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Off-Grid Days Calculator: How many days can I boondock with battery and solar?

200 Ah LiFePO4, 200 W on the roof and 800 Wh daily consumption: in summer the numbers work out indefinitely, in March or October (2.5 peak sun hours) the battery lasts about four and a half days, in continuous rain only two and a half. This calculator weighs consumption, solar output and battery against each other and shows how long you can boondock.

Units

Use the power consumption calculator.

Battery
Charging sources

0 if there is no solar system.

Continental US: winter 2–4, spring/fall 4–5, summer 5–7 depending on the region (NREL). Much less in rainy weather.

E.g. 1 h of driving with a 30 A DC-DC charger ≈ 400 Wh.

Advanced settings

Result

Off-grid days with solar

4.6 days

balance -450 Wh per day

Off-grid days without any charging
2.6 days
Available energy
2,048 Wh
Charging per day (solar + other)
350 Wh
Coverage of consumption
44%
Show calculation
  1. 1Available energy: 200 Ah × 12.8 V × (100% − 20% discharge limit) = 2,048 Wh
  2. 2Solar output: 200 W × 2.5 h × 70% = 350 Wh/day
  3. 3Daily balance: 350 Wh charging − 800 Wh consumption = -450 Wh/day
  4. 4Off-grid days: 2,048 Wh ÷ 450 Wh/day = 4.6 days
  5. 5Without any charging: 2,048 Wh ÷ 800 Wh/day = 2.6 days

 

How it's calculated

available energy [Wh] = capacity × voltage × (state of charge − discharge limit) daily balance [Wh] = solar output + other charging − consumption off-grid days = available energy ÷ (−daily balance) (for a negative balance)
200 Ah × 12.8 V × (100% − 20%) = 2,048 Wh; 350 − 800 = −450 Wh/day → 4.6 days

If the balance is positive, the solar system covers consumption every day – then the off-grid time is only limited by the weather. That's why the calculator also shows the days without any charging: that's how long the battery lasts in continuous rain.

The coverage tells you what share of consumption is recharged daily. Over 100% means surplus (the battery is full at noon, the rest is wasted).

Worked example

Fall trip with 200 Ah LiFePO4 (full), 300 W of solar at 2.5 peak sun hours, 700 Wh daily consumption and an hour of driving every other day with a 30 A DC-DC charger (≈ 200 Wh daily average).

Inputs

  • Daily power consumption: 700 Wh
  • Battery capacity: 200 Ah
  • Battery type: LiFePO4 (lithium)
  • State of charge at start: 100 %
  • Solar power: 300 W
  • Peak sun hours per day: 2.5 h
  • Other charging per day (DC-DC charger, generator …): 200 Wh
  • Usable fraction of capacity: 80 %
  • Solar performance ratio: 70 %

Result

unlimited

Off-grid days with solar

Off-grid days without any charging
2.9 days
Available energy
2,048 Wh
Charging per day (solar + other)
725 Wh
Coverage of consumption
104%

Calculation

  1. Available energy: 200 Ah × 12.8 V × (100% − 20% discharge limit) = 2,048 Wh
  2. Solar output: 300 W × 2.5 h × 70% = 525 Wh/day
  3. Daily balance: 725 Wh charging − 700 Wh consumption = 25 Wh/day
  4. The balance is positive – consumption is covered every day (unlimited under these assumptions).
  5. Without any charging: 2,048 Wh ÷ 700 Wh/day = 2.9 days

The variables explained

Daily power consumption (Wh)
All loads including inverter losses.
Battery capacity / type / usable fraction
Determine the available energy. Lead-acid: 50% usable, LiFePO4: 80–90%.
State of charge at start (%)
Full after shore power or driving; correspondingly lower after days parked.
Solar power and sun hours
Output = W × peak sun hours × performance ratio. Sun hours per region and month in the solar output calculator.
Other charging per day (Wh)
DC-DC charger (A × h × 13 V), generator or fuel cell as a daily average.

Common mistakes

  • Planning for fair weather: three rainy days in a row are normal – the “without charging” value is the honest reserve.
  • Using summer consumption in fall: the furnace fan and more lighting increase demand.
  • Seeing surplus as reserve: solar surplus that the full battery doesn't absorb is lost.
  • Ignoring the discharge limit: at 50% state of charge a lead-acid battery has practically no reserve left.

Assumptions and limits

  • Constant daily consumption and constant sun hours.
  • Solar surplus with a full battery is not stored.
  • No temperature effects on battery capacity.

Frequently asked questions

How many days can you boondock in an RV?

Without solar: usable battery energy divided by daily consumption – with 100 Ah LiFePO4 and 500 Wh a day about two days. With 200–300 W of solar, summer is usually unlimited, spring and fall 3–7 days depending on consumption.