Calculator KB-28
Solar power system calculator
Estimate an off-grid system for a country house, backup power, or a workshop.
Result
In brief: Calculate an off-grid solar system: panel power, battery capacity, and charge controller current based on daily consumption, peak sun hours, and days of autonomy. Example: Panel power: 333 W.
Calculation example
| Daily consumption | 1000 Wh |
|---|---|
| Peak sun hours | 4 h/day |
| System losses | 25 % |
| Days without sun | 2 d |
| Battery voltage | 12 V |
| Depth of discharge | 50 % |
| Panel power | 333 W |
|---|---|
| Battery capacity | 333 Ah |
| Energy reserve | 4 kWh |
| Controller current | 34.7 A |
| Monthly energy production | 30 kWh |
How it is calculated
Q = E · days / (U · DoD)
Ictl = 1,25 · P / U
A solar hour is equivalent to one hour at the panel's full power, not the length of daylight. Some energy is lost in the controller, wires, inverter, and battery. Use the darkest month if the system needs to work year-round.
What to look for
- An MPPT controller draws noticeably more power from the panels than PWM, especially when the panels have a high voltage.
- Even partial shading on a panel sharply reduces the output of the entire string.
Frequently asked questions
How many solar panels do you need for 1 kWh per day?
With 4 peak sun hours and 25% losses, you'll need about 333 W of panels. There are fewer sun hours in winter, so you'll need to increase the power several times over.
What battery do you need for a solar power system?
Q = E · days / (U · DoD). For 1 kWh per day, 2 days of backup, 12 V, and a 50% depth of discharge, you'll need about 333 Ah.
How do you choose a charge controller?
Choose a controller rated for 25% more current: 1.25 · P / U. For 333 W and 12 V, that's about 35 A. MPPT is usually more cost-effective than PWM when the panel voltage is higher than the battery voltage.
