Solar Panel and Battery Calculation in 5 Steps

Sizing solar panels and a battery is five sums, in a fixed order. Each one answers a single question, and each can be done on paper. This guide writes them out, runs three real loads through them, and names the Voltronic unit whose record meets each result. The sizing calculator on this site performs the same five sums, so you can check your arithmetic against it.

The figures every calculation starts from

A calculation needs three numbers per appliance: its running power, how many times that power it draws when it starts, and how many hours a day it runs. These are the values the calculator uses:

The five sums

1. Energy per day (kWh). Running power x hours, added up over every appliance and divided by 1,000.
2. Peak load (W). The larger of two figures: all running powers added up and multiplied by 1.25, or the start-up draw of the single largest motor plus everything else running.
3. Inverter. Its rated power must be at least the peak load. A unit more than 2.5 times the peak is oversized.
4. Battery (kWh). Energy per day x backup hours / 24. With no grid at all, a full day of energy.
5. Solar array (kWp). Energy per day / (peak sun hours x 0.8). Off-grid, divide by 0.75 instead of 0.8. The array in kWp divided by the module rating gives the number of panels.

The system pages on this site use 5.7 peak sun hours, the sizing engine's figure for the UAE; its figure for Saudi Arabia is 5.8 and for France 3.7.

Example 1: one air conditioner, a fridge and the lights, 4 hours of backup

• Energy: 1,200 x 8 + 150 x 8 + 200 x 8 = 12,400 Wh, so 12.4 kWh a day.
• Peak: the running sum is 1,550 W, and 1,550 x 1.25 = 1,938 W. The air conditioner starting draws 3,600 W, plus 350 W for the rest: 3,950 W. The larger figure wins: 3,950 W.
• Inverter: the Axpert King II TWIN 6K is rated 6000VA/6000W, above 3,950 W and under the 2.5 times limit of 9,875 W. Its record states a surge of twice the rated power for 5 seconds.
• Battery: 12.4 x 4 / 24 = 2.07 kWh. One LIO 4805 stores 2,400 Wh, counted as 2.16 kWh usable, which covers it. One LIO 4810 of 4,800 Wh doubles the window.
• Array: 12.4 / (5.7 x 0.8) = 2.72 kWp, which is 5 modules of 550 W.

Example 2: a farm with two pumps, no grid

Two water pumps of 750 W for 1.5 hours, a fridge, a freezer, and lighting.

• Energy: 2.25 + 1.2 + 1.6 + 1.6 = 6.65 kWh a day.
• Peak: one pump starting draws 750 x 3.5 = 2,625 W, and the other loads add 1,300 W: 3,925 W.
• Inverter: the InfiniSolar V II TWIN 6kw record states a rated output of 6000W, a maximum PV input of 7000W and an MPP range of 120 to 430 VDC.
• Battery: with no grid, a full day, 6.65 kWh. The LIO 4810 counts for 4.32 kWh usable: 6.65 / 4.32 = 1.5, so 2 modules.
• Array: 6.65 / (5.7 x 0.75) = 1.56 kWp, which is 3 modules of 550 W, or 1,650 W, well under the 7000W the inverter accepts.

This is the off-grid farm kit line for line.

Example 3: a shop with two air conditioners, no grid

Two air conditioners, a fridge, two sets of lighting and a computer, all 8 hours a day.

• Energy: 19.2 + 1.2 + 3.2 + 1.6 = 25.2 kWh a day.
• Peak: 3,600 W for the air conditioner that starts, 1,950 W for the rest: 5,550 W.
• Inverter: the Axpert King II TWIN 6K, 6000VA/6000W.
• Battery: 25.2 / 4.32 = 5.8, so 6 LIO 4810 modules. The LIO 4810 record states 150 A of maximum continuous discharge per module at 48 VDC.
• Array: 25.2 / (5.7 x 0.75) = 5.89 kWp, which is 11 modules of 550 W.

The off-grid shop kit shows the same result.

Three checks before you order

• The battery voltage sits inside the inverter's window. The Axpert King II TWIN 6K record states a nominal DC input of 48Vdc and overcharge protection at 66Vdc; the LIO modules are 48 VDC nominal with a full charge voltage of 52.5VDC.
• The array stays under the inverter's PV limits. Read the maximum PV power, the MPPT range and the open-circuit voltage on the Axpert King II TWIN datasheet or the InfiniSolar V II TWIN datasheet before fixing the number of panels per string.
• The charge current suits the battery. The LIO 4805 record states a maximum charge current of 50A, the LIO 4810 100A; both are on the LIO datasheet.

If two inverters both pass, the comparison pages set their records side by side. Parts are ordered from their product pages at the price shown, and orders are collected in store in Dubai or Riyadh.