Why is the inverter smaller than the array in step 3?

Because panels almost never produce their rated output. An array rated 8.2 kW at standard test conditions typically peaks well below that in real conditions, so pairing it with a 6.8 kW inverter captures nearly all the available energy while costing less. The inverter clips a small number of peak hours a year — usually a low single-digit percentage of total production — in exchange for a meaningfully cheaper unit.

Do I need a battery at all if I have net metering?

Not for economics, where full retail net metering exists — the grid acts as free, lossless storage. You need a battery for two reasons: outage backup, since grid-tied solar shuts down when the grid does, and poor export rates, where storing your own production is worth far more than exporting it at a fraction of retail.

Can I use these formulas for an off-grid system?

Steps 1, 2 and 4 transfer directly, but off-grid changes two things: size the array against your worst month rather than the annual average, since there is no grid to fall back on in December, and size the inverter from your largest simultaneous load plus surge rather than from the array. Off-grid systems also typically need 2-3 days of battery autonomy where a grid-tied backup system might need one.

Solar System Size Calculator: kWh, Panels, and Battery in One Place

The complete sizing sequence in one walkthrough — from your utility bill to a panel count, an inverter rating and a battery size, with the formula at each step.

Solar sizing gets split across a dozen articles that each answer one piece, and the pieces are easy to connect wrongly. This is the whole sequence in order: consumption, array, inverter, battery. Four steps, four formulas, one worked example carried through all of them.

400W solar panel used in the array sizing step — EcoFlow 400W — $599. Step 2 turns kilowatt-hours into a panel count.

LiFePO4 battery used in the storage sizing step — Renogy 12V 100Ah — $259, 1,280Wh. Step 4 sizes the bank.

Step 1: annual and daily kWh

Daily kWh = annual kWh ÷ 365

Pull twelve months of kWh from your utility account and add them. Twelve, not one — summer and winter can differ by 3x, and a single bill will mislead you in whichever direction it happens to sit. Our guide to calculating daily kWh usage covers the fallbacks if you cannot get a full year.

Worked example: 10,800 kWh a year ÷ 365 = 29.6 kWh per day.

Step 2: array size and panel count

System kW = annual kWh ÷ (peak sun hours × 365 × 0.8)

Panels = system watts ÷ panel watts

Peak sun hours is not daylight hours — it is roughly 3.2 in the Pacific Northwest and 6.0 in the Southwest. The 0.8 covers inverter, wiring, temperature and soiling losses; omitting it is the most common sizing error.

Worked example, at 4.5 peak sun hours: 10,800 ÷ (4.5 × 365 × 0.8) = 8.2 kW. At 400W per panel, that is 21 panels. The regional breakdown is in how many solar panels do I need to run my house.

Step 3: inverter rating

This is the step most sizing guides skip, and it is a different calculation from the array — the array is sized by energy, the inverter by power.

For a grid-tied system, the inverter is sized to the array with a DC-to-AC ratio of roughly 1.2, so an 8.2 kW array pairs with a 6.5-7 kW inverter. Clipping the rare peak is deliberate and costs very little energy.

For an off-grid or backup system, the inverter must instead cover your largest simultaneous load plus the surge of whatever motor starts under it — a well pump running at 1,000W can spike to 3,000W. Size this from an appliance list, not from the array. Our free wattage calculator returns both running and surge figures for exactly this.

There is one more ceiling to check: on a 200A service, interconnection rules usually cap backfed inverter output near 7.6 kW regardless of your array. See solar panel sizing for a 200 amp service before finalising.

Step 4: battery capacity

Usable kWh = critical daily kWh × days of autonomy

Two traps here. First, size against critical loads, not total consumption — a house averaging 29.6 kWh a day typically needs 4-8 kWh a day during an outage, and sizing to the full figure overbuilds by four times on the most expensive component. Second, usable is not nominal: LiFePO4 gives you 90-95% of its rating, lead-acid only 50%.

Worked example: 6 kWh of critical load × 2 days = 12 kWh usable. In LiFePO4 at 95% depth of discharge, that is about 12.6 kWh nominal. In lead-acid it would be 24 kWh nominal for the same result.

For a full three-day build including the winter-weather case, see solar and battery sizing for a 72-hour outage.

The whole example in one place

Step	Formula	Result
1. Consumption	10,800 ÷ 365	29.6 kWh/day
2. Array	10,800 ÷ (4.5 × 365 × 0.8)	8.2 kW — 21 × 400W panels
3. Inverter	8.2 ÷ 1.2, capped by service	6.8 kW AC
4. Battery	6 kWh critical × 2 days ÷ 0.95	12.6 kWh nominal

Sanity-check before you buy

Does the array fit? 21 panels at ~21 sq ft each is about 440 sq ft of unshaded, well-oriented roof
Does the inverter fit your service panel's backfeed limit?
Is your export rate good enough to justify sizing to 100% of consumption, or is a smaller array plus storage the better return?
Have you sized the inverter against surge watts, not just running watts?
Are any large loads about to change — an EV, a heat pump, a pool?

Every number above is a planning estimate. Confirm final equipment sizing against a load calculation performed by a licensed electrician before ordering, particularly on the inverter and service-panel side.