Every off-grid solar design starts with an honest load audit: how much energy each appliance uses in a day. This calculator lets you list appliances with their wattage and daily runtime, then sums the watt-hours and applies inverter losses so your battery and panel sizing rests on real numbers.
How it works
Each appliance contributes watts × hours/day watt-hours. DC loads add to the
battery drain directly. AC loads pass through the inverter, so their energy is
divided by the inverter efficiency to account for conversion loss:
DC daily Wh = Σ (watts × hours) for DC rows
AC daily Wh = Σ (watts × hours) ÷ efficiency for AC rows
Total daily Wh = DC daily Wh + AC daily WhPeak power is the sum of every appliance’s wattage — the worst case where all run at once — and this figure sizes the inverter.
Worked example: small cabin system
Consider a cabin with these loads:
| Appliance | Watts | Hours/day | Type | Daily Wh |
|---|---|---|---|---|
| LED lighting (4 bulbs) | 40 W | 5 h | DC | 200 Wh |
| Laptop | 60 W | 6 h | DC | 360 Wh |
| Mini-fridge | 80 W | 10 h | AC | ~889 Wh at 90% eff. |
| Phone charger | 10 W | 2 h | DC | 20 Wh |
| Kettle | 1200 W | 0.2 h | AC | ~267 Wh at 90% eff. |
DC subtotal: 580 Wh. AC subtotal (before inverter): 1,240 Wh, which becomes about 1,156 Wh at 90% inverter efficiency. Total daily draw from the battery: roughly 1,736 Wh. Peak power: 40 + 60 + 80 + 10 + 1,200 = 1,390 W (size your inverter for at least that, plus a startup margin for compressor loads).
What the output tells you
Daily watt-hours is the number to use when sizing your battery bank and solar array. To run reliably through one overcast day without the sun, your usable battery capacity should equal or exceed your daily total, factoring in the depth-of-discharge limit of your battery chemistry (typically 50% for flooded lead-acid, 80–90% for lithium).
Peak power tells you the minimum inverter continuous-output rating. Add roughly 25–50% above calculated peak for motor start-up surges from refrigerators, pumps, and air conditioners — these can pull three to six times their running watts for a fraction of a second.
Common mistakes and how to avoid them
- Forgetting standby loads. A satellite router drawing 15 W runs 24 hours — that alone is 360 Wh per day, more than a laptop. Audit everything plugged in.
- Using rated wattage for compressor loads. A fridge’s nameplate shows peak draw; its actual daily Wh is much lower because the compressor cycles off. Use a plug-in power meter to measure real consumption over 24 hours.
- Skipping the margin. Real-world losses from wiring, battery internal resistance, and temperature add up. A 20% design margin is the minimum for a reliable system; 30% is safer for year-round off-grid use.
- Confusing watts and watt-hours. Watts is a rate (how fast energy flows); watt-hours is a quantity (how much total energy). This tool calculates the quantity by multiplying rate by time.
Tips
- Add a 15 to 25 percent margin before sizing panels and batteries.
- Hunt down phantom standby loads — routers and chargers run all day.
- Peak power sizes the inverter; daily Wh sizes the battery and array.
- Re-run the audit each season if usage patterns change significantly.