What this calculator does
The Solar Load Calculator turns the question "how big does my solar system have to be?" into three concrete answers: how many solar panels, which inverter, and how much lithium battery. You start from the appliances you actually plan to run — lights, fridge, fans, TV, washing machine, a pump — and the calculator adds up their daily energy, converts that into an array size using the peak sun hours of your location, sizes the battery bank for the number of days of autonomy you want, and picks an inverter sized on your simultaneous peak load rather than on daily energy.
Everything runs in your browser and nothing you type is sent anywhere. The tool is available in English, Arabic, and Spanish.
How to use the Solar Load Calculator
- List each appliance with its rated watts, the hours per day it runs, and the quantity. For fridges, pumps and compressors, enter the average running hours per day (a fridge compressor typically runs the equivalent of 8–10 hours a day), not 24.
- Enter the peak sun hours of your location — the equivalent number of hours per day of full, unhindered sun. Use about 5 for sunny regions, 3.5–4.5 for temperate climates, or look your city up in a solar atlas.
- Enter the panel wattage. Modern residential panels are 400–550 W; small RV or hut setups often use 100–200 W modules.
- Check the overall system efficiency (default 80%). It lumps together panel temperature losses, dust, wiring, charge-controller and inverter losses.
- Choose the days of autonomy — how many days the battery must run the loads without sun. 1 day suits most daily-cycling systems; 2–3 days for backup during cloudy spells.
- Set the depth of discharge for the lithium battery. 90% is a common design value for LiFePO₄; 80% is more conservative and extends life.
- Choose the system voltage (12 V for small setups, 24 V for mid-size, 48 V for whole homes) and adjust the inverter safety factor if you like (1.25 already includes a healthy margin).
- Click "Calculate" to see the daily load, the recommended solar array in kWp and panel count, the lithium battery in kWh and ampere-hours, and the inverter size in kW and kVA.
Formulas
The calculator uses the standard off-grid sizing chain found in solar design guides:
The daily load is straightforward energy accounting. The array formula divides that energy by the local sun hours and the system efficiency, so a 1 kWp array at 5 sun hours and 80% efficiency is credited with 4 kWh per day. The battery formula grows the daily energy by the autonomy days and divides by usable depth of discharge, because you never plan to use the last percent of a lithium cell. The inverter is sized from power, not energy: it must carry everything that may run at the same time, plus a safety margin for surges.
Examples
- The default loads above — lights, TV, fridge, washing machine and two fans — come to 3.36 kWh/day with a 930 W simultaneous peak. With 5 sun hours, 80% efficiency and 550 W panels: a 0.84 kWp minimum array → 2 panels (1.10 kWp installed), about 4.4 kWh on a sunny day. Battery: 3.73 kWh → about 78 Ah at 48 V. Inverter: 1.16 kW (1.45 kVA) → pick a 1.5 kW or larger unit.
- A small home averaging 8 kWh/day with a 2.5 kW peak, 5.5 sun hours, 80% efficiency, 550 W panels, 1 day of autonomy, 80% DoD, at 48 V: array 1.82 kWp → 4 panels (2.2 kWp) producing ≈ 9.7 kWh/day; battery 10 kWh ≈ 208 Ah; inverter 3.13 kW (3.9 kVA) → a 4 kW unit.
- An RV on 12 V using 1.2 kWh/day, peak 600 W, 4.5 sun hours, 75% efficiency, 120 W panels, 90% DoD: array 0.36 kWp → 3 panels; battery 1.33 kWh ≈ 111 Ah; inverter 0.75 kW → a 1 kVA unit.
Getting the inputs right
Peak sun hours
Peak sun hours (kWh/m² per day) are not the hours of daylight — they are the equivalent hours of full-power sun. A location with 5 peak sun hours produces as much energy as if the sun sat at noon intensity for 5 hours. Sun hours vary seasonally with tilt and orientation; use the low-season value if the system must carry the load year-round.
System efficiency
Real systems lose 15–25% between panel and appliance: hot panels lose 10–15% of their-rated output, dust and shading another few percent, and each conversion step (MPPT, wiring, inverter) takes its share. The 80% default is a fair mid-point; use 85–90% only for cool, clean, shortened-wiring systems.
Lithium batteries
LiFePO₄ cells tolerate 80–90% daily depth of discharge and typically deliver 3,000–6,000 cycles — far more than lead-acid, which should not be cycled below 50%. Keep a reserve below the entered DoD: the battery management system disconnects before the cell is truly empty, and cold weather temporarily reduces the capacity available (budget roughly 10–20% less below 0 °C).
Inverter surge
Motors in fridges, pumps, washing machines and air conditioners draw 2–3 times their rated power for a second or two at start-up. Size the inverter for continuous output at the calculated figure and make sure its surge rating covers the largest motor load; inverters that cannot hold the smoke test fail on start-up surges, not steady running.
Frequently asked questions
What exactly are peak sun hours?
One peak sun hour equals 1 kWh/m² of solar irradiation. Five peak sun hours means a panel produces five times its rated power-hours of energy per day under ideal tracking conditions — in practice, multiply your array kWp by the sun hours and by efficiency to get daily kWh.
Why 80% system losses — is that not pessimistic?
It is a realistic average, not pessimism. Between the panel datasheet and your appliances sit temperature derating, soiling, wiring resistance, MPPT conversion and the inverter itself (93–96%). Most installed systems measure 70–85% end-to-end; 80% is used by countless professional sizing tools.
Can I size the battery on one day and add more later?
Yes — lithium banks scale modularly, and that is one of their advantages. Start with the autonomy days you truly need most of the year; adding batteries later mainly requires matching voltage and chemistry and a charge controller or inverter rated for the extra charge current.
Why does the inverter not account for daily energy?
The inverter's job is instantaneous power, not energy. A 3 kWh/day load that all happens at 18:00 needs a much larger inverter than the same load spread evenly. That is why the calculator sizes the inverter on the simultaneous peak of your appliance list, while the array and battery handle the energy side.
12, 24 or 48 volts?
As a rule of thumb: 12 V up to about 1 kW, 24 V to about 3 kW, and 48 V for everything bigger. Higher voltage means lower current for the same power, which cuts cable cost and heat loss and keeps the inverter efficient.
Does the result include a charge controller?
No — the three sizes shown are the array, the inverter and the battery bank. A charge controller (or a hybrid inverter with one built in) must still be selected for your array's short-circuit current and voltage; an MPPT rated for slightly more array watts is usually the right pick.
Is my data saved or sent anywhere?
No. Every calculation runs in your browser, and nothing you type is stored or sent to a server.
Related calculators
Other free calculators commonly used with this tool: Cooling Load Calculator, Heat Pump Calculator, and Water Pressure Pump Calculator, and Wet Bulb Calculator.
Want a deeper walkthrough? Read our solar sizing guide.