# How many solar panels do you actually need?

URL: https://solarrealcost.com/does-solar-pay/how-many-solar-panels-do-i-need
Updated: 2026-09-18
Review: Reviewed by SolarRealCost Editorial Team, editorial review: primary sources checked and independently fact-checked

Work it out from your own annual usage before anyone tells you. The arithmetic, why panel count alone means nothing, and when a smaller array is better.

You can work this out yourself in about five minutes, and it is worth doing before anyone quotes you — because a quote larger than your usage justifies is the most common thing to find in this market.

- Start from **twelve months of real usage**, not one bill.
- Array size = **annual kWh ÷ annual kWh per kW** for your location.
- **Panel count alone means nothing** without the wattage.
- Shade and orientation change the answer more than people expect.
- **100% offset is a convention, not a rule** — it depends on what exports are worth.

## The arithmetic

Take your annual kilowatt-hours from twelve months of bills. If you don't have a year of bills yet, see [how many kWh a house uses](https://solarrealcost.com/does-solar-pay/how-many-kwh-does-a-house-use) for EIA's national and state averages. Divide by how many kilowatt-hours a kilowatt of array produces per year where you live — that production factor is exactly what [NREL's PVWatts](https://pvwatts.nlr.gov/) gives you, for your address, roof orientation, pitch and shading. The result is the kilowatts of array you need. For what a single panel makes in a few example cities, see [how much energy a solar panel produces](https://solarrealcost.com/does-solar-pay/how-much-energy-does-a-solar-panel-produce).

Then divide by the wattage of the panels being quoted. At today's common 400 to 450 watt panels, a 7 kW array is roughly sixteen to eighteen panels.

Tool: [How many panels do you need?](https://solarrealcost.com/does-solar-pay/tools/how-many-panels). From what you actually use, the panel wattage available, and the production factor where you live.

## Why panel count on its own is meaningless

"Eighteen panels" describes nothing until you know the wattage. Two proposals with the same count can differ by more than a kilowatt of capacity. Always ask for the wattage and the total kW, and check both against the production model — the [PVWatts v8 documentation](https://developer.nlr.gov/docs/solar/pvwatts/v8/) explains what the model accounts for.

## Orientation and shade

An east or west facing array works; it simply needs more panels for the same annual output. Our guide to the [best angle for solar panels](https://solarrealcost.com/does-solar-pay/best-angle-for-solar-panels) explains how tilt and direction play in. Shade is different — it is the one constraint no design fully solves, and it should be measured rather than estimated by eye. If a proposal assumes almost no shading loss on a roof with trees, that assumption is doing a lot of work.

**Cut the usage before you size a system.** Good when: Always, and especially if the bill is high because the house leaks heat. Watch out: A system sized to a wasteful house is a bigger system than you need, sold at a bigger price.

**Bigger is a bigger sale**
Legitimate reasons for an array larger than your bills justify include a planned electric car or heat pump. The reason to be alert to is simply that a larger system is a larger commission. Ask which applies, and ask to see the production model behind it.

## Bottom line

Do the arithmetic from your own bills, run PVWatts yourself, and treat any large gap between that and a quote as a question rather than an answer. The economics are in [are solar panels worth it now](https://solarrealcost.com/does-solar-pay/are-solar-panels-worth-it-now) and the price in [what solar costs to install](https://solarrealcost.com/does-solar-pay/install-solar-panels-cost).

Start the Solar Check: https://solarrealcost.com/check

## Common questions

### How do I work out the array size?

Annual kilowatt-hours divided by how many kilowatt-hours a kilowatt of array produces per year where you live. Then divide by the panel wattage being quoted to get a count.

### Where do I find my annual usage?

Twelve months of bills, or your utility's online account. Use a real year rather than one month, because summer and winter differ enormously.

### How many panels is a kilowatt?

At today's common residential wattages of roughly 400 to 450 watts, a little over two. So a 7 kW array is about sixteen to eighteen panels.

### Should the system cover all my usage?

Not automatically. Where exported power is credited at a low rate, a smaller array you mostly consume yourself can pay back better.

## Sources

- [NLR (formerly NREL) Developer Network: PVWatts v8 API documentation](https://developer.nlr.gov/docs/solar/pvwatts/v8/)
- [NLR (formerly NREL): PVWatts Calculator](https://pvwatts.nlr.gov/)
- [U.S. Energy Information Administration: Residential Energy Consumption Survey](https://www.eia.gov/consumption/residential/)
- [U.S. Energy Information Administration: average retail price of electricity by state](https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a)
- [U.S. Department of Energy: Homeowner's guide to going solar](https://www.energy.gov/eere/solar/homeowners-guide-going-solar)