Solar Load Calculator

How to do a solar load calculation

A load calculation is the foundation of every well-designed solar system — it's the honest inventory of how much energy you actually use, and it drives the size of your array, battery and inverter. Skip it or guess, and you'll either overspend on capacity you don't need or come up short when the system can't cover your real demand. The process is simple bookkeeping: list every electrical load, note its power and how long it runs, and add up the daily energy. Doing this carefully is the single highest-value half-hour in planning a system.

It helps to think of the load calculation as the budget that every other number in your system has to balance against. Just as you wouldn’t plan monthly spending without knowing your income, you can’t sensibly choose panels, a battery or an inverter without first knowing how much energy flows through your home each day and when. The good news is that the inputs are things you can measure or look up: the wattage printed on each appliance, and a realistic estimate of how many hours it runs. Spend a little care here and the rest of the design falls out almost automatically; rush it, and every downstream choice inherits the error. The calculator below turns your appliance list into a daily total and a suggested system size, but the thinking behind it — what to count, and how real appliances behave — is worth understanding so you can trust the result.

Whether you're sizing a grid-tied array to offset your bill, an off-grid system that must stand alone, or a backup setup for outages, it all starts here. The difference is only which loads you count: everything for whole-home solar, or just the essentials for backup.

Watts, hours and watt-hours

The key unit is the watt-hour (Wh): a device's power in watts multiplied by the hours it runs. A 100 W fan running 5 hours uses 500 Wh; a 1,500 W kettle running 10 minutes uses 250 Wh. Summing the watt-hours of every load gives your daily energy use, usually expressed in kilowatt-hours (1 kWh = 1,000 Wh). This daily figure is what you match your array and battery to. Your electricity bill's monthly kWh, divided by 30, is a good cross-check on your appliance-by-appliance total.

Account for how loads really behave

• Cycling loads. Fridges and freezers don't run constantly — their compressors cycle on and off, so their average draw over a day is well below their running wattage. Use a realistic duty cycle (often 30–50%) rather than assuming they run 24 hours.
• Seasonal loads. Air-conditioning, heating and pool pumps vary hugely by season. Size for the season you actually need to cover.
• Surge loads. Motors draw a start-up surge that matters for inverter and generator sizing even though it adds little to daily energy.
• Phantom loads. Standby power from electronics and chargers adds up; it's small per device but constant.

From load to system size

Once you know your daily kWh, the array follows: divide by your local peak sun-hours and a performance ratio (~0.8) to get the kW of panels needed. The battery follows from your daily use, desired days of autonomy and depth of discharge. The inverter follows from your largest simultaneous load plus surge. Each downstream component traces back to this one number — which is why an accurate load calculation is worth the effort.

Common load-calculation mistakes

• Assuming cycling appliances run constantly. This badly overstates fridge and pump energy.
• Forgetting seasonal peaks. An air-conditioner ignored in winter planning can overwhelm a summer system.
• Using nameplate watts for everything. Many devices draw less than their maximum rating in normal use.
• Omitting phantom loads. Always-on electronics quietly add to the daily total.

Reducing your load before sizing

The cheapest kilowatt-hour is the one you never use, so it pays to trim demand before sizing hardware. Swapping incandescent bulbs for LEDs, choosing an efficient fridge, eliminating phantom standby loads, and using gas or efficient heat pumps for heating and cooking can cut a daily total substantially. Every watt-hour removed shrinks the array, the battery and sometimes the inverter all at once, so demand reduction is usually the highest-return step in the whole project. Run your load calculation, identify the biggest contributors, and ask whether each can be reduced before you commit to a system size.

Frequently asked questions

List each appliance, multiply its wattage by the hours it runs per day to get watt-hours, and sum them for your daily energy use. Enter your loads above and the calculator totals it for you and suggests the system size that matches.

Both — they cross-check each other. Your bill's monthly kWh divided by 30 gives a daily average; an appliance-by-appliance count shows where that energy goes and helps you plan backup priorities. If they disagree wildly, you've likely missed a load or a duty cycle.

For whole-home solar, yes. For backup sizing, count only what you need during an outage — typically fridge, lights, internet and phone charging — which is far smaller and cheaper to support than your entire household load.

Because its compressor cycles on and off rather than running continuously. Over a day it might only run a third to half the time, so its real energy use is well below wattage × 24 hours. Use a realistic duty cycle in your calculation.

Aim for within about 10–15%. Solar sizing already includes margins for losses and bad weather, so small estimating errors wash out. The goal is to avoid large omissions — a forgotten air-conditioner or well pump — rather than to account for every phone charger to the watt. Cross-checking your appliance total against your bill catches the big misses.