Off-grid solar systems are used where grid access is unavailable or too expensive: holiday homes, mountain houses, farms, telecom sites, irrigation and monitoring points. In a grid-connected system, any shortfall is covered by the grid; in an off-grid system there is no such backup. That is why a sizing mistake turns directly into a "the power went off" complaint. In this guide we size an off-grid system step by step using example values.
Step 1: Work out the daily energy demand
List each appliance's power and daily operating hours. For appliances with compressors, such as refrigerators, it is more accurate to use the average daily consumption rather than the rated power, because the compressor does not run continuously.
| Appliance (example) | Power (W) | Hours per day | Energy (Wh/day) |
|---|---|---|---|
| LED lighting (8 × 10 W) | 80 | 5 | 400 |
| Refrigerator (average) | 150 | 8 | 1,200 |
| Television | 100 | 4 | 400 |
| Water pressure pump | 750 | 1 | 750 |
| Chargers, laptop | 100 | 3 | 300 |
| Washing machine (average) | 550 | 1 | 550 |
| Total | 3,600 |
AC loads are supplied through the inverter, so add the inverter loss. Assuming an example inverter efficiency of 90%, the daily energy to be drawn from the battery is 3,600 / 0.90 = 4,000 Wh. Consider seasonal differences too: lighting hours are longer in winter, while the refrigerator and pump work harder in summer. Size the system for the period when consumption is highest and sunshine is weakest.
Step 2: Calculate the battery capacity
The battery calculation rests on three values: daily energy, days of autonomy (the number of days the system must run without sun) and usable depth of discharge (DoD).
- Autonomy: Let us choose 2 days as an example. Energy to be stored: 4,000 × 2 = 8,000 Wh.
- Depth of discharge: Fully discharging a battery shortens its life. Taking an example DoD of 80% for lithium (LiFePO4), the nominal capacity required is 8,000 / 0.80 = 10,000 Wh.
- System voltage: At this power level, a 48 V system reduces current by a factor of four compared with 12 V, which reduces cable size and losses. 10,000 Wh / 48 V ≈ 208 Ah. The nearest standard capacity above this is selected.
Now the same calculation with gel batteries. For gel batteries, a DoD of around 50% is generally recommended to protect cycle life. 8,000 / 0.50 = 16,000 Wh; 16,000 / 48 ≈ 333 Ah. To reach 48 V with 12 V 200 Ah gel batteries, 4 batteries are connected in series (200 Ah); to cover 333 Ah, two of these series strings are connected in parallel, giving 400 Ah in total, or 8 batteries. As you can see, gel batteries need a noticeably larger nominal capacity than lithium for the same demand.
Remember that battery capacity drops in cold conditions and decreases as batteries age. For projects where the battery sits in a cold room, check the manufacturer's temperature correction data.
Step 3: Select the inverter
An inverter is selected against two separate values: continuous power and surge (peak) power.
- Continuous power: The sum of loads that may run at the same time. In our example, the refrigerator (150 W), pump (750 W), television (100 W), lighting (80 W) and chargers (100 W) may all run together: 1,180 W in total. Add a margin and choose an inverter whose continuous rating is comfortably above this.
- Surge power: Motor loads (pumps, compressors) draw several times their rated power at start-up. Using an example factor of 3, the pump draws 750 × 3 = 2,250 W and the refrigerator 150 × 3 = 450 W. Adding the other loads (280 W) gives a worst-case peak of 2,980 W. The inverter's short-term surge rating in its datasheet must cover this. The real starting factor depends on the motor type; measure it if possible or check the manufacturer's data.
In this example, an inverter of around 3 kW whose datasheet surge rating covers the start-up peak is a reasonable choice. The inverter's input voltage must match the battery system voltage (48 V).
Step 4: Size the array for the worst month
Array power is calculated for the month with the lowest solar resource of the year. This value should be taken for the tilted plane, according to the panel tilt and orientation; sources such as PVGIS provide monthly values. As an example, assume 2.5 peak sun hours per day on the tilted plane in the worst month (December). To cover temperature, cable, charge controller and battery charge-discharge losses, use an example overall efficiency factor of 0.75.
Required array power = 4,000 Wh / (2.5 hours × 0.75) = 4,000 / 1.875 ≈ 2,133 Wp. With an example 400 W panel, 2,133 / 400 ≈ 5.3, so 6 panels (2,400 Wp) are selected. To improve winter performance, off-grid arrays are often tilted more steeply than grid-connected ones.
Step 5: Check the MPPT charge controller
Example panel values: Voc 37.0 V, Vmp 31.0 V, Isc 13.7 A, Voc temperature coefficient −0.28%/°C. Assume the lowest site temperature is −10 °C.
- Open-circuit voltage in the cold: From 25 °C to −10 °C is a 35 °C difference. 37.0 × (1 + 0.0028 × 35) = 37.0 × 1.098 ≈ 40.6 V. If the example controller's maximum PV input voltage is 150 V, then 150 / 40.6 ≈ 3.7, so at most 3 panels can be connected in series. The cold Voc of 3 panels in series, about 121.9 V, stays below the 150 V limit.
- Operating voltage in the heat: With an example Vmp coefficient of −0.35%/°C and a cell temperature of 70 °C, 31.0 × (1 − 0.0035 × 45) ≈ 26.1 V; with 3 panels in series that is about 78.4 V. This stays well above the charging voltage of a 48 V battery, so the MPPT can operate.
- Configuration: The 6 panels are connected as 3 in series × 2 in parallel (3S2P). PV input current is 2 × 13.7 = 27.4 A; the controller's PV input current limit must cover this.
- Charge current: 2,400 W / 48 V = 50 A. An MPPT controller with a charge current of at least 50 A, preferably a little more (for example 60 A), is therefore selected.
For more on series-parallel connection and MPPT voltage windows, see our stringing and MPPT guide.
Gel or lithium?
| Property | Gel battery | Lithium (LiFePO4) |
|---|---|---|
| Recommended usable DoD | Generally around 50% | Generally 80–90% |
| Nominal capacity for the same demand | Higher | Lower |
| Weight and volume | Heavy, takes up a lot of space | Lighter and more compact |
| Charge efficiency | Lower | Higher |
| Cycle life | Shorter | Usually significantly longer |
| Charging in the cold | Capacity drops but it can be charged | Should not be charged below 0 °C without heating |
| Management | Correct charge voltage settings matter | BMS (battery management system) required |
| Upfront cost | Lower | Higher |
For short-term or occasional use, gel batteries can be economical. For systems that cycle deeply every day, the long life and high DoD of lithium usually balance out the total cost. Base the comparison on the manufacturer's cycle life and warranty terms.
Common mistakes
- Sizing the array on the annual average, leaving the battery permanently flat in winter.
- Ignoring start-up current, so the inverter shuts down whenever the pump starts.
- Calculating with the full battery capacity and ignoring the DoD limit.
- Setting the number of panels in series without allowing for the rise in Voc in cold weather.
- Running high power at a low system voltage and ending up with thin cables and high losses.
Off-grid design with PVAGE
PVAGE's off-grid module lets you build a load list from an appliance library, work out the daily energy demand with simultaneity, and run a start-up (surge) analysis for motor loads. You set the number of autonomy days and choose gel or lithium batteries, and you see the MPPT configuration. For off-grid systems it also suggests line current, fuse and cable cross-section, and provides a battery connection diagram. A quick calculation option is available for a short preliminary assessment. See the off-grid system design feature page for details.
Explore the feature: Off-Grid Solar Calculator and Battery Sizing