Solar for an ADU or guest house

If the ADU is fed from your main panel through a subpanel, it’s just another load on your meter. Solar anywhere on the lot offsets everything, and sizing is straightforward: add the ADU’s expected usage to yours and design for the total. This is the simplest and, for an owner-occupied ADU or family member, usually the best setup.

If the ADU has its own utility meter, which is common when it’s a rental, net metering is credited per meter. An array on the ADU roof tied to the ADU meter offsets only the ADU. An array on the main house offsets only the main house. Unless your utility allows aggregation of meters on one property, you can’t credit both from one array, and you’d need two separate interconnections to solar both.

That’s not a reason to skip solar; it’s a reason to decide the meter question before you pour the ADU foundation. A separately metered rental lets the tenant pay their own bill, but it also means the landlord’s solar can’t help with it. Some owners sub-meter the ADU privately instead: one utility meter, one array, and a lease that passes through a share of the electric cost. Whether that’s allowed depends on local landlord rules, so check them.

A 600- to 900-square-foot all-electric ADU with a heat-pump mini-split, induction range and heat-pump water heater typically uses somewhere in the range of 3,000–6,000 kWh a year in Colorado, more if the tenant has an EV. That’s roughly a 2–4.5 kW array at 1,300–1,600 kWh per kW, which fits on most ADU roofs if a face points south or west.

A tenant’s habits are outside your control, so we size to a realistic average rather than a best case. If the ADU shares your meter, the array size is the combined total and the ADU is just part of it; if it’s separate, we design specifically for that meter and leave headroom for an EV charger, which more tenants ask about every year.

New ADU construction is the cheapest moment to prepare for solar. We ask the builder for a roof plane facing south or west with no plumbing vents in the middle of it, a conduit sleeve from the attic to the electrical panel, a panel with spare breaker space and a bus rated for backfeed, and a 240-volt circuit to the parking spot for a future charger. Each of those costs little during framing and a lot afterward.

Standing-seam metal on the ADU roof makes the array penetration-free and pairs well with the modern, flat-roofed or low-slope designs most Denver ADUs use. Low-slope roofs work with tilted racking; we just need parapet heights and drainage worked out with the architect. See our new-construction page for the full checklist.

A common case: the main house is a 1920s bungalow with a hip roof full of dormers and a big silver maple over it, and the new ADU behind it has a clean, unshaded shed roof. When the ADU shares the house meter, we put the array on the ADU and run conduit to the main panel. Electrically it works exactly like a detached-garage array, and it often out-produces anything the old house roof could hold.

The reverse is also common: the ADU is tucked in the shade of the main house and the house roof is wide open. Then the house carries the panels and the ADU is simply a load. Either way, the rule is to put the array where the sun is and connect it to the meter that needs the offset.