How much battery do I need for my off-grid system?

It depends on your daily load in watt-hours, your target depth of discharge (DoD), and how many days of autonomy you want. A common starting point is: divide your daily load by your DoD to get the minimum usable capacity, then add a safety margin. For example, 1,000 Wh/day at 80% DoD requires at least 1,250 Wh of nominal battery capacity. This calculator sizes both battery and solar together against real monthly irradiance data so you can see which months fall short.

What is depth of discharge (DoD) and why does it matter?

Depth of discharge is the percentage of a battery's capacity that has been used relative to its total capacity. Discharging a battery too deeply too often shortens its lifespan — lithium chemistries typically allow 80–100% DoD, while lead-acid batteries are usually limited to 50% for longevity. Setting a conservative DoD target in the calculator gives you a buffer above the hard cutoff, so the system flags months where your battery would dip into that buffer zone.

How does cloud cover affect solar panel output?

Cloud cover reduces the amount of solar irradiance reaching your panels. Overcast (OVC) skies can cut output to as little as 20% of clear-sky production, while scattered clouds (SCT) typically allow around 70%. The Scenario Planner in this calculator lets you apply a sky-condition slider — from clear (SKC) to fully overcast (OVC) — to stress-test your system against poor-weather days without needing to run a new API fetch.

What's the difference between PVGIS and PVWatts data?

Both are free, government-backed datasets of historical solar irradiance, but they draw on different satellite and ground-station sources. PVGIS (from the EU's Joint Research Centre) has strong coverage for Europe, Africa, and Asia. PVWatts (from NREL) is optimized for the Americas and also supports tracking array types (1-axis, 2-axis). This calculator lets you choose which source to use per panel plane, and supports hourly TMY data from PVWatts for 8760-hour simulations.

Can this calculator size an RV or cabin solar system?

Yes — it's designed for exactly that. Enter your location (or any location you'll be traveling to), your panel array wattage and tilt, and your electrical loads with their daily hours. The calculator returns month-by-month results showing whether your system is adequate, approaching your DoD limit, or undersized for that month. The Scenario Planner can also simulate a specific trip duration under chosen weather conditions.

TMY stands for Typical Meteorological Year — a synthetic dataset assembled from many years of historical weather records to represent a statistically typical year at a given location. It provides hourly values for solar irradiance, temperature, and wind. Using TMY data gives a more realistic picture of annual system performance than a single year of measurements, but it is not a forecast — actual production in any given year will vary. This calculator uses TMY data for both its monthly irradiance lookups and its optional 8760-hour hourly simulation mode.

Off-Grid Solar & Battery Calculator

Location & Solar Data

Search Address

Format:

Latitude

Longitude

Tilt Optimization

Calculated from your latitude using NOAA solar position geometry. Click any value to apply it to the Tilt field below.

Fixed installation

Annual optimum

Summer (May–Aug)

Winter (Nov–Feb)

Adjustable mount — monthly optimum

For DIY adjustable mounts, change tilt monthly to maximize harvest.

How these recommendations are calculated

Panel Tilt °

Angle of your panel from horizontal. 0° = flat; 90° = vertical. Rule of thumb: match your latitude for the best year-round average.

Panel Azimuth °

Direction your panel faces, measured clockwise from north (true north). 180° = due south (optimal in the northern hemisphere). 0°/360° = north | 90° = east | 270° = west.

*Panel Array Rated Power W at STC

Rated wattage of your solar panel, or combined power of your array.

Temp. coefficient (γ) %/°C

Magnitude of your panel's power-vs-temperature coefficient (γ_Pmax), from the datasheet — enter as a positive number (e.g. 0.40 for −0.40 %/°C). Typical crystalline silicon: 0.35–0.45 %/°C. Reduces output above 25 °C cell temp; slightly increases it below.

Leave blank to skip — no additional thermal derating is applied. Requires the 8760-hour TMY simulation toggle below to take effect. Affects hourly-panel results only — the monthly Daily Solar Wh/day and Scenario Planner do not yet incorporate temperature.

Mount Type

Get Hourly Data

Enable 8760-hour simulation

Fetches a full year of typical hourly AC output from PVWatts and runs an hour-by-hour battery simulation. Typical Meteorological Year (TMY) data is observed historical weather drawn from many years — useful for sizing, not a forecast.

Monthly Irradiance (kWh/m²/day)

Fetched from PVWatts or PVGIS, or enter manually. PVWatts: Solar Radiation column. PVGIS: H(i) column.

