Power fit tool
Pick your use case, list your loads, and see the battery, inverter and solar sizes to look for. Then compare the budget, mid and premium picks.
FitMyPower fit tool
1. Pick your use case
2. List your loads
Each row starts from a published typical wattage (source shown per row). Replace it with the number on your appliance's label if you have it. Hours per day are example values you should change.
| Appliance | Running W | Starting W | Hours per day | Qty | Remove |
|---|
3. Backup days, sun and cold
Days start at a suggested value per use case (set your own). Find your peak sun hours with NREL PVWatts; 4 is only a starting value. Enter 0 to skip solar.
How the planner sizes your system
- Daily energy = sum of running watts × hours per day × quantity (DOE method, checked Nov 2, 2026).
- Battery capacity = daily energy × days ÷ 0.92 inverter efficiency ÷ 0.80 usable depth, ÷ 0.80 again if the battery sits near 0°C. 0.92 is the inverter derate in NREL's PVWatts manual (a grid-tie figure; it is an estimate for a battery inverter). 0.80 depth and the cold loss come from Victron's LiFePO4 data (checked Nov 2, 2026). Other batteries rate depth differently; check yours.
- Continuous inverter watts = running watts of everything that could be on at once. Surge watts = that plus the biggest starting jump of any one load (Honda's method; summarized). Start big motor loads one at a time.
- Solar input = daily energy ÷ (peak sun hours × 0.77). 0.77 is the PVWatts V1 total derate in NREL's manual, checked Nov 2, 2026.
These are planning numbers from published typical figures, not a design. Anything tied to home wiring needs a licensed electrician.
Sources (checked Nov 2, 2026)
- U.S. DOE Energy Saver, "Estimating Appliance and Home Electronic Energy Use" (typical wattages list and watts × hours ÷ 1000 method), as reproduced by The College of New Jersey Energy Conservation office DOE's own page (energy.gov/energysaver/estimating-appliance-and-home-electronic-energy-use) returned 404 on Nov 2, 2026, so the DOE original could not be re-opened, so this is not checked against the original.
- Honda Power Equipment, "What wattage generator do I need?" (approximate starting and running watts)
- Honda Power Equipment, "What size generator do I need?" (continuous need = running watts on at once; occasional need = largest starting load) Read through a search summary on Nov 2, 2026; the page blocks direct fetch, so it was not read in full.
- NREL PVWatts Version 5 Manual (Dobos, 2014), Table 6: PVWatts V1 total DC-to-AC derate 0.77, inverter derate 0.92
- NREL PVWatts Calculator (look up the peak sun hours for your address)
- Victron Energy, Lithium Battery Smart technical data (cycle life rated at 80% DoD; 100Ah at 25°C falls to 80Ah at 0°C)