Sizing an off-grid solar system means matching three different requirements: daily energy consumption, instantaneous power demand and the autonomy needed when solar production is insufficient. Oversizing one component does not automatically fix the others: PV modules, charge controller, battery bank, inverter, cables and protection devices must be checked as one coordinated system.
This guide presents a practical method using a camper van or small cabin as an example. The figures explain the process; they are not a universal bill of materials and do not replace component datasheets or a final electrical design.
Functional diagram of an off-grid photovoltaic system
| 1. PV modules | → | 2. DC isolation and protection | → | 3. MPPT charge controller |
| Generate direct-current energy | Fuses, breakers and suitable protection | Optimises the array and charges the battery | ||
| 6. 12/24/48 V DC loads | ← | 4. Battery bank and main fuse | → | 5. Pure sine inverter → 230 V AC loads |
| Lighting, pumps and compatible electronics | Stores energy and provides autonomy | Supplies appliances and AC outlets |
Key rule: the charge controller sits between the PV array and the battery. A power inverter connects to the battery through its own protection, not to the LOAD output of a small solar controller.
1. Calculate energy in Wh, not only power in watts
Power tells you how much an appliance draws at a given moment. Daily energy also includes how long it runs:
Daily energy in Wh = power in W × operating hours per day
Example loads in a camper van:
| Load | Power | Daily use | Energy | Type |
| 2 LED lamps | 30 W total | 2 h | 60 Wh | DC |
| Water pump | 60 W | 15 min | 15 Wh | DC |
| 12 V refrigerator | 48 W while running | 8 equivalent h | 384 Wh | DC |
| Television | 50 W | 2 h | 100 Wh | AC |
| Microwave oven | 900 W input | 15 min | 225 Wh | AC |
The DC loads total 459 Wh/day. The AC loads total 325 Wh/day, but inverter losses must be included. Assuming 90% efficiency, they require about 361 Wh from the battery. The total is therefore about 820 Wh/day; allowing for standby consumption, tolerances and small unlisted loads, the example can reasonably be designed around 900 Wh per day.
For refrigerators, pumps and compressors, do not automatically multiply rated power by 24 hours. Measure the real duty cycle or use the manufacturer's energy-consumption data. For a microwave, use the electrical input shown on the rating label, which may be higher than the advertised cooking power.
2. Size the PV array with site data, not a universal “5 sun hours” assumption
Solar yield changes with location, month, tilt, orientation, temperature and shading. An autonomous system should be checked against the most difficult period in which the required service must remain available.
The European Commission's official PVGIS tool can simulate hourly PV production, consumption and battery state using multi-year solar-radiation data. Enter the location, installed PV power, battery capacity, discharge limit and daily consumption in the Off-grid PV section.
Open PVGIS and select the OFF-GRID PV tab
For an initial manual estimate:
PV power in Wp = daily energy / (equivalent sun hours × overall system efficiency)
In this example we use 900 Wh/day and a conservative overall factor of 75%, covering temperature, wiring, controller, battery, soiling and mismatch losses. The actual factor must be checked for the real design.
| Equivalent sun hours in the design month | Calculated minimum PV power |
| 2.5 h | 480 Wp |
| 3 h | 400 Wp |
| 4 h | 300 Wp |
| 5 h | 240 Wp |
If the critical month provides about 3 equivalent sun hours, the starting point is therefore roughly 400 Wp. A good summer result does not guarantee winter operation: verify the final choice with PVGIS and consider whether a generator or another backup source is available.
3. Choose the system voltage: 12, 24 or 48 V
For the same power, higher voltage means lower current. This can reduce cable size and voltage drop, but the battery bank, controller, inverter and DC loads must all be designed for the selected voltage.
| 1200 W inverter load | Ideal DC current before losses |
| 12 V system | About 100 A |
| 24 V system | About 50 A |
| 48 V system | About 25 A |
In this example, with an inverter above 1 kW, a 24 V system is often more practical than 12 V. Do not change system voltage halfway through the design: series connections, BMS, charge controller, inverter and DC appliances must remain consistent.
4. Size the battery bank in Wh, then convert to Ah
Required nominal capacity depends on daily consumption, autonomy days, usable depth of discharge and battery efficiency:
Nominal battery capacity in Wh = daily consumption × autonomy days / (usable DoD × battery efficiency)
For 900 Wh/day and 3 days of autonomy, the following are two design examples. The DoD and efficiency values are conservative assumptions; always use the values stated by the battery manufacturer.
| Battery example | Assumed DoD | Assumed efficiency | Nominal capacity | At 12 V | At 24 V |
| Lead-acid AGM/GEL | 50% | 90% | About 6.0 kWh | About 500 Ah | About 250 Ah |
| LiFePO4 | 80% | 95% | About 3.55 kWh | About 296 Ah | About 148 Ah |
Capacity in Ah is not the only limit. Check minimum temperature, ageing allowance, continuous current, surge current, BMS rating, expected cycle life and the ability of the PV array to recharge the bank fully. A very large battery with too little PV remains chronically undercharged; a small battery is exposed to deep cycling and may not support inverter peaks.
