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OffgridCalc

Solar & Off-Grid Calculator: How long will my power station last and how fast does it charge from solar?

A 200 W panel, four sun hours and a 1,000 Wh power station: is that enough for a cooler, laptop and lights? This calculator adds up your loads, compares them with the solar output and shows how long the battery lasts without sun, how many days you can stay off-grid with solar and how long the panel needs to fully charge it.

Pick devices from the presets or enter your own values – all results update instantly, with a step-by-step calculation.

Solar

What does your panel deliver?

Rated power (Wp) of all panels combined. 0 if you only want the battery runtime.

Peak sun hours, not daylight hours: continental US winter 2–4, spring/fall 4–5, summer 5–7.

Storage

Battery or power station

Rated capacity from the label. Ah × volts = Wh (100 Ah × 12.8 V = 1,280 Wh).

Loads

What runs during the day – and for how long?

Devices

Power from the label or a meter and actual runtime per day. Cycling devices (cooler, furnace) only with their real runtime.

Advanced settings

Default 75%: losses from temperature, orientation, charge controller and cabling. Flat-mounted panels 60–70%, optimally tilted with MPPT up to 80%.

Power station typically 85–92% (BMS reserve, converter losses). LiFePO4 battery 80–90%, lead-acid 50%.

Result

Deficit per day

85 Wh

600 Wh/day solar output − 685 Wh/day consumption

One day uses of the usable energy685 / 900 Wh
Total consumption per day
685 Wh
Solar output per day
600 Wh
200 W × 4 h × 75%
Off-grid without sun
1.3 days
usable: 900 Wh
Off-grid with solar
10.6 days
at 4 h sun hours per day
Charge time empty → full
6 h 40 min
6.7 sun hours ≈ 1.7 days at 4 h per day · never full with loads running (no surplus)
Solar coverage of consumption
88%
  • Deficit of 85 Wh per day – the battery is empty after about 10.6 days. For an even balance at 4 h sun hours you'd need about 230 W of solar or 85 Wh less consumption.
Show calculation
  1. 1Cooler: 45 W × 10 h = 450 Wh
  2. 2Laptop: 65 W × 3 h = 195 Wh
  3. 3LED lights: 10 W × 4 h = 40 Wh
  4. 4Total consumption per day: 685 Wh/day
  5. 5Solar output: 200 W × 4 h × 75% = 600 Wh/day
  6. 6Daily balance: 600 Wh/day − 685 Wh/day = -85 Wh/day
  7. 7Usable energy: 1,000 Wh × 90% = 900 Wh
  8. 8Off-grid days without sun: 900 Wh ÷ 685 Wh/day = 1.3 days
  9. 9Off-grid days with solar: 900 Wh ÷ 85 Wh/day deficit = 10.6 days
  10. 10Charge time empty → full: 1,000 Wh ÷ (200 W × 75%) = 6.7 sun hours ≈ 1.7 days at 4 h per day

 

How it's calculated

The calculator compares three quantities: what your devices use per day, what the solar panel delivers per day, and how much energy is stored in the battery or power station. Balance, off-grid days and charge time follow from that.

consumption [Wh/day] = Σ power [W] × runtime [h/day] solar output [Wh/day] = panel power [W] × sun hours [h/day] × 0.75 daily balance [Wh/day] = solar output − consumption
Example: 200 W × 4 h × 0.75 = 600 Wh solar output; 685 Wh consumption → −85 Wh/day deficit

The factor 0.75 is the system efficiency: a 200 W panel never delivers 200 W in everyday use because module temperature, orientation, charge controller and cabling all cause losses. 60–75% is realistic for panels lying flat on a van or tiny house, up to 80% with an optimally tilted panel and an MPPT controller. You can adjust the value under “Advanced settings”.

