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Part VII: Solar ElectricityChapter 11. Grid-Tied and Off-Grid PV

Sizing a Home Solar PV System from Your Electric Bill

Turn a year of kilowatt-hours into an array size, with room for the site and the roof you actually have.

Difficulty
Beginner
Cost
Planning only
published

A photovoltaic array should start from the kilowatt-hours you already use, not from the number of modules that fit on the roof. The bill says what you use. The roof and the shade say what you can collect. This guide turns a year of bills into a direct-current array size and then asks whether the site can hold it.

It stops before wiring. Landing conductors in a panel, choosing an inverter topology, and signing an interconnection agreement are later work, most of it electrical and most of it permitted. The number you leave with is a size worth taking to that work, or a finding that the roof is the wrong size and the better project is to use less electricity.

Difficulty

Level
Beginner
Time
An evening with twelve bills and a tape
Skills
Reading a meter history and measuring a roof from the ground or the attic

Cost estimate

Planning only

Modules are only one line in a system. Racking, the inverter or microinverters, wiring, and a permitted interconnection dominate the cost. A full grid-tied system is a several-thousand-dollar project at the small end and a large construction project at the big end. Do not budget from a module price per watt alone.

Materials

ItemQtyNotes
Twelve months of electric bills1 yearKilowatt-hours, not dollars. A single July bill will oversize or undersize the array depending on your climate.
Site survey1Which roof planes face near south, and what shades them in winter and summer.
Roof measurementsThe candidate planeLength, width, pitch, and the obstacles: vents, skylights, chimneys.

The arithmetic

On a clear day the sun delivers about 1000 watts per square meter when it is high and the sky is clean. A module's nameplate watts are measured near that condition. Real days are shorter, cloudier, hotter, and dustier, and the inverter and the wiring take a share.

A practical sketch folds all of that into peak sun hours and a derate. Peak sun hours are the number of equivalent full-sun hours per day for your tilt and your town. They are not the hours between sunrise and sunset. Many US sites land somewhere between about 3 and 6 depending on climate and tilt. Look up a current solar resource number for your location. The calculator starts at 4.5 only so the box is not empty.

Annual kilowatt-hours from the array are about the array's DC kilowatts, times peak sun hours per day, times 365, times a derate. A derate around 0.75 to 0.85 is a fair planning range for a grid-tied system that is not badly shaded: temperature, inverter, soiling, and wiring. Shade is not a derate you should bury in that number. Shade is a reason to shrink or move the array.

Rearranged, DC kilowatts equal the yearly kilowatt-hours you want to offset, divided by peak sun hours, divided by 365, divided by the derate.

East and west roofs produce less than a south roof at the same tilt. A common rule of thumb is on the order of 15 to 20 percent less over the year, and the real figure depends on pitch and latitude. If the only open roof faces west, raise the array size or lower your offset goal, and say which one you did.

From a bill to a roof layoutA year's electricity use points to an array size, then to modules on a roof that still leave a clear walkway at the ridge and the edge.Year of kWhfrom the billArray kWRidge walkway
A year of kilowatt-hours becomes an array size, then a set of modules that still leave a walkway. The walkway is part of the layout, not leftover space.

Array size from the bill

Yearly kilowatt-hours, divided by peak sun hours per day, divided by 365, divided by a derate. Peak sun hours are the equivalent full-sun hours for your tilt and climate, not the length of the day. Look them up for your site. The 4.5 hour starting value is only a placeholder.

6.85 kW dc

About 9,000 kWh a year after the derate, from roughly 18 modules.

Module glass is on the order of 360 square feet, counting each module as 20 square feet. Walkways, ridge setbacks, and shade will make the roof area you need larger than that.

This is a planning sketch. It is not a production guarantee, a structural check, or a permit drawing.

Read the bill before you touch the calculator

Add the kilowatt-hours for twelve consecutive months. If the utility shows net use and also shows solar buyback, you want the household's consumption, not a bill that is already partly offset by an old array.

Look at the shape. Electric heat or a well pump can make winter enormous. Air conditioning can make summer enormous. An array sized on an annual total will overproduce in the mild months and underproduce in the extreme ones. Net metering policy decides whether those mild months can be banked. If your utility credits exports at a low rate, a smaller array that covers the daytime base load can be the better match. That is an economics question. The calculator's 100 percent offset is a math setting, not a recommendation.

Subtract the loads you are about to eliminate. If you are about to replace electric resistance heat with a heat pump, or fix a well that short-cycles, size the array for the house you are going to have. The heat-loss guide and the air-sealing weekend belong ahead of a large array when the bill is mostly heat.

Fit it on a roof that can carry it and that firefighters can walk

A typical residential module is on the order of 20 square feet and around 400 watts. The calculator multiplies module count by 20 square feet as glass area. The roof area you need is larger. You will leave gaps, you will miss the vents, and you should leave clear paths.

Many fire codes require unobstructed pathways and ridge setbacks so firefighters can vent a roof. The exact width is a local rule. Plan on pathways before you plan on covering every square foot the calculator asked for. If the roof cannot hold the full offset after setbacks and shade, a smaller system is a normal outcome.

Shade from a chimney, a vent stack, or a tree branch across a string of modules can cut far more than the shaded fraction, depending on how the modules are wired. Treat any soft shade on the candidate rectangle as a problem to solve in the layout, not as a rounding error. The site survey's winter and summer notes both matter, because the sun is in a different part of the sky.

Structure is the other limit. Modules and racking add weight and they add uplift. A roof near the end of its shingles should be renewed when the array goes on, not five years later under the rails. If you cannot tell whether the rafters are sound, stop and have someone qualified look at them. This page cannot see your attic.

What this number does not authorize

The kilowatts are not a wiring diagram. A grid-tied system needs listed equipment, a rapid-shutdown method where the electrical code requires one, grounding, a disconnect the utility accepts, and an interconnection application before you operate it. In many places the homeowner may not legally make the final connection at the service, even on their own house. Hire an electrician for that work. Pull the permit the building department and the utility both expect.

Off-grid sizing is a different problem. It starts from the worst string of cloudy days and from a battery, not from an annual average and a grid that absorbs the extra. Do not use this calculator for a cabin that has to ride through a week of storms.

  1. Add twelve months of kilowatt-hours

    Use consumption, not the dollar total and not a single season. Note whether electric heat or cooling dominates.

  2. Look up peak sun hours

    Find a current figure for your town and for the tilt you can actually build. Replace the placeholder in the calculator.

  3. Choose an offset you can use

    One hundred percent is a starting knob. Lower it if exports are poorly credited or if the roof is small.

  4. Lay the modules on the real rectangle

    Subtract shade, vents, and the fire pathways your local code requires. If the modules do not fit, reduce the offset and say so.

  5. Stop at the permit package

    Take the kilowatts, the roof sketch, and the bills to an electrician and to the utility's interconnection rules. Do not buy modules as a substitute for that step.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.