5. How to choose the charge controller
For an MPPT controller, at least two independent limits must be checked:
- Maximum battery-side charge current: for a quick estimate, divide PV power by battery voltage.
- Maximum PV input voltage: the sum of module Voc values in series, corrected for the lowest expected temperature, must remain below the controller limit.
With 400 Wp, theoretical current is about 33 A on 12 V and 17 A on 24 V before real charging conditions. On a 24 V system, a 20-30 A controller may be suitable, but the final choice depends on charging voltage, model limits and the oversizing allowance permitted by the manufacturer. With PWM controllers, array short-circuit current and panel-to-battery voltage compatibility must also be checked.
6. How to size the inverter
The inverter must support both the simultaneous continuous load and the starting surge of compressors, pumps and motors. In the example, the microwave and television require about 950 W together. A 1200-1500 W pure sine wave inverter gives a sensible margin, provided its surge rating suits the actual appliances.
Include inverter efficiency and no-load consumption in the energy balance. A heavily oversized inverter can waste energy during long periods of light load. Pure sine wave output is the safer choice for modern electronics, motors, refrigerators and power supplies.
7. Cables, fuses and protection are part of the design
- A correctly rated fuse close to the battery positive terminal.
- DC-rated isolation and protection for the actual voltage and current.
- Cables sized for current, length, temperature and allowable voltage drop.
- Connectors and terminals suitable for the selected cable section.
- Surge protection and earthing where required by the design and local rules.
- Correct separation between DC circuits and the 230 V AC output.
Low-voltage systems can carry very high current. A 1200 W load on 12 V exceeds 100 A once losses are included. Undersized cable is not merely inefficient; it can overheat.
Worked example summary
| Item | Indicative design value |
| Daily consumption | About 900 Wh |
| PV array | About 400 Wp with 3 equivalent sun hours and a 75% overall factor |
| System voltage | 24 V is advisable to reduce inverter-side current |
| Lead-acid battery, 3 days | About 24 V 250 Ah at 50% DoD |
| LiFePO4 battery, 3 days | About 24 V 150 Ah at 80% DoD |
| MPPT controller | Indicatively 20-30 A, after checking Voc and MPPT range |
| Inverter | 1200-1500 W pure sine wave with adequate surge rating |
This summary is a calculation exercise, not a universal shopping list. Location, season, shading, hourly load profile and component specifications can materially change the result.
Common sizing mistakes
- Adding watts without multiplying by operating hours.
- Using the same “5 sun hours” for every location and season.
- Quoting battery capacity in Ah without stating voltage.
- Ignoring module Voc on cold days.
- Connecting a power inverter to the controller's LOAD output.
- Ignoring surge demand, standby consumption and conversion losses.
- Mixing batteries of different age, capacity, chemistry or state of charge.
- Neglecting fuses, isolators and cable voltage drop.
From calculation to a pre-matched system
Those who prefer to start from components already selected to work together can compare the MR WATT off-grid photovoltaic kits. Even with a kit, real consumption, local solar yield and required autonomy still need to be verified.
Frequently asked questions
How much PV power is needed for 1 kWh per day?
It depends on the design month and location. With 75% overall efficiency, roughly 270 Wp is required with 5 equivalent sun hours, or about 535 Wp with 2.5 hours. For year-round service, use the critical month in PVGIS rather than a summer average.
Should I use a 12 V or 24 V system?
For small DC loads, 12 V can be simple. Once inverter power approaches or exceeds 1 kW, 24 V significantly reduces current: 1200 W corresponds ideally to 100 A at 12 V and 50 A at 24 V before losses.
How many days of battery autonomy should I design for?
It depends on the required reliability, local climate and whether a backup generator is available. Insert the actual autonomy target in the formula, then use PVGIS to check how often the battery may become empty in the worst month.
How do I know whether an MPPT controller is large enough?
Check maximum charge current, permitted PV power at the selected battery voltage, MPPT operating range and the cold-corrected maximum string Voc. None of these limits may be exceeded.
Can an off-grid system run a microwave oven?
Yes, if the inverter, battery, BMS, cables and fuses can supply the continuous power and surge. Read the electrical input from the microwave rating label; it can be higher than the cooking power shown on the front.
Safety note: this guide is educational. A real installation should be checked and installed by competent personnel in accordance with component manuals and applicable electrical regulations.