What size solar system for 1500 kWh per month?
Using 1500 kWh a month means 18,000 kWh a year. What that takes in panels depends almost entirely on where you are — the range below is nearly two to one.
This is an estimate from the standard industry rule of thumb, not an engineering calculation. For anything you are about to spend money on, have the figure confirmed by a load calculation.
1500 kWh per month, by region
| Region | Peak sun hours | System size | Panels at 400 W |
|---|---|---|---|
| Desert southwest | 6 hrs | 10.3 kW | 26 panels |
| South and southern California | 5.5 hrs | 11.2 kW | 29 panels |
| Central and mid-Atlantic states | 4.5 hrs | 13.7 kW | 35 panels |
| Northern states and New England | 4 hrs | 15.4 kW | 39 panels |
| Pacific Northwest and Great Lakes | 3.5 hrs | 17.6 kW | 45 panels |
That spread is the single most important thing on this page. The same 18,000 kWh of annual consumption needs roughly 10.3 kW in the desert southwest and 17.6 kW in the Pacific Northwest — nearly double the hardware for identical usage. Any national figure for "how many panels do I need" is averaging across that gap.
The panel counts assume 400 W modules, which is the mainstream residential size at the moment, and about 18 square feet of roof each. They also assume a south-facing, unshaded roof. East or west facing costs you roughly 15 percent, and meaningful shading costs far more than its area suggests — one chimney shadow crossing a string can take down more than the panels it actually covers unless the system uses microinverters or optimisers.
How solar system size is worked out
System size in kilowatts equals your annual kilowatt-hours divided by how many kilowatt-hours a kilowatt of panels produces where you live. That production figure is roughly your average daily peak sun hours multiplied by 365 and then derated for real-world losses — inverter efficiency, wiring, heat, dust and panel ageing together cost around 20 percent of the theoretical figure.
The calculation is shown step by step under the result rather than hidden behind the answer. That is deliberate: most sizing guides online are lead capture forms that give you a number and a phone call, and a number you cannot check is a number you cannot argue with when a contractor disagrees. Every adjustment is listed, so you can see what changed the answer and by how much.
Nothing you type is sent anywhere. The arithmetic runs in your browser, there is no form to fill in, and no one calls you afterwards. That is the whole reason this page exists — you should be able to what size solar system for 1500 kwh per month without trading your phone number for it.
What going too big or too small actually costs
Oversizing past your own consumption is usually a poor investment, because what happens to the surplus depends entirely on your utility. Under a full net metering arrangement it banks at retail value and the extra pays for itself. Under net billing it exports at a wholesale rate that can be a quarter of what you pay, and the extra panels take three times as long to break even.
An undersized array still saves money — every kilowatt-hour it makes is one you did not buy. The cost of going small is mostly the fixed overheads: permits, inspection, mounting hardware and the truck roll are much the same for a 4 kW system as for an 8 kW one, so a small array carries a higher cost per watt.
Both directions cost money, which is why it is worth ten minutes to what size solar system for 1500 kwh per month before you take a quote at face value. The asymmetry is worth knowing though: for most equipment on this site, being slightly under is a comfort problem you notice on a handful of extreme days a year, while being significantly over is a permanent efficiency and equipment-life problem you pay for every month.
Where a rule of thumb stops being enough
Every number on this page comes from the standard industry rule of thumb, and a rule of thumb is not an engineering calculation. The real method is a Manual J load calculation, which models your wall and attic insulation, your window area and orientation, how leaky the building envelope is, and how much your ducts lose. It routinely lands a size away from any square-footage estimate — in either direction, which is the part people forget.
So treat this as a sanity check rather than a specification. Ten minutes to size it properly tells you whether a quote is in the right neighbourhood, which is genuinely useful — it is enough to catch a contractor who has quoted you two sizes up because that is what was on the truck. It is not enough to order equipment from.
Two things move the answer more than panel wattage does: your roof orientation and your shading. A south-facing roof in the northern hemisphere produces materially more than an east or west one, and a single chimney shadow crossing a string can cost far more output than its area suggests unless the system uses microinverters or optimisers. Any quote that has not looked at your actual roof is a guess.