📆 Battery Autonomy Planning

Days of Autonomy Calculator:
Partial Sun Credit, Generator Runtime, Per-Load Timeline

The only days-of-autonomy calculator with chemistry-specific DoD, temperature derating, cloudy-day partial sun credit, generator runtime cross-check, and a per-appliance breakdown showing exactly which load drains your bank fastest.

⚡ Calculate Your Autonomy

Step 1 — Battery Chemistry
LFP / LiFePO4 | DoD: 90% | Temp loss at 32F: 20%
Step 2 — Your Battery Bank Size
Enter EITHER your bank in Ah + voltage, OR your total bank size in kWh. The kWh entry overrides Ah if both are filled in.
Ah
kWh rated
°F
Temperature matters: AGM loses 40% capacity at 32°F. LFP loses 20%. Enter your coldest expected storage temp for accurate real-world autonomy.
Step 3 — Your Daily Loads
LoadWattsHrs
🏠 Essential Loads
🏠 Additional Loads
Step 4 — Recharge Credits (Optional)
During cloudy weather your panels still produce 10-30% of rated output. Enter this partial-sun daily kWh to reduce your net drain. Also enter any planned generator use.
kWh/day
Watts
hrs/day
Partial sun tip: A 400W array in heavy overcast typically produces 0.2-0.8 kWh/day. In light clouds it can produce 1-2 kWh/day. Use a conservative estimate here to avoid overestimating autonomy.

☀ Your Autonomy Timeline

📆

Select your battery chemistry, enter your bank size, choose your loads, and add any recharge credits. Hit Calculate to see your full autonomy timeline with depletion checkpoints and per-load breakdown.

What Days of Autonomy Actually Means in Practice

Days of autonomy is the number of days your battery bank can power your loads without any recharging — no solar production, no generator, no grid connection. It is the answer to the most critical question in off-grid power planning: how long can I survive a stretch of zero-production weather?

The straightforward formula is usable battery capacity divided by daily load. But “usable battery capacity” is where most online calculators fall apart. They take your rated battery size and divide by your load without adjusting for the chemistry-specific depth of discharge (LFP can use 90% of rated capacity; lead-acid should only use 50%), the temperature-related capacity loss (a 200Ah AGM battery stored in a 25-degree-Fahrenheit garage delivers about 110Ah, not 100Ah at 50% DoD), or the partial solar production that occurs even on cloudy days. These three factors together can make a 2x or 3x difference in your actual real-world autonomy versus the theoretical calculation.

The Partial Sun Factor Competitors Ignore

No solar system truly has zero production on cloudy days. A clear day might produce 100% of rated output. A thin overcast might produce 60-80%. A heavy overcast might produce 10-25%. A day of continuous heavy rain might produce 5-10%. For most US locations, the probability of multiple consecutive days of total zero production is very low — even major storm systems typically break and allow some production. If your system produces even 0.5 kWh per day during a cloudy stretch, and your net load is 2 kWh per day, your effective autonomy more than doubles because the drain is only 1.5 kWh per day instead of 2. Our calculator lets you enter this partial sun credit so your autonomy estimate reflects realistic cloudy-day conditions rather than a theoretical worst case.

How the Days of Autonomy Calculator Works

Enter your battery chemistry, bank size (in Ah at a voltage, or directly in kWh), storage temperature, and daily loads. Optionally add partial sun recharge credit and generator hours. The calculator applies the chemistry-specific DoD, temperature derating, and subtracts all daily credits to find your true net drain. It then divides your usable bank capacity by this net drain to give you your real-world autonomy in days and hours. The per-load breakdown table shows you exactly which appliance consumes the most of your autonomy, helping you decide which loads to shed first if clouds persist longer than expected.

Three Real Autonomy Calculation Examples

🏠

Weekend Cabin — Vermont

Northeast Kingdom, VT | AGM batteries | Cold storage 20°F | No generator

Dave has a hunting cabin in Vermont with a 300Ah AGM bank at 12V (3.6 kWh rated) and a 400W solar array. During hunting season in November, the batteries live in an unheated outbuilding where temps drop to 20 degrees F. He runs an LED lighting system, a small 12V cooler, and his phone charger.

