar system showing panels, charge controller, battery bank, and inverter. — An off-grid system routes solar DC through a charge controller to the battery bank, then to an inverter for AC loads or directly to 12V DC devices.

Example: Sizing a Weekend Cabin System

Consider a small cabin with these loads: LED lights (40W for 5 hours = 200Wh), a 12V 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.

The cabin sits in Colorado with 4.5 worst-month peak sun hours. Adding 20% for losses: 930 x 1.2 = 1,116Wh needed from panels. That is 1,116 / 4.5 = 248W of solar. Two 200W panels (400W total) give a solid 60% buffer above minimum.

For batteries, 2 days of autonomy: 930 x 2 / 0.80 = 2,325Wh. At 12V, that is 194Ah. 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), a 30A MPPT controller ($100-150), and a 1,000W pure sine inverter ($100-200). Complete system: roughly $800-1,250.

Common Off-Grid System Sizes

System Size	Daily Output	Battery Bank	Typical Use
400W panels / 200Ah 12V	1,200-1,600Wh	2x 100Ah LiFePO4	Small cabin, shed, or boat
800W panels / 400Ah 12V	2,400-3,200Wh	4x 100Ah LiFePO4	Full-time RV or van life
2,000W panels / 400Ah 24V	6,000-8,000Wh	8x 100Ah LiFePO4 (24V)	Off-grid tiny house
5,000W panels / 800Ah 48V	15,000-20,000Wh	16x 100Ah LiFePO4 (48V)	Full-size off-grid home

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

Loss-adjusted: 2,310 × 1.20 = 2,772Wh

Solar array: 2,772 / 3 PSH = 924W → 3 × 400W panels (1,200W)

Battery bank (2 days autonomy): 2,310 × 2 / 0.80 = 5,775Wh ÷ 24V = 241Ah at 24V

Interpretation

Three 400W panels provide a 30% buffer over minimum in Oregon's overcast winters. The 241Ah 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

Loss-adjusted: 2,640 × 1.22 = 3,221Wh

Solar (winter): 3,221 / 3.5 = 920W → 3 × 400W panels (1,200W)

Battery (3 days autonomy): 2,640 × 3 / 0.80 = 9,900Wh ÷ 12V = 825Ah at 12V

Interpretation

For 3 days without sun, the battery bank is substantial — nine 100Ah batteries at 12V. That's heavy and expensive ($2,200+). Reducing autonomy to 2 days cuts the bank to 6 batteries. The 1,200W array recharges the bank in about 2 sunny days from empty.

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 (typically 12V, 24V, or 48V). Higher voltages reduce current for the same power, allowing thinner cables and smaller fuses, which is why large systems use 24V or 48V.

Loss-Adjusted Usage

Your raw daily energy consumption multiplied by a loss factor (typically 1.15 to 1.25) to account for charge controller, battery, wiring, and inverter inefficiencies that consume energy before it reaches your appliances.

Charge Controller

An electronic device between the solar panels and battery bank that regulates charging voltage and current. MPPT controllers are more efficient than PWM, especially when panel voltage is much higher than battery voltage.

Check whether your planned battery bank can handle your loads with our battery runtime calculator. Try it now →

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 — a 20% buffer on panels 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.

Last updated: March 22, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

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.