What is the first number I need?
Start with how much energy you use. For a grid-connected home, twelve months of electric bills show use month by month, including the winter and summer peaks. For an off-grid cabin or barn, you build the number from a list of loads instead.
Energy is measured in kilowatt-hours. Power, measured in watts or kilowatts, is how fast something uses energy. Keeping the two straight avoids most beginner mistakes.
Remember that sizing is an estimate. Ask the installer for a range of expected output, not a single figure, and keep your own bills and meter readings so you can compare real results with the promise.
How do I build a load list?
List every device, its power in watts from the nameplate, and how many hours a day it runs. Multiply watts by hours to get watt-hours per day, then add up the list. Include seasonal items such as stock tank heaters, fans, brooders and well pump cycles.
Note which loads are motors, because they draw extra at startup and need a larger inverter. Note which must run at the same time, since that sets the peak power the system must supply.
Count seasonal peaks honestly. If water pumping, stock heaters or a greenhouse fan run hardest in the season with the least sun, the system must be sized for that season or backed by another source.
- Device, watts and hours per day for each item.
- Seasonal items listed separately.
- Motors flagged for startup surge.
- Loads that overlap marked for peak power.
How do sun hours affect size?
Panels only produce at full rate in strong sun. Installers use a figure often called peak sun hours, which varies by location, season, tilt and shade. Ask your installer which source and which month they used. A system sized on a summer figure may fall short in a dark winter.
Government and university solar resource tools exist, and your installer or state energy office can point you to them. Do not guess a number from memory.
What losses should I expect?
Real systems produce less than the panel labels suggest. Wiring, the inverter, heat, dust, snow, shade and, for battery systems, charging and discharging all take a share. Good designs include a margin for these losses.
Ask for the assumptions in writing. A quote that lists no losses is a quote that deserves questions.
How does a battery change the size?
If you want power at night or in outages, you size storage for the energy you need to cover, and the array must be large enough to recharge the batteries as well as run daily loads. Battery type, usable capacity and days of autonomy all matter, and a longer cloudy stretch means a bigger system or a generator backup.
Decide how many days you want to ride through, and be honest about cost. Each added day raises the price noticeably.
How do I check an installer's proposal?
Ask for the load or bill data used, the sun hour source, the loss assumptions, the expected annual output, the inverter and battery specs, the warranty terms and the permit plan. Compare at least three proposals, and ask each to explain any large differences.
A licensed electrician or your state energy office can review a proposal. Reducing loads first, such as efficient lighting and appliances, often lowers the size you need and the cost.
Keep your assumptions in writing and review them after the first year. Compare actual output and use against your plan, and adjust habits or equipment if the numbers differ from what you expected.
What are common sizing mistakes?
The most common mistakes are sizing from the roof instead of the loads, using a summer sun figure for a winter problem, forgetting motor startup surges, leaving out losses and underestimating how much storage a few cloudy days require. Another is counting loads you plan to add later without saying so, which leaves the system too small or the budget too thin.
A second group of mistakes comes from loads. A freezer, a pump and a heater can each be minor alone, but together they set the peak power your inverter must deliver. Check which devices run at once, and consider staggering them.
Finally, do not ignore efficiency. Every appliance replaced with a more efficient one reduces the size and cost of the system. Spend a little on reducing loads first, then size what remains.
Key takeaways
- Size from your actual energy use, not from the available roof.
- Build an honest load list with watts, hours and motor surges.
- Ask which sun hour figure and which losses were used.
- Compare several written proposals and ask about assumptions.
Sources and further reading
Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.