SolarSizer

SolarSizer Methodology & Editorial Standards

Last updated: October 6, 2026

The calculators on this site are planning tools. Each figure below is either a formula in assets/app.js or a number already printed on a linked page.

How the calculator works

sizeSystem sets array watts to daily watt-hours ÷ sun hours ÷ 0.86, then rounds up to the next 100 W. 0.86 is 1 − 0.14, the 14% default system losses in Table 6 of the NREL PVWatts Version 5 manual. Bank watt-hours are daily watt-hours × days of autonomy ÷ the depth of discharge selected on the form. The form opens at 4 sun hours, 2 days, 24 V, and 80% (LiFePO4), with a 50% (AGM / lead-acid) option. The same function also prints an array for winter sun of 2.5 hours, using that same ÷ 0.86 and the same 100 W round-up.

sizeController sets amps = array watts ÷ bank volts ÷ 0.85, prints that current to one decimal, then picks the next size in 10, 20, 30, 40, 60, 80, and 100 A. The comment in that function calls 0.85 a charge-inefficiency margin. On how to size a solar charge controller, that 0.85 is not Canadian Solar's × 1.25 on module short-circuit current, and the page does not relabel the result as a code current.

sizeInverter multiplies continuous watts by 1.25 and uses surge factors of 1 (resistive), 3 (motor), 4 (compressor), and 2 (mixed). In the same function the surge recommendation is that surge need × 1.1, rounded up to the next 100 W, and DC amps are continuous watts ÷ 0.92 ÷ bank volts. The result line calls the 0.92 term about 8% inverter loss.

panelsForBattery uses amp-hours × volts ÷ sun hours ÷ 0.85. That form opens at 4.5 sun hours.

These are planning tools, not professional advice. Calculator outputs are planning estimates from the formulas above; they are not measurements.

Where the numbers come from

Guide pages link the publication next to the figure. The publications already cited on this site include NREL PVWatts, Victron Energy datasheets, Morningstar datasheets, Trojan Battery, Battle Born Batteries, the U.S. Department of Energy, Canadian Solar, and Jordan and Kurtz.

The MPPT and PWM charge controller spec table copies cells from the linked datasheets. Every row links its source document. The figures were copied on October 6, 2026. Nothing on that page is estimated. A missing value on a spec table is written as "not listed". The compilation is published under CC BY 4.0. The datasheets linked in the table stay the manufacturers' documents.

How pages are written

Compiled with AI assistance from the sources below. Spot an error? Email [email protected].

Page text is drafted with AI agents (Cursor and local models) from the cited sources.

Built and owned by Jeremy Panasuk, New Auburn, WI.

Affiliate policy

As an Amazon Associate I earn from qualifying purchases. The Amazon links below are affiliate links; they don't change the math.

Amazon Associates links on this site use the tag generatorsi0d-20. The disclosure sits before the first affiliate link. The links never change calculator output. There are no paid placements.

Off-grid solar kits on Amazon

Corrections

Spot an error? Email [email protected] with the page URL.

Safety scope

Jeremy is a data/software person, not a licensed electrician or solar installer. A licensed electrician sizes the conductors and the overcurrent devices. Trojan's warning on terminals, quoted on the 12 V open-circuit page: do not overtighten terminals.

Sources