How this off-grid calculator works
An off-grid solar system has no grid to fall back on, so it has to be sized for the worst conditions it will meet, not for the average year. The calculator follows the classic stand-alone design method used in IEEE 1562 and the Sandia design handbook, with real climate data for your location.
- Loads. List your appliances (or enter a daily total). They’re summed in watt-hours per day, along with an estimate of the peak simultaneous load and the start-up surge of motors such as pumps and fridge compressors.
- Energy chain. AC energy is divided by the inverter efficiency and the inverter’s idle consumption is added for 24 hours. The share of energy used in the evening and at night passes through the battery and pays its round-trip loss.
- Sun. NASA POWER long-term monthly irradiance, temperature and wind for your location are transposed onto your panel tilt and converted into the kWh each kWp actually delivers to the battery per day, month by month, including cell-temperature losses and charge-controller efficiency.
- Design month. The array is sized for the darkest month (or the fourth-darkest if a generator covers deep winter, or the darkest summer month for a seasonal cabin), plus a safety margin.
- Battery, inverter, controller. The bank must cover your chosen days of autonomy within the chemistry’s depth-of-discharge limit and the capacity loss at your lowest battery temperature. It is then split into series and parallel units.
The formulas
Worked example: a year-round cabin near Bucharest
The default load list adds up to 3.18 kWh/day: eight LED lights, an efficient fridge, a TV, a laptop, a router, a 750 W water pump for an hour a day and two washing cycles a week. With a 92%-efficient inverter and 15 W idle draw, the battery bus must supply 3.81 kWh/day. Cycle 60% of that through a 95%-efficient LFP battery and the array has to deliver 3.94 kWh/day.
On panels tilted at 55°, December brings 2.12 kWh/m²/day of sunlight, and each kWp delivers only 1.76 kWh/day after temperature, soiling, wiring and controller losses. With a 10% margin that means 2.46 kWp, so six 450 W panels (2.70 kWp). In August the same array harvests 10.7 kWh/day, leaving a summer surplus of about 1,670 kWh a year.
For two days of autonomy at 80% DoD and 10 °C, the bank needs about 10.4 kWh nominal. At 48 V that is four 12.8 V 100 Ah batteries in series, three strings in parallel: 15.4 kWh (300 Ah), giving almost three days of real autonomy. Peak load (pump, fridge and washing-machine heater together) calls for a 4 kW inverter with at least 4.4 kW surge, and a 80 A MPPT controller. The rough hardware-plus-installation estimate is about €10,900.
How to read the results
- Design month and its sun-hours explain the array size. Look at the array size by month chart. If one or two months drive the size, a generator or seasonal tilt adjustment may be cheaper than more panels.
- Real autonomy can exceed your target because batteries come in whole units.
- Months needing backup is zero when you design for the darkest month. With the generator option it shows how many kWh a generator must supply.
- Battery life combines rated cycles with calendar ageing. Off-grid LFP banks rarely wear out from cycling; lead-acid banks often die from chronic undercharging in winter.
Typical values
| Location | Dec. sun on 55° panels | kWh/kWp/day (Dec.) | kWp for ≈ 3 kWh/day of loads |
|---|---|---|---|
| Helsinki, Finland | 0.3–0.6 kWh/m² | 0.3–0.5 | 10+ (generator usually cheaper) |
| Berlin, Germany | 0.9–1.2 kWh/m² | 0.7–1.0 | 4.5–6 |
| Bucharest, Romania | 2.0–2.3 kWh/m² | 1.6–1.9 | 2.3–2.7 |
| Madrid, Spain | 3.5–4.2 kWh/m² | 2.8–3.3 | 1.3–1.6 |
Figures are from this calculator’s model with the default load list, 12% PV losses, MPPT control and a 10% margin.
Tips for a reliable off-grid system
- Cut loads first. Every kWh/day saved removes about 0.5–0.8 kWp of panels and 2–3 kWh of battery in mid-latitude winters. Buy an efficient fridge, use LED lighting and avoid electric heating. Cook and heat water with gas, wood or a solar thermal collector.
- Tilt steeply. 55–65° in Europe boosts December harvest and sheds snow. Check the tilt angle calculator with winter selected.
- Prefer LFP. It beats lead-acid on usable capacity, efficiency and cycle life. Just don’t charge it below 0 °C.
- Go 48 V for anything above a small cabin, and size cables for low loss with the cable size calculator.
- Match strings to the controller. The array’s cold-weather open-circuit voltage must stay below the MPPT controller’s maximum input. The string sizing calculator checks this.
- Consider a small generator for the darkest weeks. Designing for the fourth-darkest month often halves the array and battery cost in northern climates.
Limitations
Monthly mean climate data miss runs of consecutive cloudy days or year-to-year variation, which is why the days of autonomy and margin inputs exist. Loads are assumed constant through the year; seasonal loads such as heating fans or summer cooling should be entered for the month you design for. Shading, snow cover and battery self-discharge are not modelled separately. Costs are coarse estimates that vary widely by country and installer.
Method & assumptions
Daily energy E = Σ (power × hours) of all AC loads. At the battery bus: E_dc = E ÷ η_inverter + idle × 24 h. Energy the array must deliver: E_pv = E_dc × [(1 − f) + f ÷ RTE], where f is the share used at night through the battery.
Array = E_pv ÷ (Y_design) × (1 + margin), where Y_design is the kWh per kWp per day delivered in the design month, from NASA monthly irradiance transposed to your tilt, cell temperature and losses, times controller efficiency (MPPT 97%, PWM ≈ 80%).
Battery nominal Wh = E_dc × days of autonomy ÷ √RTE ÷ DoD ÷ temperature factor; Ah = Wh ÷ system voltage; series units = V_system ÷ V_unit, parallel strings = ⌈Ah ÷ Ah_unit⌉. Inverter ≥ 1.25 × peak load, surge ≥ largest motor start. Controller current = array W ÷ V_system × 1.25.
Frequently asked questions
How big a solar system do I need to go off-grid?
How many batteries do I need for an off-grid solar system?
Should I use a 12 V, 24 V or 48 V off-grid system?
What are days of autonomy?
MPPT or PWM charge controller?
Why is the array so large compared with my summer needs?
Sources & references
- IEEE Std 1562 — Guide for Array and Battery Sizing in Stand-Alone Photovoltaic (PV) Systems
- IEC TS 62257 — Recommendations for renewable energy and hybrid systems for rural electrification
- NASA POWER — long-term monthly solar and meteorological climatology
- Sandia National Laboratories — Stand-Alone Photovoltaic Systems, A Handbook of Recommended Design Practices (SAND87-7023)
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