Solar Access — Shading Factor

Fraction of irradiance that reaches your panel after near-field shading (trees, structures). Tilt & orientation are already accounted for by the fetched irradiance. 1.00 = no shading | 0.90 = occasional shade | 0.75 = significant shade.

Annual assumption 0–1

Additional Arrays

Add extra planes for split arrays (e.g. east + west roofs, or a boat bimini plus deck). Each plane can be fetched independently. Global Solar Access and irradiance overrides apply to all planes unless you opt into per-plane values.

Storage Setup

Battery Bank

*Fields with an asterisk are required

Chemistry

Capacity unit:

*Battery Capacity Wh

*Battery Capacity Ah at C/20

*Battery Voltage V DC

Use advertised, rated, or nominal output voltage.

Chemistry Parameters (auto-filled, overridable)

Peukert Exponent (k)

Round-Trip Efficiency %

DoD Design Target %

Hard Cutoff DoD %

Rated Hour H

External Charging (optional)

Top-off nightly

Battery supports pass-through charging

Pass-through on (default): charger charges the battery while loads run simultaneously — the battery is the central bus. Off: charger output first serves loads directly; only surplus reaches the battery (use for systems where the battery cannot simultaneously accept charge and supply loads, e.g. some isolated charging circuits or older lead-acid banks). Affects the 8760-hour simulation; the monthly model reports the same energy balance either way.

Shore or grid power refills the battery to 100% each night. Infinite duration shown when daily deficit stays within usable capacity; otherwise the row flags “Top-off insufficient” and duration remains finite.

Supplemental charger

Power entry:

Charger Power W

Duration h/day

Window

Efficiency %

Battery Voltage (optional) V DC

Enter your battery bank voltage if you have DC loads at a different voltage. Used to detect DC-DC conversion losses; leave blank if unknown or all loads are AC.

Battery Charging

Charge Controller

Load Management

Select the power conversion devices your loads run through. Inefficiencies increase the effective draw on your battery.

Inverter

DC-DC Converter

Wiring

Wiring Loss %

Electrical Loads

*Fields with an asterisk are required

Advanced: Standby / Parasitic Draw

All real systems have a small continuous draw from the charge controller, BMS, clocks, and other standby electronics. Enabling this adds it to your load calculation and may change Charging/Deficit status results.

Standby Draw W (continuous)

Month	Irradiance kWh/m²/day	Solar Access	Typical Daily Charge Status	Daily Solar Wh/day	Net Balance Wh/day	Recharge fraction/day	Days to Full from empty	Duration days, clear	Duration days, overcast	Load/Capacity %	Net DoD %	Solar vs Load %	Solar Noon 15th, std time

Scenario Planner — trip / event check

Stress-test a specific trip against a chosen month and sky condition. Uses the irradiance you’ve already fetched — no new data call. Add an extra ad-hoc load (e.g. a tent A/C) and slide the cloud cover from clear to overcast to see when your battery hits the DoD cutoff.

Month

Start date

Duration (days)

Starting SoC (%)

Add an extra ad-hoc load

Power (W)

Start time (std)

Duration (h)

Geometric + Cloud Conditions Monthly average irradiance with sky-condition derate and cosine solar model. Works without fetching hourly data.

TMY Data Uses the 8760-hour dataset from your last hourly fetch. Real modelled weather — sky slider does not apply.

Sky condition (forecast)

Formulas & Sources

Daily Solar Input (Ah/day)

Panel Wattage × Irradiance × Solar Access × System Efficiency ÷ System Voltage

where:

Solar Harvest Efficiency = Controller Efficiency × (1 − Wiring Loss) × Round-Trip Efficiency

Effective Load = Raw Load ÷ (Inverter Efficiency × DC-DC Efficiency)

Irradiance in kWh/m²/day equals peak sun hours — a 1:1 equivalence at STC means Panel Wattage × kWh/m²/day = Wh/day directly.

Example: Panel = 200 W, Irradiance = 5.2 kWh/m²/day, Solar Access = 0.85, System Eff = 0.89 (MPPT 97% × wiring 97% × RTE 95%), Voltage = 12 V
→ 200 W × 5.2 kWh/m²/day × 0.85 × 0.89 ÷ 12 V = 786.8 Wh/day ÷ 12 V = 65.6 Ah/day

Peukert Correction (discharge only)

Effective Capacity = Rated Capacity × (Rated Hour ÷ (Load Current × Rated Hour ÷ Rated Capacity))^{(Peukert Exponent − 1)}

Applied to discharge only; charge correction is complex and rarely material. Load Current is approximated as the 24-hour average: total daily Wh ÷ System Voltage ÷ 24 hours. This is accurate for steady loads. Intermittent high-draw loads (e.g., a pump running 30 minutes) experience a higher instantaneous current and therefore a greater Peukert penalty than this average reflects — treat those results as optimistic.

