How to use this calculator
- Enter daily load and sun hours. Use the daily AC kWh load and local design-month peak sun hours.
- Set losses and margins. Enter inverter efficiency, PV derate and PV sizing margin.
- Choose battery basis. Pick 12, 24 or 48 V, then set autonomy days and usable battery capacity.
- Enter inverter loads. Add peak continuous AC load, surge load and inverter margin.
- Review system sizes. Check PV watts, battery Ah and kWh, charge-controller amps and inverter continuous/surge ratings.
How it works
Off-grid sizing starts with energy, not panel count. The calculator converts
daily AC load to DC battery energy by dividing by inverter efficiency:
E_dc = E_ac / eta_inv
PV array watts then come from peak sun hours and the entered array derate:
P_pv = E_dc / (PSH x D_pv) x margin
where E_dc is in watt-hours per day.
Battery capacity uses autonomy days and usable depth of discharge: battery Wh = E_dc x days / usable_fraction The battery amp-hour result is that installed watt-hour capacity divided by the selected DC bank voltage. Use the solar string sizing calculator to check cold Voc, hot Vmp, MPPT voltage and parallel-string current. Use the voltage drop calculator after this screen for long DC or AC runs, and use conduit fill, electrical box fill and conduit bend offset for the installation-side checks. For longer raceways with bends, add the conduit pull tension calculator.
Worked example
Verified against the live calculator
A small off-grid load using 3.5 kWh/day AC at 90%
inverter efficiency needs 3.89 kWh/day from the battery. With
4.5 peak sun hours, a 75% array derate and a
20% PV margin, the array screens near 1,383 W.
For 2 autonomy days at 80% usable battery capacity
on a 48 V bank, installed storage is about 9.72 kWh,
or 203 Ah.
Reference data
Nominal DC voltage changes current. Higher voltage does not reduce energy required, but it can materially reduce controller current and voltage drop.
| Nominal bank | Typical planning use | Sizing note |
|---|---|---|
| 12 V | Small cabins, RVs, small DC loads | Higher current at modest inverter wattage. |
| 24 V | Medium cabins, pumps, telecom or shed systems | Lower current than 12 V while staying common for small equipment. |
| 48 V | Larger inverter systems and whole-cabin loads | Lower current and voltage drop for the same watts. |
Source: Planning guidance only. Verify battery, inverter and charge-controller datasheets for the exact voltage window and current limits.
Frequently asked questions
How do you size an off-grid solar system?
Start with daily energy use in kWh/day, divide by inverter efficiency to estimate DC battery energy, then size PV watts from peak sun hours and array derate. Battery capacity comes from daily DC energy times autonomy days divided by usable battery capacity.
How many solar panels do I need off grid?
Required PV watts = daily DC watt-hours / (peak sun hours x array derate), with a margin added. Divide that result by the wattage of one panel to estimate panel count, then use the solar string sizing calculator to check series/parallel layout, MPPT voltage and controller input current.
How do I size an off-grid battery bank?
Battery installed Wh = daily DC Wh x autonomy days / usable capacity fraction. Battery Ah = installed Wh / battery voltage. Lithium systems often use a higher usable fraction than lead-acid systems, but the final value should come from the battery datasheet and reserve target.
Is 12 V, 24 V or 48 V better for off-grid solar?
Higher DC voltage reduces current for the same power, which helps with controller current, conductor size and voltage drop. Small RV or cabin loads may use 12 V, medium systems often use 24 V, and larger inverter systems commonly use 48 V.
Does this replace a code-compliant solar design?
No. It is an early sizing screen only. Final solar design must check module string voltage, controller limits, battery chemistry, conductors, fuses, disconnects, grounding, product listings, adopted code and permits.
Method & assumptions
- Assumes the daily load is AC energy in kWh/day and applies inverter efficiency to estimate battery DC energy.
- PV sizing uses peak sun hours and one combined derate for temperature, dirt, mismatch, wiring and controller losses.
- Charge-controller current is a simplified MPPT output-current screen: PV watts divided by battery voltage, then multiplied by the entered current margin.
- Does not check module series/parallel string voltage, low-temperature Voc, conductor ampacity, voltage drop, overcurrent protection, grounding, rapid shutdown, battery BMS limits, enclosure ratings, product listings or local permits. Use the solar string sizing calculator for the PV series/parallel voltage window.