An off-grid system is a chain: panels charge a battery through a controller, and an inverter turns the battery’s DC into AC for the load. Each link has a size, and all four depend on how much energy you use. This calculator sizes them together from nine inputs, each with a default you can change.
How it works
- Array. The panels must replace the day’s energy plus the loss in the battery and inverter. Array (W) = daily load / efficiency / (peak sun hours × performance ratio).
- Battery. Capacity (Ah) = daily load × days of autonomy / (DoD × V × efficiency), the same formula as the battery bank calculator.
- Inverter. The continuous rating is the peak simultaneous load plus a 25 % margin.
- Charge controller. Output current = installed array watts / battery voltage × 1.25, which is the current an MPPT controller can push into the battery at full sun, with margin.
Worked example
A cabin uses 5,000 Wh per day with a 2,000 W peak. It has 5 peak sun hours in its worst month, a performance ratio of 0.75, 400 W panels, a 48 V bank, 2 days of autonomy, 50 % DoD and a 0.9 efficiency.
- Energy the array must supply: 5,000 / 0.9 = 5,556 Wh.
- Array: 5,556 / (5 × 0.75) = 1,481 W, which is 3.7 panels, so 4 panels (1,600 W).
- Battery: 5,000 × 2 / (0.5 × 48 × 0.9) = 463 Ah, or 22.2 kWh nominal.
- Inverter: 2,000 × 1.25 = 2,500 W.
- Controller: 1,600 / 48 × 1.25 = 41.7 A, so the next standard size up, such as a 50 A controller.
Choosing the design month
An off-grid system fails in its worst month, not its average one. In the northern hemisphere, that is usually December. If you design from an annual average of 5 hours and winter gives 2, the batteries will go flat for weeks. Either design for winter, which means a larger and more expensive array that is oversized for summer, or add a generator and accept some fuel use in the weak months.
What to check next
- Controller input voltage. The array’s open-circuit voltage, especially at low temperature, must stay below the controller’s limit. This depends on how panels are wired in series and parallel, and the calculator does not design strings.
- Cable sizes. Battery cables carry large currents. Use the solar DC cable size calculator and check ampacity.
- Protection. Fuses or breakers are needed on the battery, the array and the inverter, selected against the applicable code.
- Surge loads. Pumps and compressors need more inverter capacity than their running power. See the inverter size calculator.
Common mistakes
- Sizing on average sun.
- Sizing the battery on watts instead of watt-hours.
- Forgetting that a larger battery needs a larger array to recharge it.
Questions
How many solar panels do I need to run a house off-grid?
It depends entirely on daily energy use and the worst-month sun. A home using 20 kWh per day needs around 6 kW of panels at 5 peak sun hours, and much more in a poor winter.
Is a generator cheaper than extra batteries?
Often, for the last few days a year. Compare the cost of the extra days of autonomy with the cost of a generator and its fuel.
Does the calculator assume MPPT or PWM?
The controller figure is a minimum output current. An MPPT controller is the common choice at these sizes.