How to Size a Solar PV System from Your Electrical Load

Many solar projects start from a round number: "I need a 5 kW system." The trouble is that a kilowatt figure on its own says little about whether the array, the battery, and the inverter will actually carry your loads through a hot afternoon or a dark evening. A more reliable path starts from the load itself. This guide walks through that path step by step: build a load list, turn it into daily energy, separate typical from peak demand, and then size the PV array, the optional battery, and the inverter or MPPT controller.

The formulas below are general engineering relationships. The worked example uses simple round inputs and is illustrative only; your site, climate, and equipment will give different figures. For a scheme sized against your country's weather, you can run the free online PV design with your own load.

Step 1: Choose the architecture first

Before any arithmetic, decide which kind of system you are designing, because it changes what has to be sized. load2pv designs two exclusive architectures and never mixes them in one scheme:

A home rooftop scheme needs an inverter sized for AC demand. A site DC scheme needs MPPT controllers sized for the DC load, and no inverter at all. Mixing the two in one design usually creates sizing errors that are hard to see on paper.

Step 2: Build a load list

List every load that the system must supply, with its power and the hours it runs on a normal day. Use measured values or nameplate ratings, and note which loads can start at the same time. Motors, pumps, and compressors deserve a separate note, because their starting current can be several times their running current.

A useful load list has at least these columns:

• Load name and quantity
• Power in kW (running, not starting)
• Hours per day of operation
• When it runs: daytime, night, or both
• Starting behaviour: resistive, electronic, or motor

Step 3: Turn the list into daily energy

Daily energy is the sum of power multiplied by hours for every load:

Daily energy (kWh/day) = Σ (power in kW × hours per day)

Here is an illustrative load list with round numbers:

Daily energy is what the array has to harvest, on average, to keep up with consumption. It also tells you how much energy a battery would have to hold for a given number of backup hours.

Step 4: Separate typical load from peak load

Two power figures matter, and they must not be swapped:

• Typical load is the average demand while the system runs; the load2pv form defines it as the daytime average. It drives daily energy and battery hours.
• Peak load is the highest demand the inverter or MPPT must cover at one moment, including loads that start together. Peak load must always be at least the typical load.

In the illustrative list, the peak if every load ran at once would be 3.75 kW, before any motor starting allowance. The typical daytime load might be closer to 2 kW, depending on how the loads overlap. These are the two figures the load2pv online tool asks for, together with your country and the system type.

Step 5: Size the PV array

A common first estimate of array size is:

Array size (kWp) ≈ daily energy (kWh/day) ÷ (peak sun hours × performance ratio)

Peak sun hours describe the local solar resource. The performance ratio bundles real-world losses such as high cell temperature, soiling, cable drop, and conversion losses. Both depend on the site, so treat any single value as an assumption to be checked.

With illustrative inputs of 24 kWh/day, 5 peak sun hours, and a performance ratio of 0.75, the first estimate is 24 ÷ (5 × 0.75) = 6.4 kWp. The number of modules then follows from the module rating: array kWp divided by the module's rated power.

A climate-aware design goes further than this ratio. City weather sets irradiance and cell temperature, and the strings must stay inside the MPPT voltage window: cold-start open-circuit voltage (Voc) below the equipment maximum, and hot-noon maximum-power voltage (Vmp) above the MPPT minimum after soiling and cable drop. In hot, dusty MENA sites, that window often decides the string length more than the kWp figure does. The About page explains how load2pv checks it.

Step 6: Size the battery, if you need one

Storage is optional. If you only need daytime supply, or the grid covers the night, you may not need a battery at all. If you do need backup, a standard relationship is:

Battery capacity (kWh) = typical load (kW) × backup hours ÷ (depth of discharge × efficiency)

With illustrative inputs of a 2 kW typical load, 4 backup hours, 0.8 depth of discharge, and 0.95 efficiency, the result is 2 × 4 ÷ (0.8 × 0.95) ≈ 10.5 kWh. The battery's discharge power must also be at least the peak load, otherwise it cannot carry the loads that start together, even if it holds enough energy.

If storage is not selected, no battery should be added quietly. PV and the MPPT or inverter still follow the peak load.

Step 7: Size the inverter or MPPT

The power stage is sized from peak load plus a margin, not from typical load:

In the illustrative example, a peak of 3.75 kW plus a margin points to an inverter class above that figure. Which margin is right depends on the loads that start together and on the equipment's surge rating, so it is worth checking against the datasheet.

Step 8: Check the result against reality

Before buying equipment, compare the sized system with what you know about the site:

1. Does daily energy match past bills, generator fuel use, or metered data?
2. Is the peak load realistic, including motors that start together?
3. Is there enough roof or ground area for the array, after shading and access paths?
4. Do the strings stay inside the MPPT window on the hottest and coldest days?
5. If there is a battery, do the backup hours match what the site actually needs?

A first pass like this is useful for sizing and budgeting. It is not a stamped construction-permit set, and the final design should be reviewed by a qualified engineer for the site.

Next step

If you already have a load list, you can turn it into a scheme in a few minutes. Start the free online design with your country, system type, and typical and peak load, then use your Design ID to request official files if the preview looks right.

Worked examples use illustrative round numbers. Online output is an indicative scheme, not a construction-permit set.