How This Calculator Works
Sizing an off-grid system is a chain of steps: measure how much energy you use each day, decide how many days the battery must cover without sun, account for battery chemistry and conversion losses, and then size the solar array, charge controller, and inverter to match. The calculator does each step with the standard formulas below, so you can check every number by hand.
Using the calculator in four steps
- Start from a preset (camper van, cabin, or home) that resembles your setup, then edit it. Presets also set a typical system voltage.
- List every electrical load with its real power draw and how many hours per day it runs. Mark devices that run through the inverter as AC, and those wired directly to the battery as DC. For cycling appliances like fridges, use measured average hours, not 24. If your inverter stays on all day, add a line for its idle draw (typically 10–40 W).
- Set your climate with the peak sun hours slider, using the value for the worst month you plan to use the system. Our peak sun hours guide explains where to find it.
- Choose battery chemistry and autonomy days, then read the results. Use "Print / Export Spec" to save a copy for your installer.
1. Inverter Conversion Losses
Direct current (DC) loads run straight from your 12, 24, or 48 V battery. Alternating current (AC) appliances need a pure sine wave inverter, which typically loses 5–15% of the energy as heat. The calculator divides AC energy by 90% efficiency to account for this. Inverter sizing guide →
2. Battery Depth of Discharge (DoD)
Regularly discharging AGM or other lead-acid batteries below about 50% state of charge sharply shortens their life. LiFePO4 batteries can routinely use 80–90% of their capacity and typically last thousands of cycles. The calculator uses 85% for LiFePO4 and 50% for AGM. LiFePO4 vs AGM →
3. Peak Sun Hours (PSH)
Peak sun hours are the day's solar energy expressed as hours of full 1,000 W/m² sunlight, not hours of daylight. A long sunny summer day typically gives 4–6 PSH depending on latitude, haze, and panel angle, while a winter day at high latitude may give 1–2. Peak sun hours guide →
Formulas Used
(AC Wh ÷ 0.90 inverter efficiency) + DC Wh
E_total × Autonomy days ÷ Depth of discharge
Battery storage (Wh) ÷ System voltage
E_total ÷ (Peak sun hours × 0.78 derate factor)
Solar array (W) ÷ System voltage × 1.25
Sum of AC loads × 1.35, rounded up to 500 W
The 0.78 derate factor combines typical losses from panel heating, dirt, cable resistance, charge controller conversion, and battery charging. The inverter estimate conservatively assumes every AC load could run at once; if some never overlap, a smaller inverter may be enough.
Worked example: the default setup
The calculator opens with a small remote-work setup: a 45 W DC compressor fridge running 10 hours, 25 W of LED lighting for 5 hours, a 75 W laptop for 5 hours (AC), a 30 W satellite internet terminal for 8 hours, and an 18-minute daily 1,000 W cooking session (AC), on a 12 V LiFePO4 system with 4.5 peak sun hours and 1.5 days of autonomy.
| DC loads | 450 + 125 + 240 = 815 Wh |
| AC loads incl. inverter loss | (375 + 300) ÷ 0.90 = 750 Wh |
| Daily energy | 1,565 Wh/day |
| Battery storage | 1,565 × 1.5 ÷ 0.85 = 2,762 Wh → 230 Ah at 12 V |
| Solar array | 1,565 ÷ (4.5 × 0.78) = 446 W (two 400 W panels) |
| MPPT controller | 446 ÷ 12 × 1.25 = 46 A → 50 A |
| Inverter | (75 + 1,000) × 1.35 = 1,451 W → 1,500 W |
In practice, a 12 V 200 Ah LiFePO4 battery would give about 1.2 days of autonomy, and a 300 Ah battery about 2 days. Rounding to available products is where your judgment comes in. See the battery bank sizing guide for temperature and ageing margins.
Frequently asked questions
Should I choose a 12 V, 24 V, or 48 V system?
As a rule of thumb, 12 V suits vans and small systems under about 1–1.5 kWh per day and inverters up to about 1.5–2 kW. 24 V suits cabins and medium systems, and 48 V suits full-time homes. Higher voltage means lower current, which allows thinner cables, smaller fuses, and cheaper charge controllers for the same power. Our wire sizing guide shows the difference.
Why is the recommended battery so much bigger with AGM?
Lead-acid batteries should only be discharged to about 50%, while LiFePO4 can safely use about 85%. To get the same usable energy, an AGM bank needs roughly 70% more rated capacity, and it will weigh several times more.
Which peak sun hours value should I use?
Use the value for the worst month in which you need the system to be self-sufficient, for your panel tilt. Free tools such as NREL PVWatts or the EU's PVGIS give monthly values for any location. Using an annual average will leave you short of power in winter.
Can I connect more solar panels than the calculator recommends?
Yes, and it is often a good idea: extra array capacity helps in winter and recharges the battery faster after cloudy days. Make sure the charge controller is rated for the extra current, or that the manufacturer allows over-paneling, and that the string's cold-weather open-circuit voltage stays below the controller's limit. See the MPPT sizing guide.
Is my data saved or sent anywhere?
No. All calculations run in your browser and nothing you type is sent to our server. See our Privacy Policy for details, including how advertising cookies work on this site.
Can I build my system directly from these results?
Treat the results as a well-founded starting point for planning and budgeting. Before buying, check each component's datasheet, and have the final design, cabling, and fusing reviewed by a licensed electrician in line with your local electrical code. See our engineering disclaimer.