InputValueNotes
Battery bank300Ah AGM @ 12V3.6 kWh rated
DoD50%AGM chemistry limit
Storage temp20°FUnheated building
Temp derating~50%AGM loses ~50% at 20F
Usable capacity0.9 kWh3.6 x 0.50 x 0.50
Daily load0.65 kWhLights + cooler + phone
Partial sun (Nov)0.2 kWh/dayShort overcast days
Net drain: 0.65 – 0.2 = 0.45 kWh/day. Autonomy: 0.9 / 0.45 = 2.0 days. Without the partial sun credit: 0.9 / 0.65 = 1.4 days. The temperature derating is devastating for AGM in cold climates — the same 300Ah bank stored at 70F would provide 5.5 days of autonomy for this load. This is exactly why many cold-climate cabin owners either insulate/heat their battery enclosure or switch to LFP (which only loses 20% at 32F).
🏭

Full-Time Off-Grid Home — Oregon Coast

Lincoln City, OR | LFP batteries | Mild storage 55°F | 2kW generator backup

The Johnson family lives off-grid year-round on the Oregon Coast. They have a 400Ah LFP bank at 24V (9.6 kWh rated) and a 2.0 kW solar array. January on the Oregon Coast means heavy clouds — their array produces about 0.8 kWh per day on average during winter. They have a 2,000W Honda EU2200i generator for backup.

InputValueNotes
Battery bank400Ah LFP @ 24V9.6 kWh rated
DoD90%LFP chemistry
Storage temp55°FClimate-controlled room
Usable capacity8.4 kWh9.6 x 0.90 x 0.97
Daily load5.2 kWhFull household, no AC
Partial sun (Jan)0.8 kWh/dayHeavy coastal overcast
Net drain: 5.2 – 0.8 = 4.4 kWh/day. Autonomy without generator: 8.4 / 4.4 = 1.9 days. Act point (50% bank): Day 0.95 — basically after one night. Generator runs 4.4 / 2.0 = 2.2 hours to restore one day’s autonomy, using about 1.1 gallons of fuel. In January on the OR coast, the Johnsons run the generator about 3-4 times per week for 2 hours each run, burning about 8-10 gallons/month. They keep 30 gallons of stabilized fuel in their shed.
⛽

RV Boondocking — Southwest Desert

Quartzsite, AZ | LFP batteries | Warm storage 75°F | 600W solar, no gen

Maria boondocks in her Class C for 2-3 weeks at a time near Quartzsite, Arizona. She has 200Ah LFP at 12V (2.4 kWh rated) and a 600W rooftop array. Arizona desert in January has 5.5 peak sun hours and even on overcast days she gets 0.5-1.0 kWh production. She wants to understand her theoretical worst-case autonomy.

InputValueNotes
Battery bank200Ah LFP @ 12V2.4 kWh rated
DoD90%LFP chemistry
Storage temp75°FDesert climate, no derating
Usable capacity2.16 kWh2.4 x 0.90
Daily load1.8 kWhFridge, lights, laptop, phone
Partial sun credit0.5 kWh/dayConservative cloudy-day min
Net drain: 1.8 – 0.5 = 1.3 kWh/day. Autonomy: 2.16 / 1.3 = 1.7 days worst-case. On a typical sunny Quartzsite day (5.5 hrs x 0.6 kW = 3.3 kWh), her array produces nearly double her daily load — she ends each good day at 100% state of charge. The biggest autonomy consumer is her 45W 12V fridge running 24 hours = 1.08 kWh/day alone (2.0 days of bank on its own). Shutting off the fridge and using a cooler would triple her autonomy on cloudy stretches.

Expert Tips for Extending Your Off-Grid Autonomy

1

Know Your 50% Depletion Point — Act Before Then

The most important output from this calculator is not your maximum autonomy days — it is the day when your battery bank hits 50% depletion. That is your action trigger. For LFP, 50% depletion means you still have 50% of your usable capacity remaining, giving you time to run the generator, drive somewhere with shore power, or aggressively reduce loads before you are in crisis mode. For lead-acid chemistries, 50% depletion of usable capacity means you are at the DoD limit and must stop discharging immediately to prevent damage. Build a mental model: every morning, check your battery monitor. If you are below the action threshold, take action before the next night of drain depletes you further.

2

Partial Sun Production Is More Reliable Than You Think

The biggest mistake in autonomy planning is modeling cloudy days as zero-production days. For most of the continental US, a completely cloudy day with zero measurable solar production is rare — even a solidly overcast sky lets through 10-25% of normal irradiance. A 600W array on a heavy overcast day in the Pacific Northwest still produces 60-150 watts continuously — that is 0.6-1.5 kWh over 10 hours of daylight. This production does not fill your battery but it dramatically slows the drain. Model your worst-case cloud scenario (not absolute zero) and your autonomy estimates will be more realistic and less anxiety-inducing.

