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SolarCalcWorks

Solar Array Size Calculator

Work out how much array your daily energy use actually needs, using the NREL PVWatts loss model rather than a folk derate factor. For off-grid systems the calculation also passes through the battery, which most array calculators quietly skip.

  • NREL PVWatts loss model
  • Off-grid and grid-tied
  • Module count and annual output
  • Requires your real peak sun hours

Solar Array Size Calculator

1,776 W5 modules
Inputs
System type
Array size5 × 400 W

1,776W

Why this size

1,480 W covers the load exactly; the rest is your 20 % design margin. 5 modules gives 2,000 W installed, delivering about 6,757 Wh a day to the load — roughly 2,466 kWh a year at this insolation.

Array needed
1,776 W
1,480 W + margin
Modules
5
2,000 W
Must generate
6.66 kWh
per day at the modules
System derate
85.9 %
14.1 % losses
  • NoteFor an off-grid system, size on your worst month rather than the annual average. December insolation at mid-latitudes can be half the June figure, and an array sized on the average simply runs a generator through the winter.
  • NoteLoss defaults follow the NREL PVWatts model (14.1 % total). Shading is the largest single component and the most site-specific — measure yours rather than accepting 3 %.
Calculation

1Work backwards from the load

5,000 Wh ÷ 0.92 inverter ÷ 0.95 battery ÷ 0.8590 system
= 6,660 Wh at the modules

Every conversion between the panel and the appliance costs something, and each loss divides rather than subtracts. Off-grid, the energy also passes through the battery, which is why the round-trip term appears here and not on a grid-tied system.

2Divide by the sun you actually get

6,660 Wh ÷ 4.5 peak sun hours = 1,480 W
× 1.20 margin = 1,776 W

A peak sun hour is an hour at 1000 W/m², the irradiance modules are rated at. It is the standard way of collapsing a whole day's varying insolation into one number.

3Where the losses come from

soiling 2 % · shading 3 % · mismatch 2 % · wiring 2 % · connections 0.5 %
degradation 1.5 % · nameplate 1 % · availability 3 %
multiplied, not added → 85.90 % reaches the output

These are the NREL PVWatts defaults for a typical installation. Shading is both the largest component and the most site-specific — measure yours rather than accepting 3 %, because a single afternoon shadow across one string costs far more than that.

Loss defaults follow the NREL PVWatts model. Peak sun hours are site data, not a constant — this tool asks for yours rather than assuming one. Sources and limitations.

NextSize the charge controller for this arrayArray watts and bank voltage decide the controller, and the string layout decides whether it survives winter.

The calculation, worked backwards from the load

AC energy at the appliance
÷ inverter efficiency   → DC energy out of the battery
÷ round-trip efficiency  → DC energy into the battery (off-grid only)
÷ (1 − system losses)   → energy the modules must generate
÷ peak sun hours       → array watts

Each loss divides rather than subtracts, which is why they compound faster than people expect. An off-grid system passing 5 kWh a day through a 92 % inverter, a 95 % battery and a 14 % system derate needs its modules to generate 6.66 kWh — a third more than the load.

Where the 14 % comes from

The default is NREL’s PVWatts figure, which is the multiplicative product of ten named components:

Component Default
Soiling 2 %
Shading 3 %
Snow 0 %
Mismatch 2 %
Wiring 2 %
Connections 0.5 %
Light-induced degradation 1.5 %
Nameplate rating 1 %
Age 0 %
Availability 3 %

Multiplied together these give a derate of about 0.8592, or 14.08 % total loss. They are PVWatts defaults for a typical installation, not a standard, and every one is adjustable in the calculator.

Off-grid needs more array than grid-tied

For the same load, an off-grid array is larger, for a reason that is easy to miss: the energy passes through a battery, and a battery returns only 80 to 95 % of what it accepts.

Grid-tied systems have no such term. Their energy goes straight to the load or to the meter.

The calculator applies the round-trip term only when you select off-grid, and shows both the base figure and the version with your design margin, so you can see what the margin is buying.

Design margin

The default 20 % is design practice, not a rule. It covers three things nobody plans for:

  • Ageing. Modules lose roughly 0.5 % of output per year, so a 25-year array ends around 88 % of nameplate.
  • Reality. Soiling between cleanings, a panel that ends up slightly shaded, a summer that is not the meteorological average.
  • Load creep. Off-grid loads always grow. The freezer, the second laptop, the espresso machine.

Set it to zero if you want the bare figure, but understand what you are removing.

What this calculator does not do

It does not know your peak sun hours. That is a property of your location, orientation and season, and there is no honest default. Get the figure from NREL PVWatts using your actual coordinates and tilt.

It does not model tilt or azimuth. Those effects belong inside the peak sun hour figure. Get a PVWatts result for your real orientation rather than sizing on a south-facing figure and adjusting.

It does not size the controller, bank or inverter. Those follow from the array and the load, and each has its own tool here.

What to calculate next

NextSize the charge controllerArray watts and battery voltage set the controller.

Frequently asked questions

How many solar panels do I need to run a house?

Work from consumption, not from house size. A US home averaging 30 kWh a day, grid-tied, at 4.5 peak sun hours and PVWatts default losses, needs roughly 7.8 kW of array — about 20 modules at 400 W. The same house off-grid needs meaningfully more, because the energy also passes through an inverter and a battery. Your own electricity bill has the consumption figure on it, and it is far more reliable than any square-footage rule.

What are peak sun hours and where do I find mine?

One peak sun hour is one hour of irradiance at 1000 W/m², the condition modules are rated at. A site with 4.5 peak sun hours receives 4.5 kWh/m² across a day considerably longer than 4.5 hours. It varies by location, season and panel orientation. Look yours up with the NREL PVWatts calculator, which uses measured meteorological data for your coordinates. There is no useful national average, which is why this tool requires the input rather than assuming one.

Why do I lose 14% of my array output?

That figure is the NREL PVWatts default, and it is the multiplicative product of soiling 2 %, shading 3 %, mismatch 2 %, wiring 2 %, connections 0.5 %, light-induced degradation 1.5 %, nameplate tolerance 1 % and availability 3 %. They are combined multiplicatively rather than added. Shading is both the largest component and the most site-specific — a single afternoon shadow across one string costs far more than the 3 % default.

Should I size my array for summer or winter?

For off-grid, size for your worst month. December insolation at mid-latitudes can be half the June figure, and an array sized on the annual average simply runs a generator through the winter. For grid-tied with net metering, the annual average is the right basis, because summer surplus offsets winter shortfall through the meter.

Does panel tilt and direction matter?

Substantially, and this calculator does not model it — peak sun hours is the input where that effect belongs. A south-facing array at latitude tilt is the reference case in the northern hemisphere. East or west orientation typically costs 10 to 20 % annually; a steeper winter tilt trades summer output for winter output, which is usually the right trade off-grid. Get a peak sun hour figure for your actual orientation from PVWatts rather than adjusting afterwards.

Last reviewed 2026-08-19. Calculations reference NFPA 70 (NEC) 2023 where a code section applies. Sources, and what these tools deliberately do not model.