How many plug-in solar panels do I need?

Most buying guides start at the panel and work backwards. That order fails, because a 400 W panel means nothing without a sun figure behind it. Two inputs settle the question: how much energy the house uses in a day, and how many hours of usable sun the location gets on average.

The first number is yours to measure. Our load calculator turns a list of appliances into daily energy in Wh, running watts and startup surge, and it uses the same method described on that page so you can check it by hand. If you already know your daily figure from a utility bill, divide the monthly kWh by 30 and multiply by 1,000.

The second number belongs to your location, not to you. Peak sun hours are not daylight hours: they are the hours in a day when sunlight is strong enough to equal 1,000 W per square meter, which is the condition panels are rated at. A long summer day in Seattle can deliver fewer of those hours than a shorter day in Arizona.

There is a third input that people forget, and it decides whether the installation is legal rather than whether it works: the circuit. A plug-in system feeds power back through an outlet, so the outlet rating and the inverter certification both belong in the plan. The arithmetic and the wiring are two separate checks, and passing the first does not excuse you from the second.

The calculator returns six things, and only one of them is the answer you came for. Read them in this order.

The order matters, because a result read out of sequence invites the wrong purchase. Someone who looks only at the panel count and skips coverage can buy a kit that balances on paper in June and falls short in December. Someone who looks only at coverage and skips the inverter count can end up with an array that no single outlet circuit can carry, which is a wiring problem disguised as a sizing answer.

Array watts is the raw requirement before panels are rounded up. It is the honest number: it tells you what the house actually demands from the sun. Panel count is that figure divided by your panel size and rounded up, because you cannot buy two thirds of a panel. Total watts is what you would really be bolting on, and it is almost always a little above the raw requirement.

Inverter count splits the array across units, because one outlet circuit cannot carry the whole array. Daily generation and coverage close the loop. If generation lands below your daily use, the system is undersized and the shortfall is power you still buy. If it lands above, the surplus either gets credited by the utility or is given away, depending on where you live and what your meter does.

Two refinements make the result more useful. If your daily use is not flat, run the calculator twice, once with a heat-wave week and once with a mild week, and treat the pair as a range rather than one number. And if the array lands right on a panel boundary, test the next panel size up before buying: the extra cost of a bigger panel is usually smaller than adding a panel, a rail section and a second inverter channel.

The figures below are long-run planning averages, rounded, for a fixed tilt facing south. They are a starting point for a calculation, not a forecast for one roof. Use the NREL PVWatts calculator with your own location for the number you would defend.

Note the last column: the same house jumps from two panels to three somewhere around 4.0 sun hours. That single step is the difference between a kit that covers a refrigerator and one that covers a refrigerator plus a router and a television. It is also why a national average is a trap.

If you would rather skip the arithmetic, the table below is a shortcut. It reads across from daily energy to a panel count, assuming 400 W panels, 80% efficiency and four sun hours. Treat it as a sanity check on the calculator rather than a replacement for it, because your own sun hours are what move the answer.

Divide 2,160 by 4.5 and you get 480. The calculator says 600. The gap is not padding, it is the losses between the panel and your meter, and skipping them is the most common sizing error.

Four things take a cut. The inverter loses three to five percent turning direct current into alternating current. Wiring loses one or two percent. Heat costs roughly 0.4% per degree above 25 °C, which on a summer roof is not a small number; that 0.4% is the planning value, since datasheets for common silicon modules quote −0.3% to −0.45% per °C, and the number that counts is the one on your own panel. Dust, shading and imperfect tilt can take another ten to twenty percent together.

The calculator folds all of that into one efficiency input, defaulting to 80% and adjustable in the panel on the right. Drop it to 70 if the panels are shaded or sit flat against a wall. If your panels sit at a poor angle, or a balcony railing shades them for two hours after lunch, that default is optimistic and you should lower it. A system sized on 80% in a shaded spot will simply underperform, quietly, every day.

The efficiency figure is the one input people set once and never revisit, which is a mistake. Two identical 400 W panels can differ by a third in daily output because of how they are mounted: one tilted toward the sun on an open rail, the other flat against a shaded balcony wall. If the install is fixed and cannot be improved, the honest response is to lower the input and let the calculator ask for more panels, rather than keep an assumption the roof will never deliver.

Everything above is arithmetic. This section is where the arithmetic meets a socket, and the socket usually wins.

A standard 15 A, 120 V branch circuit is rated for 1,800 W, and the electrical code limits a continuous load to 80% of that, about 1,440 W. That outlet already feeds whatever else is plugged into the same circuit, so a kit that pushes the full 1,440 W leaves no room for a television, a lamp or a charger. This is why most plug-in inverters top out around 600 to 800 W, and why a 1,600 W kit is usually split across several inverters and several circuits.