3

The Per-Load Breakdown Tells You Exactly What to Shed

Use the per-load autonomy breakdown in this calculator to build your load-shedding protocol before you ever need it. If your fridge alone drains the bank in 2 days, and your laptop alone drains it in 6 days, the protocol is obvious: the fridge is the first thing to address in a crisis. Options include switching to a cooler with ice, pre-cooling food before clouds hit, or upgrading to a more efficient 12V compressor fridge. Having a written load-shedding plan — Tier 1 (cut non-essentials), Tier 2 (cut comfort loads), Tier 3 (emergency fridge shutdown) — turns a cloudy week from a crisis into a manageable situation you have already planned for.

16 Frequently Asked Questions About Solar Battery Autonomy

What is a good number of days of autonomy?+
The right autonomy target depends on your location and application. For the US Southwest (Arizona, New Mexico, Nevada) where extended cloudy periods are rare: 2-3 days is comfortable. For the Pacific Northwest or New England where multi-day overcast is common in winter: 4-5 days or a reliable generator backup. For full-time liveaboard sailors on ocean passages who cannot use a generator offshore: 5-7 days. For weekend cabins where a grid-powered drive home is the backup: 2 days is plenty. More autonomy costs more money in batteries. Size for your actual climate and backup options rather than a generic rule of thumb.
How does temperature affect battery autonomy?+
Temperature reduces the chemical activity inside battery cells, which reduces how much energy they can deliver. LFP batteries lose about 20% of their capacity at 32 degrees Fahrenheit. AGM and Gel lose about 35-40%. Flooded lead-acid loses nearly 50% at 32 degrees F — a 200Ah FLA battery at 32F only delivers about 100Ah of energy before it is exhausted. The loss increases as temperature drops further. At 0 degrees F, an AGM battery may only deliver 50-60% of its rated capacity. This is why batteries stored in cold environments (unheated garages, sheds, outdoor compartments) produce dramatically less autonomy than the same bank kept at room temperature. If your batteries must live in a cold environment, either insulate and heat the battery compartment or switch to LFP which derates less.
How much power does a solar panel produce on a cloudy day?+
It depends on cloud thickness. Thin cirrus clouds reduce production by 10-20%. Scattered cumulus clouds cause variable output averaging 40-70% of clear-sky production. A solid, continuous overcast (think Seattle in January) produces about 10-25% of rated panel output. Heavy rain and storm conditions can drop to 5-10%. Completely zero production requires extremely dense cloud cover or heavy snowfall over the panels. For autonomy planning, a conservative but realistic partial sun credit for continuous overcast is 10-15% of your array’s rated wattage over the full daylight hours. For a 400W array with 10 hours of winter daylight, that is 400W x 0.12 x 10hr = 0.48 kWh — meaningful but not enough to balance a full household load.
Can I use a generator to extend my autonomy?+
Yes, and this is one of the most practical strategies for off-grid living in cloudy climates. The key is to run the generator efficiently: charge your batteries at the highest rate your battery chemistry and BMS allow, not at a low trickle that wastes fuel. A 2,000W Honda EU2200i generator connected to a 30A battery charger will put about 30A into a 12V bank — that is 360W or 0.36 kWh per hour. Running it for 2 hours restores 0.72 kWh, enough for a full additional day of autonomy for a minimal load system. A larger Victron MultiPlus inverter-charger can accept generator power at up to 100A on a 12V system (1,200W), filling the bank twice as fast and consuming half the generator fuel per kWh restored. Size your charger to your generator, not just to your bank.
What is the difference between rated capacity and usable capacity?+
Rated capacity (printed on the battery label) is the total energy the battery contains. Usable capacity is how much of that you can safely extract without damaging the battery or shortening its life. LFP: rated 200Ah at 12V = 2.4 kWh rated, 2.16 kWh usable (90% DoD). AGM: rated 200Ah at 12V = 2.4 kWh rated, 1.2 kWh usable (50% DoD). The difference is dramatic — an AGM and an LFP battery of the same rated size deliver very different real-world autonomy. Temperature further reduces usable capacity below the DoD ceiling. A 200Ah AGM at 20 degrees F at 50% DoD only delivers about 0.6 kWh — just 25% of its rated capacity label. Always calculate autonomy from usable capacity after both DoD and temperature derating.
How do I calculate how long a specific appliance will run on my battery?+
