How to size a home solar system

Start with energy, not panels

The most common question we hear is "how many panels do I need?" The better question is "how much energy do I want to produce in a year?" Once that is answered, the number of modules follows.

Alberta's micro-generation rules are built around the idea that your system offsets your own consumption. So the starting point is your actual electricity use over a full year, which you will find on your bills in kilowatt-hours (kWh). Twelve months matters because Edmonton use swings a lot between seasons.

If you have lived in the home for less than a year, or your household has changed, the designer can estimate use from the size of the house, how it is heated and what appliances you run. Your utility account online may also show monthly history going back further than your paper bills.

Account for what is coming

If you are about to buy an electric vehicle, switch to a heat pump or finish a basement suite, your past bills understate your future use. A good designer asks about these plans and includes a reasonable estimate.

Be realistic. An EV's yearly energy use depends on how far you drive and how much charging happens at home. A heat pump's depends on your house and how it is used alongside your furnace. Your installer can explain how expected new loads are documented in the micro-generation application, since the rules expect sizing to be justified.

If the EV is part of the plan, read solar plus EV charging as well.

Site yield: what your roof can actually produce

The same system produces different amounts on different roofs. The designer models your site to estimate its yield: how much energy each unit of installed capacity will produce in a typical year. That model accounts for:

• Direction and tilt of each roof face.
• Shading from trees, chimneys, dormers and neighbouring buildings, through the seasons.
• Snow cover in winter, which is real in Edmonton but matters less than people think because winter output is small anyway.
• Losses in wiring and the inverter, and gradual module aging.

Divide your target yearly energy by the site yield and you get a first-pass system size. This is where good design work pays off. A rough rule of thumb from somewhere else can be well off for a shaded Edmonton lot.

Check it against the real limits

The first-pass size then meets three practical limits.

1. Roof space. Modules have to fit on good roof faces with the setbacks and pathways the current rules require, and around vents, chimneys and skylights.

2. Your electrical panel. The code limits how much solar can be connected to a panel based on its ratings. A small or older service may cap the system size unless it is upgraded or connected differently. See service upgrades.

3. Micro-generation rules and budget. The system should be sized to meet your use, not to exceed it. And of course, the budget shapes how much of your use you choose to cover. Many people start with a system that covers part of their yearly use and plan room to expand.

Inverters and equipment choices

Size is not only about module count. The inverter has to suit the array. A single string inverter is simple and works well on an unshaded roof facing one direction. Microinverters or optimizers handle roofs with several faces or partial shade better, because each module works on its own.

The inverter's AC output is what connects to your panel, so it is also the number that matters for the panel's limits. Designers sometimes pair a slightly larger array with a smaller inverter, since modules rarely reach full output at the same moment. That is a normal design choice, and your proposal should explain it.

If you think you might add a battery later, ask whether the inverter you are choosing can work with one, or whether a second inverter would be needed. Planning for it now can save a costly change. See home battery storage in Alberta.

Why a well-sized system still uses the grid

Even a system that produces exactly what you use in a year will not match your use month by month. In Edmonton, production peaks in summer, while use often peaks in winter with lighting, furnace fans and holiday cooking.

This is normal and expected. A system that tried to cover December use on its own would need to be enormous and would produce a huge surplus all summer, which the rules do not support and the economics do not reward.

That is why net billing exists. Summer surplus goes to the grid for credit, and winter shortfall comes back from the grid. See Alberta micro-generation explained for how the credits work.

Questions to ask any solar proposal

• What annual production is estimated, and what modelling tool and assumptions were used?
• How was shading assessed, and in which months does it matter?
• Does my panel need an upgrade, and is that included?
• How were future loads like an EV accounted for?
• What happens if I want to expand later?

Alberta Electrical designs residential solar and commercial solar systems around real energy use, and our energy audits can find savings that make a smaller system go further.

Common questions