usable energy [Wh] = capacity [Wh] × usable fraction (power station ≈ 90%) off-grid days without sun = usable energy ÷ consumption off-grid days with solar = usable energy ÷ deficit (with a surplus: unlimited)
1,000 Wh × 0.9 = 900 Wh; 900 ÷ 685 = 1.3 days without sun; 900 ÷ 85 = 10.6 days with 4 sun hours
charge time [sun hours] = capacity [Wh] ÷ (panel power × 0.75) charge time [days] = charge time [sun hours] ÷ sun hours per day
1,000 Wh ÷ (200 W × 0.75) = 6.7 sun hours ≈ 1.7 days at 4 h/day

Sun hours doesn't mean the time the sun is up but peak sun hours: the daily irradiation in kWh/m² equals the number of hours at full rated output. The continental US averages 4–5 peak sun hours per day, from about 2–3 in winter in the north to 6–7 on clear summer days in the Southwest.

Worked example

Weekend trip in the van: 400 W folding panel, 4 sun hours, 2,000 Wh power station. Loads: compressor cooler, laptop, LED lights, mobile router, two smartphones and a coffee maker for nine minutes in the morning.

Inputs

  • Solar panel power: 400 W
  • Sun hours per day: 4 h
  • Battery / power station capacity: 2,000 Wh
  • Devices:
  • Kühlbox: 45 W × 12 h/day
  • Laptop: 65 W × 4 h/day
  • LED-Licht: 10 W × 5 h/day
  • Mobiler Router: 10 W × 8 h/day
  • Smartphones (2×): 20 W × 2 h/day
  • Kaffeemaschine: 1,200 W × 0.15 h/day
  • Solar system efficiency: 75 %
  • Usable fraction of capacity: 90 %

Result

50 Wh

Surplus per day

Total consumption per day
1,150 Wh
Solar output per day
1,200 Wh
Off-grid without sun
1.6 days
Off-grid with solar
unlimited
Charge time empty → full
6 h 40 min
Solar coverage of consumption
104%

Calculation

  1. Kühlbox: 45 W × 12 h = 540 Wh
  2. Laptop: 65 W × 4 h = 260 Wh
  3. Kaffeemaschine: 1,200 W × 0.15 h = 180 Wh
  4. Mobiler Router: 10 W × 8 h = 80 Wh
  5. LED-Licht: 10 W × 5 h = 50 Wh
  6. Smartphones (2×): 20 W × 2 h = 40 Wh
  7. Total consumption per day: 1,150 Wh/day
  8. Solar output: 400 W × 4 h × 75% = 1,200 Wh/day
  9. Daily balance: 1,200 Wh/day − 1,150 Wh/day = 50 Wh/day
  10. Usable energy: 2,000 Wh × 90% = 1,800 Wh
  11. Off-grid days without sun: 1,800 Wh ÷ 1,150 Wh/day = 1.6 days
  12. The balance is even or positive – with these sun hours consumption is covered every day.
  13. Charge time empty → full: 2,000 Wh ÷ (400 W × 75%) = 6.7 sun hours ≈ 1.7 days at 4 h per day
  14. With loads running: 2,000 Wh ÷ 50 Wh/day surplus = 40 days

The variables explained

Solar panel power (W)
Rated power from the label (Wp). Add up multiple panels. Folding panels only reach the rated value when aimed directly at the sun.
Sun hours per day (h)
Peak sun hours. Continental US: winter 2–4, spring/fall 4–5, summer 5–7 depending on the region (NREL). Only 10–25% of that under overcast skies.
Battery / power station (Wh)
Rated capacity in watt-hours. Convert Ah ratings: Ah × voltage (100 Ah × 12.8 V = 1,280 Wh).
Devices: power (W) and runtime (h/day)
Actual power draw and actual operating time. Cycling devices (cooler, furnace fan) only run 30–50% of the time – that runtime counts.
Solar system efficiency (%)
Losses between module and battery (default 75%). Includes temperature, orientation, charge controller, cabling.
Usable fraction of capacity (%)
Power stations typically deliver 85–92% of rated capacity (BMS reserve, converter losses). Lead-acid batteries only 50%.