Divide your usable battery capacity (in kWh) by the appliance’s daily kWh consumption. If your 12V fridge draws 45W continuously (45W x 24hr = 1.08 kWh/day) and your usable battery bank is 2.16 kWh, the fridge alone would drain the bank in 2.0 days. For a non-continuous load like a 65W laptop used 8 hours per day (0.52 kWh/day), the bank lasts 4.1 days on the laptop alone. Use the per-load breakdown in this calculator to rank all your appliances by their autonomy impact — it immediately shows you which single change would most extend your autonomy during a cloudy stretch.
Should I design for worst-case or average autonomy?+
Design for a realistic worst case, not the absolute worst case. The absolute worst case for solar is a month of continuous total cloud cover — which essentially never happens anywhere in the continental US. A realistic worst case is 5-7 consecutive days of heavy overcast with minimal production. For most US locations, designing for 3-5 days of autonomy with a generator for emergencies creates a robust, cost-effective system. For the Pacific Northwest and Great Lakes region, 5-7 days is more appropriate. Designing for absolute worst case (zero production for 14 days) would require an enormous and expensive battery bank that sits mostly full 95% of the year. A well-sized bank plus a reliable generator backup is almost always more cost-effective than trying to reach absolute independence through batteries alone.
What happens when my batteries are fully depleted?+
For LFP batteries with a BMS: the BMS will disconnect the battery from the load at the configured low-voltage cutoff, protecting the cells from damage. Your loads will shut off abruptly. For AGM and Gel: the battery will continue to discharge past the safe DoD limit if nothing stops it, causing sulfation and permanent capacity loss. For flooded lead-acid: same as AGM, plus the electrolyte level can drop dangerously, potentially damaging the plates. The practical lesson: always have a low-voltage alarm or automatic generator start (Victron CCGX can trigger a generator via a relay at a set voltage) so you know when you are approaching the action point before you hit the cutoff. Never rely on the BMS disconnect as your normal operating limit — treat it as the emergency backstop, not the management tool.
What is load shedding and how do I do it?+
Load shedding is deliberately turning off non-essential electrical loads to extend the time your battery bank can supply the remaining critical loads. A practical three-tier protocol for off-grid users: Tier 1 (bank below 75%) — turn off entertainment (TV, stereo), reduce lighting to essentials, stop unnecessary device charging. Tier 2 (bank below 60%) — turn off the hot water heater, washing machine, and any heating or cooling loads you can tolerate without, switch the fridge to a lower duty cycle (many 12V compressor fridges have an eco mode). Tier 3 (bank below 40%) — run generator immediately, or turn off the fridge entirely if ice or cooler is available. Write this protocol on a card and post it next to your battery monitor so any household member can implement it without consulting you.
How does depth of discharge affect battery life?+
This is one of the most important relationships in battery chemistry. For lead-acid chemistries, the relationship is dramatic: routinely discharging to 80% DoD (only 20% remaining) instead of 50% DoD reduces cycle life by 50-60%. Consistently discharging to 100% DoD on AGM can reduce cycle life to under 100 cycles. For LFP, the relationship is more gentle — LFP can handle 100% DoD with modest additional cycle loss versus 80% DoD, though the manufacturer’s cycle rating at 80% DoD is typically much higher than at 100% DoD. In practice, LFP systems set up with a conservative low-voltage cutoff of 10-15% remaining often achieve 4,000-5,000 cycles, while systems regularly hitting 5% remaining may only get 2,500-3,000 cycles. The DoD limits built into our calculator represent the best trade-off between autonomy and long battery life.
Why is my actual autonomy less than what the calculator shows?+
Several common reasons: your actual loads are higher than estimated (a 12V fridge in a hot, sunny van with a poorly insulated compartment can draw 70-80W average instead of the 45W baseline); your battery bank has aged and lost capacity (a 3-year-old AGM at 50% DoD may only deliver 40% of original rated capacity); parasitic loads from inverters, charge controllers, and BMS on standby consume 1-5 watts continuously without being counted in your appliance tally; and your temperature is colder than estimated. Use a battery monitor (Victron BMV-712) to measure actual kWh consumed per day over a full week, then enter that measured figure into the calculator for an accurate result.
What is the ideal battery temperature for maximum capacity?+
