Solar Power Basics: The Equation, the Four Parts, and How to Size a System That Runs Your House

Sunlight knocks electrons loose in the silicon of a panel and they flow as direct current. A charge controller takes that DC and feeds it into a battery at the right voltage. An inverter takes DC from the battery and makes the 120 volt alternating current the house runs on. That is the whole system: panel, controller, battery, inverter, with wire, fuses and a disconnect between them. A power station is all four in one box; a home system is the same four parts, bigger, on the wall.

It sits between the panels and the battery and manages the charge: it drops the panel voltage to what the battery wants, tapers the current as the battery fills, stops at full so the battery is not overcharged, and blocks current from flowing backward into the panels at night. Without one, a panel cooks a battery on a sunny day and drains it after dark. It is the cheapest part of the system and the one people skip.

Lithium iron phosphate (LiFePO4) for almost everyone. It gives 3,000 to 6,000 charge cycles against 300 to 1,000 for lead-acid, you can use 80 to 90 percent of its capacity instead of 50, it needs no watering or equalizing, it weighs a third as much, and it is the most heat-stable lithium chemistry. It costs more up front and far less per cycle. Lead-acid still makes sense for a system that sits unused most of the year, or below freezing where lithium cannot charge without a heater.

A PWM controller connects the panel to the battery and pulses the connection, which drags the panel down to battery voltage and wastes the difference; on a cold bright day that is a quarter of the power. An MPPT controller finds the panel's best voltage and current point and converts it to the battery's voltage, harvesting 10 to 30 percent more, and it lets you run panels in series at higher voltage over thinner wire. PWM is fine for a 100 watt panel on a trickle charge; anything over 200 watts gets MPPT.

It turns the battery's DC into the 120 volt AC that ordinary appliances use. Buy pure sine wave, not modified sine, because motors, chargers, medical devices and anything with electronics run hot or fail on the cheap kind. Size it for the largest things you will run at once plus 20 percent, and check the surge rating against the biggest motor: a refrigerator that draws 150 watts running pulls 600 to 1,000 for a second on startup, and a well pump three to five times its running watts. A hybrid inverter combines the inverter, the charger and the grid connection in one box and is what a house system uses.

The billing arrangement for a grid-tied system: your meter runs backward when the panels make more than the house uses, and the utility credits you for it. It is what makes rooftop solar pay off, and it is why those systems shut down in a blackout: a grid-tied inverter is required to stop exporting the instant the grid drops, so it cannot energize a line a crew is working on. Net metering rules and rates are set by each utility and state, and most have been getting less generous.

A modern refrigerator uses about 1 to 1.5 kWh a day. At 4 sun hours and 75 percent system efficiency, one 400 watt panel makes about 1.2 kWh, so one to two panels cover the fridge, with a 2 to 3 kWh battery to carry it through the night and a cloudy day. In a northern winter at 3 sun hours, two or three panels. The number is not the panel; it is your daily watt-hours divided by your sun hours, with a margin for weather, and the calculator on this page does that arithmetic for your own list.

By design. A grid-tied inverter with no battery synchronizes to the grid and must shut down the moment the grid goes away, so it cannot backfeed a line and electrocute a line worker. Your roof can be in full sun during an outage and the house is dark. To have power in a blackout you need a battery with a hybrid or off-grid inverter and a transfer switch that isolates the house from the grid, or a generator on the same transfer switch.

A peak sun hour is one hour of full noon sun; the day's weaker morning and evening light is folded into the total. Annual averages run from about 6.5 a day in Arizona and 6 in California, through 5.7 in Colorado and 5 to 5.5 across Texas, Florida and the Southeast, to 4.5 in the Midwest and Northeast, 4 in Oregon and Washington, and 3 in Alaska. Winter is the number that matters for a backup system: half the annual figure in the north, two-thirds in the south. Size for your worst month, not your average.

By power. Under about 1,500 watts, 12 volts is fine and every RV and camp part is made for it. Between 1,500 and 3,000 watts, 24 volts halves the current and the wire size. Above 3,000 watts, and for any whole-house system, 48 volts: a 2,000 watt inverter pulls 167 amps at 12 volts, 83 at 24 and 42 at 48, and 167 amps needs welding-cable-sized wire and enormous fuses. Higher voltage means thinner wire, smaller losses and cheaper copper.

For most short outages, yes, and it is the right first purchase. A 1 kWh station runs a refrigerator overnight and charges everything else; a 2 kWh station covers a day; a 4 kWh station runs a fridge, a freezer and a furnace blower for a day or two and expands with extra batteries. Add a 400 W folding panel and it recharges through the day. What it does not do is run a well pump, central air, or a house through a week of gray weather; that is the 48 volt system and a generator. The portable solar power field guide picks the size and the panel.