Solar load calculator: watt-hours per day
Every solar, battery and generator size starts here. List each appliance’s watts and the hours it runs a day; the calculator adds inverter losses and gives the daily energy your system has to supply.
Enter your measurements
More options: Inverter eff. · Idle W · Inverter on
- Per day in kWh
- 1.94 kWh
- Amp-hours per day at your system voltage
- 161 Ah · 12 V
- AC loads (before inverter losses)
- 1,440 Wh
- DC loads
- 335 Wh
- AC watts if everything runs at once
- 210 W
- Per month (30 days)
- 58.1 kWh
Appliance by appliance
| Appliance | Watts × hours | Per day | From battery |
|---|---|---|---|
| Fridge (compressor hours) | 150 W × 8 h (AC) | 1,200 Wh | 1,333 Wh |
| LED lights | 10 W × 5 h × 4 (DC) | 200 Wh | 200 Wh |
| Laptop | 60 W × 4 h (AC) | 240 Wh | 267 Wh |
| 12 V fan | 45 W × 3 h (DC) | 135 Wh | 135 Wh |
AC loads: 1,440 Wh ÷ 90% inverter efficiency = 1,600 Wh
+ DC loads 335 Wh = 1,935 Wh a day
1,935 Wh ÷ 12 V = 161 Ah a day from the battery
- Your inverter must carry at least 210 W continuous if all the AC loads can run at once, plus any motor starting surge — check it with the inverter size calculator.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much power Starlink uses per day | 1.8–2.4 kWh a day (about 2,667 Wh from a battery via an inverter) |
| How much power the Starlink Mini uses | 600–960 Wh a day on 12 V DC |
| How many watts a refrigerator uses per day | About 1,600 Wh a day (200 W running a third of the time) |
The same loads at other inverter efficiencies
| Inverter efficiency | Energy per day (from the battery) | Amp-hours per day at your system voltage | Per month (30 days) |
|---|---|---|---|
| 80% | 2,135 Wh | 178 Ah | 64.1 kWh |
| 85% | 2,029 Wh | 169 Ah | 60.9 kWh |
| 90% | 1,935 Wh | 161 Ah | 58.1 kWh |
| 95% | 1,851 Wh | 154 Ah | 55.5 kWh |
Watt-hours per day is the one number every other calculator on this site needs: the battery stores it, the panels replace it, a generator burns fuel to make it. Get this list right and the rest is arithmetic. New to the units? Solar words explained.
How the load calculator works
For each appliance, multiply its power in watts by the hours it runs a day. That is the formula Virginia Cooperative Extension gives for appliance energy (it divides by 1,000 for kWh). AC appliances then lose a slice in the inverter, so the calculator divides their total by the inverter efficiency. DC loads — 12 V lights, fans, a compressor fridge, USB charging — come straight off the battery.
Wh per day = watts × hours per day × quantity from the battery = DC Wh + AC Wh ÷ inverter efficiency amp-hours per day = Wh per day ÷ battery voltage
Where to find the watts
- The nameplate. Most appliances list watts on a label on the back or bottom, or in the manual. If it only lists amps, multiply by the voltage (120 V in the US) — the extension-service method.
- A plug-in or DC meter. A usage monitor measures real watts and, over a day, real watt-hours. It is the only accurate way to measure anything that cycles, like a fridge or freezer.
- A planning chart. The appliance wattage chart lists typical running and starting watts from a generator maker’s chart and the extension service’s nameplate list. Use it when you are still choosing appliances.
Typical daily loads
| Appliance | Typical watts | Hours/day | Wh/day |
|---|---|---|---|
| Starlink Standard (average) | 75–100 | 24 | 1,800–2,400 |
| Starlink Mini (average) | 25–40 | 24 | 600–960 |
| CPAP, no heated humidifier | 30–60 | 8 | 240–480 |
| Laptop | 50 | 4 | 200 |
| LED bulb | 8–15 | 5 | 40–75 each |
| Box fan | 50–200 | 8 | 400–1,600 |
| Microwave (input watts) | 600–1,200 | 0.25 | 150–300 |
The two Starlink lines are worked out in full on how much power Starlink uses per day and how much power the Starlink Mini uses. For a battery pack to run the Mini, see what size power bank for a Starlink Mini. A household fridge — the load people most often get wrong — is on how many watts a refrigerator uses per day.
Mistakes that make systems too small
- Using the hours a fridge is plugged in (24) instead of the hours it runs. Using the nameplate watts for 24 h can triple the real figure; using too few hours runs the battery flat.
- Forgetting standby loads. An inverter left on, a router, a TV on standby, a propane fridge’s control board — a few watts each, all day, adds up.
- Leaving out the inverter loss. A 90% inverter turns 1,000 Wh of AC use into 1,111 Wh from the battery.
- Mixing peak and daily numbers. Peak watts size the inverter; watt-hours size the battery and panels. This calculator gives both.
What you’ll need
- Plug-in electricity usage monitorBudget price
Plugs in between the wall and a 120 V appliance and shows real watts and kWh over a day — the honest way to fill in a load list, especially for fridges that cycle on and off.
See options on Amazon (opens in a new tab) - Inline DC watt meterBudget price
Sits between a 12 V device and its supply and shows watts, amps and watt-hours used — handy for measuring a 12 V fridge, Starlink Mini or fan.
See options on Amazon (opens in a new tab) - Battery monitor with shuntMid-range price
Counts amp-hours in and out, so you see true state of charge instead of guessing from voltage — which barely moves on a LiFePO4 battery until it is nearly empty.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How do I calculate watt-hours per day?
Multiply each appliance’s watts by the hours it runs and add them up. A 60 W laptop for 4 hours is 240 Wh. For AC appliances on an inverter, divide the total by the inverter efficiency (0.9 for 90%) to get what comes out of the battery.
How do I convert watt-hours a day to amp-hours?
Divide by the battery voltage. 1,200 Wh a day on a 12 V system is 100 Ah a day; on 24 V it is 50 Ah. The amp-hours to watt-hours calculator does it either way.
How much power does Starlink use per day?
Starlink lists the Standard kit at 75–100 W on average, which is 1.8–2.4 kWh over 24 hours, and the Mini at 25–40 W, or 0.6–0.96 kWh. See how much power Starlink uses per day.
Should a load list use running watts or starting watts?
Running watts and hours for energy (watt-hours). Starting watts only matter for the inverter or generator size, because the surge lasts a second or two — use the inverter size calculator or generator size calculator for that.
What counts as a DC load?
Anything that runs on 12 V (or 24/48 V) without the inverter: 12 V lights, fans, water pumps, compressor fridges and USB or 12 V chargers. They avoid the inverter loss, which is why van and boat systems use DC wherever they can.
Related guides
Sources
- Virginia Cooperative Extension, BSE-301: Estimating Appliance and Home Electronic Energy Use (2020) (opens in a new tab) — Wattage × hours ÷ 1,000 = kWh per day; watts = amps × volts when only amps are listed; a refrigerator runs at full wattage about a third of the time; typical nameplate wattages.
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- ENERGY STAR — Certified Televisions data set (on-mode power) (opens in a new tab) — Certified models, on-mode watts, retrieved October 2026: 32 in about 27–36 W; 43 in about 45–78 W; 50 in about 61–90 W; 55 in about 64–114 W; 65 in about 85–135 W; 75 in about 120–176 W.
- Starlink — Standard kit specification sheet (opens in a new tab) — Power consumption: average 75–100 W.
- Starlink — Mini specification sheet (opens in a new tab) — Power consumption: average 25–40 W; input rating 12–48 V, 60 W.