The sweet spot for all battery chemistries is between 60 and 80 degrees Fahrenheit. Below this range, capacity decreases as described above. Above this range, capacity is slightly higher but battery life degrades faster. A battery operating continuously at 95 degrees Fahrenheit loses life about twice as fast as the same battery at 77 degrees. Batteries stored in hot environments (van floor in summer sun, outdoor solar cabinet in Texas) need thermal management. Insulation with reflective material reduces heat gain. For LFP specifically, the cells can cycle safely up to about 120 degrees F but life is meaningfully reduced above 95 degrees. For maximum life and capacity, keep batteries between 60-80 degrees F year-round.
How do I monitor my state of charge accurately?+
Voltage alone is unreliable for state of charge (SOC) estimation — especially for LFP which has a very flat voltage curve between 20% and 90% SOC. A coulomb counter (also called an amp-hour meter or battery monitor) that measures actual current flowing in and out of the battery is the only accurate real-time SOC method. The Victron BMV-712 ($150-$180) is the industry standard for off-grid systems, connecting via Bluetooth to the Victron Connect app. It tracks SOC, amps in/out, voltage, time remaining at current consumption, and historical statistics. Alternative: the Renogy One monitoring system or a simple AmpHour meter are lower-cost options. Never rely on inverter or charge controller voltage readings for SOC — they are affected by wire resistance and switching noise and can be off by 5-20% SOC.
Can I use this calculator for a grid-tied battery backup system?+
Yes. For grid-tied backup (Powerwall-type systems), the key difference is that you are only calculating autonomy during a grid outage — not ongoing daily cycling. Enter your critical loads only (the loads you want to keep running during an outage), your battery bank size, and the appropriate DoD for your chemistry. Leave the solar credits at zero if you want worst-case autonomy (nighttime outage with no production), or enter your typical daytime production if you are doing a daytime outage scenario. Grid-tied backup batteries are typically sized for 8-24 hours of critical loads, not days of autonomy, because the grid is the primary power source and extended outages are rare in most US locations. For hurricane-prone areas (Florida, Gulf Coast), 3-5 days of critical loads is more appropriate given extended outages after major storms.
What resources can I consult for off-grid power system standards?+
The National Electrical Code (NEC) Article 706 covers energy storage systems. The National Renewable Energy Laboratory (NREL) publishes extensive research on battery performance and off-grid system sizing. The US Department of Energy Energy Saver guide covers residential solar planning including battery storage. For RV-specific standards, the RVIA and NFPA 1192 govern RV electrical systems. For marine applications, ABYC E-11 and E-13 (lithium battery standard) apply. For homesteaders and off-grid cabin owners, the Off-Grid Solar Practitioners Forum and DIY Solar Forum (diysolarforum.com) are strong community resources for real-world autonomy experience.
How does a well pump affect my autonomy calculation?+
Well pumps have high wattage (750W-1,500W for 1/2-1 HP submersible pumps) but short duty cycles — typically running only during water demand and stopping when the pressure tank is full. The most accurate way to estimate daily well pump consumption is to measure the actual kWh used over a week with a monitoring meter at the pump panel. A rough estimate: a 1/2 HP pump running 3 cycles per day of 10 minutes each = 30 minutes of 750W = 0.375 kWh per day. But this can vary dramatically with water usage, pressure tank size, and pump depth. Well pumps are also tricky for off-grid systems because they require inverters with high surge capacity (2x-3x the running wattage for motor starting). A well pump that draws 750W running may surge to 1,500-2,250W for 1-3 seconds on startup — your inverter must handle this surge without fault.

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Legal Disclaimer and Editorial Transparency

The Days of Autonomy Calculator on USCalculators.com provides estimates for educational and planning purposes only. Actual battery autonomy depends on your specific battery condition and age, actual appliance consumption versus nameplate wattage, real-world temperature conditions, partial solar production variability, and usage patterns. Temperature derating values are based on published battery chemistry data and may vary by specific product and manufacturer.

Battery systems above certain watt-hour thresholds may require permits under NEC Article 706 and local codes. Consult a licensed electrician for any permanent battery installation. Generator operation indoors is fatal — never run a gasoline or propane generator inside a building, garage, or enclosed space. See CDC.gov for generator safety guidelines.

Editorial policy: USCalculators.com is an independent educational resource with no affiliate relationships with battery manufacturers, solar installers, or generator brands.