Example: Rated capacity = 100 Ah, k = 1.05 (LiFePO₄), Rated hour H = 20, daily load = 1,200 Wh, Voltage = 12 V
I = 1,200 Wh/day ÷ 12 V ÷ 24 h = 4.17 A
→ 100 Ah × (20 h ÷ (4.17 A × 20 h ÷ 100 Ah))^0.05 = 100 Ah × 24.0^0.05 = 117 Ah (slow draw raises effective capacity above rated)

Source: Peukert, W. (1897). Über die Abhängigkeit der Kapazität von der Entladestromstärke.

Overcast Sky Factor: 3%

Overcast-day output is modeled as 3% of clear-sky output for the same irradiance.

Example: Clear-sky harvest = 200 W × 5.2 kWh/m²/day = 1,040 Wh/day
→ 1,040 Wh/day × 0.03 = 31 Wh/day on a fully overcast day

Source: field data from several hundred off-grid systems.

Temperature Derating (γ_Pmax)

PV panel output decreases linearly with cell temperature above the Standard Test Condition reference of 25 °C. The power temperature coefficient γ_Pmax (typically −0.30 to −0.45 %/°C for crystalline silicon) is printed on every module datasheet.

P(T) = P_STC × [1 + γ × (T_cell − 25 °C)]

where γ is the signed coefficient (negative for standard panels) and T_cell is the cell temperature in °C reported by PVWatts’ thermal model (accounts for irradiance, ambient temperature, and wind cooling).

PVWatts default replacement (Choice B): PVWatts already bakes in a default γ = −0.47 %/°C. Rather than stacking the user’s value on top (which would double-count), the calculator undoes the PVWatts default first and applies the user’s coefficient:

P_corrected(h) = P_PVWatts(h) × [1 + γ_user × (T_cell(h) − 25)] ÷ [1 + (−0.0047) × (T_cell(h) − 25)]

Example: July noon, Great Falls MT — T_cell = 58 °C, panel γ = −0.40 %/°C, P_PVWatts = 820 W/kW
PVWatts factor: 1 + (−0.0047) × (58 − 25) = 0.845
User factor: 1 + (−0.0040) × (58 − 25) = 0.868
→ P_corrected = 820 × 0.868 ÷ 0.845 ≈ 842 W/kW (less conservative than PVWatts’ default for this panel)

Source: IEC 61215 temperature coefficient definition; PVWatts V8 technical reference (NREL/TP-6A20-68586).

Day / Night Load Split

Switch to Advanced mode and set a load Operating Window to see the day/night split here.

Solar Noon (15th of month, standard time)

Solar noon is the moment the sun crosses the local meridian — its highest point in the sky that day, and the midpoint of the solar window. It is not the same as clock noon: it drifts by site longitude (4 minutes per degree east/west of your time-zone meridian) and by the Equation of Time (up to ±16 minutes across the year, from Earth’s axial tilt and orbital eccentricity).

Solar Noon (local min past midnight) = 720 − EoT(N) − 4 × (Longitude − 15 × TZ Offset)

where N is the day-of-year for the 15th of the month (epoch used for the monthly display). TZ Offset is the site’s standard-time UTC offset — DST is intentionally ignored so the displayed time is consistent year-round. Site time zone is resolved from your entered latitude/longitude.

Example: New York City — Lon = −74.0°, TZ offset = −5 (EST), Feb 15 (N = 46), EoT(46) = −14 min
→ 720 min − (−14 min) − 4 min/° × (−74.0° − 15 °/h × (−5 h)) = 734 min − 4 min/° × 1.0° = 730 min = 12:10 PM

Source: Equation of Time — NOAA Solar Position Calculator methodology.

Battery Duration (days)

Usable Capacity ÷ |Net Daily Deficit|

where Usable Capacity = Effective Capacity × DoD Target. Shown as ∞ when the system is in daily surplus (charging).

Example: Battery = 300 Ah × 12 V = 3,600 Wh, DoD target = 80%, net daily deficit = 1,440 Wh/day
Usable = 3,600 Wh × 0.80 = 2,880 Wh
→ 2,880 Wh ÷ 1,440 Wh/day = 2.0 days

Safety Notice