Common mistakes

  • Confusing daylight hours with peak sun hours: 10 hours of sun in the sky is about 5–6 peak sun hours in summer, 2 in winter.
  • Counting the cooler at 24 hours: it cycles – 8–12 hours of runtime in summer, less in spring.
  • Underestimating the coffee maker or kettle: 1,200 W for 9 minutes is 180 Wh – as much as 18 hours of LED lighting.
  • Using the full rated capacity: 1,000 Wh on the label is about 900 Wh at the power station's outlets.
  • Seeing surplus as reserve: once the battery is full at noon, the remaining solar output is wasted.
  • Planning for the best day: three overcast days in a row are normal – the “without sun” value is the honest reserve.

Assumptions and limits

  • Constant daily consumption and constant sun hours across all days.
  • The system efficiency (default 75%) covers all losses from module to battery.
  • Solar surplus with a full battery is not stored.
  • Charge time applies to an empty battery without input limits; many power stations cap the solar input (e.g. 200 W) – then it takes longer.
  • No temperature effects on battery capacity; no aging.

Frequently asked questions

How do I calculate a solar panel's output per day?

Panel power in watts × peak sun hours per day × system efficiency. A 200 W panel delivers 200 × 4 × 0.75 = 600 Wh per day at 4 sun hours and 75% efficiency. In winter with 2 sun hours it's only 300 Wh.

How many watts of solar do I need for my consumption?

Daily consumption ÷ (sun hours × 0.75). For 685 Wh per day at 4 sun hours: 685 ÷ 3 ≈ 230 W. The calculator shows this value as a hint as soon as the balance is negative. Plan 20–30% reserve for overcast days.

How long does a 1,000 Wh power station last?

Usable energy (≈ 900 Wh) ÷ daily consumption. At 685 Wh per day about 1.3 days without sun. A single device: 900 Wh ÷ power – a 45 W fridge runs about 20 hours, a 65 W laptop just under 14 hours.

How long does it take to charge a power station with solar?

Capacity ÷ (panel power × 0.75) gives the required sun hours. 1,000 Wh on a 200 W panel: 1,000 ÷ 150 ≈ 6.7 sun hours, i.e. just under two days with 4 sun hours each. If loads run at the same time, only the surplus counts.

What are peak sun hours and how many are there in the US?

One peak sun hour equals 1 kWh/m² of irradiation – the time the sun would have to shine at full strength to deliver the day's energy. Continental US: about 1,400–2,200 kWh/m² per year, i.e. 4–6 peak sun hours per day on average; December around 2–3, June 5–7.

Why does my 100 W panel only deliver 60–70 W?

The rated power applies at 1,000 W/m² irradiation, 25 °C module temperature and perpendicular light. In real life the module is hotter, lying flat or angled away from the sun, partly dirty or shaded; add charge controller and cable losses. Together that's the system efficiency of about 75%.

Is 100 W of solar enough for a cooler?

A compressor cooler at 45 W and 10 hours of runtime needs 450 Wh per day. 100 W delivers only 300 Wh at 4 sun hours – it just works out in summer with 5–6 sun hours, not in spring and fall. 200 W is the safe choice.

How do I convert Ah to Wh?

Wh = Ah × nominal voltage. A 100 Ah LiFePO4 battery (12.8 V) holds 1,280 Wh, a 100 Ah AGM battery (12 V) 1,200 Wh – of which only 50% is usable with lead-acid, 80–90% with LiFePO4.

Can I use residential solar modules with a power station?

Yes, if the power station tolerates the module voltage: standard 400 W modules deliver 35–45 V open-circuit; many power stations only accept 12–60 V at the solar input and cap the power (e.g. 200 W). The calculator uses the panel power – check your device's maximum solar input.

Sources and background