Does days of autonomy account for partial solar on cloudy days?

No — this calculator assumes zero solar production for a worst-case estimate. Overcast days still produce 10-25% of rated output, so real autonomy will be longer.

Days of Autonomy Calculator

How many days can your off-grid system run without sunlight? Enter your battery and daily usage — get your autonomy days.

Energy usage & battery

Daily energy consumption?

kWh/day

Battery capacity?

kWh

Depth of discharge?

Climate zone?

Days of autonomy

2.0 days

AssessmentUndersized — high risk of running out during cloudy periods

Usable battery energy16.0 kWh

Recommended for your climate3+ days

Battery needed for recommendation30.0 kWh

3–4 days of autonomy recommended. Include a generator or grid connection as backup for extended cloudy stretches.

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How to Use This Calculator

Enter your daily energy consumption

Type your average daily kWh usage. Find this on your utility bill (monthly kWh ÷ 30) or from your inverter's monitoring app if you're already solar-equipped. Off-grid homes typically consume 5-15 kWh/day; cabins use 1-5 kWh/day depending on whether they have refrigeration and climate control.

Set battery capacity and DoD

Enter your total battery capacity in kWh. For a battery bank, add all batteries together. Set depth of discharge for your battery type — LiFePO4 batteries can run at 80-90% DoD; lead-acid should stay at 50%.

Choose your climate zone

The climate zone selector provides context about how many consecutive days without significant solar production you might face. Phoenix rarely goes more than 1-2 days; Seattle can have 7+ consecutive cloudy days in winter. The recommendation updates based on your selection.

Read the assessment

The result shows your calculated days of autonomy, compares it to the climate recommendation, and tells you the battery size needed to hit the recommendation if you're currently undersized.

The Formula

Usable kWh = Battery kWh × DoD (%) Days of autonomy = Usable kWh ÷ Daily kWh consumption Battery needed = Daily kWh × Recommended days ÷ DoD (%)

Days of autonomy assumes zero solar input — a true worst-case. In reality, even on cloudy days your panels produce 10-25% of rated output, extending real autonomy beyond this calculation. The conservative calculation is intentional: size for the worst case, enjoy the buffer in normal conditions.

Example

Pacific NW cabin — 3 days autonomy needed?

A cabin near Portland, OR uses 5 kWh/day. The owner has a 20 kWh LiFePO4 battery at 80% DoD. Portland is in the cloudy climate zone — recommended 5+ days of autonomy.

Daily usage5 kWh/day

Battery20 kWh

DoD80%

Usable energy16 kWh

Autonomy3.2 days

Recommended5+ days

Battery needed for rec.31.3 kWh

The current 20 kWh battery gives 3.2 days — short of the 5-day recommendation for the Pacific NW. Options: add another 15 kWh battery bank, install a generator for backup, or reduce daily consumption below 3.2 kWh/day. Most off-grid installers in cloudy climates recommend generator backup alongside battery storage rather than sizing the battery bank for worst-case winter periods alone.

FAQ

How many days of autonomy do I need for off-grid living? ▼

It depends on your climate. Sunny climates like Arizona or Nevada: 2-3 days is typically sufficient. Moderate climates like Texas or Colorado: 3-5 days. Cloudy climates like the Pacific Northwest or New England: 5-7 days minimum. Extremely cloudy regions (UK, Alaska, northern Canada): 7-14 days or a generator backup is nearly essential. Most full-time off-grid homeowners in moderate climates target 4-5 days.

Does days of autonomy account for partial solar production on cloudy days? ▼

This calculator assumes zero solar production — a true worst-case calculation. In practice, overcast days still produce 10-25% of rated panel output, so your real autonomy will be longer. For a more accurate calculation, estimate your worst-case daily solar production (say 1 kWh/day on very cloudy days) and subtract it from your daily consumption before entering the net figure.

Should I add a generator instead of more batteries? ▼

For cloudy climates, a generator is often more cost-effective than sizing batteries for 7+ days of autonomy. A propane or diesel generator running 2-3 hours per cloudy day can recharge a 20-30 kWh battery bank at a fraction of the cost of doubling the battery bank. The sweet spot is 3-5 days of battery autonomy plus a generator for extended cloudy periods. Auto-start generators can handle this automatically without manual intervention.

What is the difference between days of autonomy and battery backup hours? ▼

Battery backup hours (from our Battery Backup Calculator) measures how long your battery lasts at a specific wattage — useful for outage planning. Days of autonomy is an off-grid concept: how many full days of typical energy consumption your battery can cover without solar input. Days of autonomy assumes you're using your normal daily load continuously, not a reduced emergency load.

How do I calculate my daily kWh if I'm planning a new off-grid system? ▼

Use our Appliance Energy Calculator to add up all your planned loads: each appliance's wattage × hours per day = watt-hours/day. Divide by 1000 for kWh/day. A realistic off-grid estimate for a comfortable 2-bedroom home: refrigerator (1.2 kWh), lighting (0.3 kWh), entertainment (0.5 kWh), cooking (1.0 kWh), water pump (0.5 kWh), misc (0.5 kWh) = roughly 4-5 kWh/day without electric heating or AC.