Two practical consequences. First, a big array does not mean a big single plug: plan for multiple inverters, each on its own circuit. Second, an inverter has to be certified for the way it is connected. A unit certified only as a grid-tied inverter has not been tested in the outlet-connected configuration, so check the certification of the specific model rather than the brand.

There is also a paperwork step that catches people out. Where a state has passed plug-in solar legislation, the statute usually sets out when and how the utility has to be told: some require a notification within a set number of days after installation, some require an application first, and the size cap and the certification requirement differ from state to state. Where no such law exists, the same equipment may need an interconnection application, or may not be permitted at all. Read your own state's rule before buying, because this page cannot tell you which of the three applies to your address. The panel count does not change either way, but the plan does, and that is worth checking before the purchase rather than after.

Take one house: a refrigerator, four LED bulbs, a router and a television, which our load calculator puts at 2,160 Wh a day with a 755 W startup surge. Now price the array in three places, holding the 400 W panel and 80% efficiency constant.

Three things worth noticing. Arizona needs half the array of Washington for the same daily energy. Virginia and Arizona both land on two panels, which means Virginia buys far less surplus for the same hardware. And Washington's third panel only exists because of a rounding step: 844 W of requirement does not fit into two 400 W panels, so it becomes three.

That last point is the practical lesson. If you are near a panel boundary, a slightly larger panel can save you a whole unit, a whole mount and a whole inverter channel.

The reverse case is worth running too. If you already own a 400 W panel, the question flips: how much of your day can it cover? In Arizona it produces about 1,920 Wh a day, which is most of the 2,160 Wh house. In Washington the same panel produces about 1,024 Wh, less than half. Same hardware, same house, a difference no product page will mention, because the product page does not know where you live.

Using daylight hours instead of peak sun hours. Twelve hours of daylight in June is not twelve hours of output. Sizing on daylight overstates generation by roughly a factor of three.

Forgetting the surge. Panels and inverters are matched on continuous output, but a compressor still asks for a startup spike. A system that covers the energy can still trip when the refrigerator restarts.

Sizing to the best month. A system that balances in June will run a deficit in December. If you want year-round coverage, size on your weakest month, not your strongest.

Ignoring the circuit. The most common real-world failure is not an undersized array, it is 1,600 W of panels and one 15 A outlet. The array was right; the wiring plan was not.

Buying the kit before the number. Panel count is an output of your own load and location. Buying first and hoping is how people end up with a kit that covers a third of what they wanted.

A sixth mistake deserves its own line because it is invisible in the short term: sizing for the load you have today and not for the load you will have in two years. A house that adds a chest freezer or a second work-from-home desk moves up a panel or two, and the mount, the cabling and the inverter channel are all cheaper to plan once than to retrofit.

The eight kits below cover the range this page calculates: a single-panel starter, the 800 W class that fits one circuit, and the larger arrays that need more than one inverter. Every entry is a grid-tie plug-in kit with a microinverter, not an off-grid panel set. Check the certification and the circuit plan on the listing before you buy, and check the price there too, because we do not print prices.

We have not bought or tested any of these. The specifications come from the product listings, the selection comes from the arithmetic above, and the certification column is the one to verify yourself. None of these links change what the calculator says.

One note on reading the cards. The first three are matched kits, meaning panels and inverter arrive together and the panel count is fixed by the seller. The last two are inverters on their own, for someone building an array from separate panels. The middle ones sit between those poles. Mixed listings like that are common in this category, and the difference matters more than the wattage, because a kit removes a compatibility question that an inverter alone leaves open.

The five questions below are the ones that come up most often once the numbers are on screen. They use the same assumptions as the calculator, so each answer can be checked against the rail rather than taken on trust.

This page is arithmetic, not electrical advice. Connecting a generating source to a house involves code requirements, utility interconnection rules and equipment certification that vary by state and by city. Have a licensed electrician review the circuit and the inverter before anything is plugged in, and check with your utility about interconnection and metering.

No system on this page can be sized correctly without knowing your own daily energy and your own sun hours. If either input is a guess, the output is a guess with a decimal point.

Once the panel count is fixed, the next question is what to do with the production: the battery sizing for a fixed array works out how much nameplate storage it takes to hold a day of output, and whether the station’s input can accept the array at all.

If you want the whole chain explained rather than just the panel count, see how plug-in solar works.

The sun hours behind this calculator are tabulated for one city in every state in the sun hours table.

For what a kit actually contains and how a balcony differs from a roof, see the guide on balcony solar kits.