Plug-In Solar Savings Calculator

Your estimate normally comes from an hour-by-hour solar model; editing sun hours or production factor selects a labeled manual estimate. Every input and assumption behind it is listed below, and the full write-up with sources and worked examples is linked at the bottom.

The hourly model

• The engine: PVWatts Version 8 (engine 8.5.0), the free, open solar model from the National Laboratory of the Rockies (formerly NREL), simulates 8,760 hours of typical-year weather (NSRDB data) with your panel watts, inverter limit and inverter efficiency.
• Your location, or a US starter: enter your ZIP and the model runs for your area's weather and sun, hour by hour. With no ZIP, the model runs the same model for the largest city in every state plus DC and averages the results, weighted by state population (2020 census). One city per state is a limited sample, so treat the no-ZIP figure as a starting point.
• Your specs: panel watts and inverter output limit are yours to edit. The model applies the inverter limit hour by hour, so panels oversized past the inverter clip the midday peak but keep morning and evening production. Hand-entered custom specs use an assumed 96% inverter efficiency and front-side production only (no rear-side gain).
• Mounting: Ground models a sunny, unshaded 35-degree tilt facing south on a fixed open rack. Balcony models vertical panels on a south-facing railing, with no rear-side gain, since the back faces the railing.
• Custom-spec losses, built from components: unshaded, premium module type: 2% soiling on tilted panels (0% for vertical balcony panels, which assume occasional cleaning), 2% wiring, 0.5% connections, 1.5% light-induced degradation and a 1% nameplate-tolerance allowance, totaling 6.81% for ground and 4.91% for balcony, plus the model's standard ground reflection (albedo 0.2).

Kits and mounting

• Picking a kit: choosing a kit fills in its panel watts and inverter output. Change the panel watts or inverter output and the picker switches to Other / Custom. Changing your ZIP, share of solar used, rate, bill, sun hours or production factor keeps the kit you picked.
• Mount and kits: EcoFlow kits work with either mount and stay selected. The 700W APsystems Balcony Kit and the 350W Balcony Solar Kit are sold for balcony, railing and fence mounting and model vertically. For any other kit, picking a mount it does not come in switches the picker to Other / Custom and keeps its panel watts and inverter limit: Hoymiles and the 880W APsystems kit are ground-only products, the Mango kit ships four ground-mount panel stands, and a balcony configuration is unconfirmed for Craftstrom.
• Kit losses: kit estimates use component-built losses (unshaded): 5.87% for the Hanersun ground kits (the Hanersun panels publish 0 to +3% power tolerance, so they carry no nameplate deduction), 6.81% for the other ground kits and 4.91% for the balcony kits, with no mismatch or availability deduction for normal operation.
• Inverter efficiency: the model input is the datasheet peak figure where the exact inverter is confirmed: 95.5% for the Hoymiles HiFlow Pro kits and 96% for the APsystems EZ1-LV kits. It is a rated maximum used as a model input, not a claimed annual average. Craftstrom, EcoFlow and Mango do not publish a comparable figure, so those kits use an assumed 96%.
• Bifacial panels: kits with bifacial panels, which also catch light on their back side, use the panel's bifaciality factor: 0.80 for the Hoymiles 440W and APsystems 880W kits (the nominal value Hanersun rates for this panel series, 80% plus or minus 5%) and 0.65 for Craftstrom kits (the factor used in the published Craftstrom guide; Craftstrom does not publish one, so it is an unverified assumption). EcoFlow, Mango and the 700W APsystems Balcony Kit are modeled front-side only, and balcony estimates include no rear-side gain.
• Custom specs: custom specs are modeled hour by hour once a ZIP is entered. A converted kit keeps its inverter efficiency and bifacial settings; hand-entered specs use an assumed 96% inverter efficiency and no rear-side gain. With no ZIP, custom specs run the US-average hourly model, which can take up to a minute the first time.

If the model can't run

• Service at its limit or unreachable: the status says what could not run. An applicable hourly estimate stays shown: a selected kit can use its precomputed US average, and custom specs can keep an applicable US hourly estimate. Only when custom specs have no applicable hourly estimate does a service limit trigger the labeled simple daily estimate - peak sun hours x 365 x panel kW x production factor. If the model simply cannot be reached, no estimate is shown, only a message and a Retry hourly model button. Nothing retries on its own.
• The production factor folds every production-side effect into one number, including the inverter limit's effect and rear-side gain where applicable, and it is applied once to the panel watts. Editing sun hours or production factor switches to a labeled manual estimate: sun hours x 365 x panel kW x production factor. Changing ZIP, panel watts, inverter output or mounting resets these values to model defaults. Bill, rate, self-use and rate-rise edits keep your manual values.

From energy to dollars

• Savings = used solar x your rate: only the share of solar your home uses while it is being made counts. Anything unused flows back to the grid with no savings or export credit in this estimate, and no tax credits or incentives are included.
• Bill comparison: the monthly bill starts at $151, rounded from the 2025 US residential average of $150.68 in EIA's final annual data. Replace this example with your own average bill. The percentage is first-year monthly savings divided by that bill, even when the rising-rate option shows 25-year average dollar savings. The bill does not set household energy use, cap savings or determine how much solar your home uses. The editable $0.18/kWh starter rate is a separate example, not the matching 2025 US average rate.
• Rounding: Monthly savings are shown with cents; annual and 25-year savings are rounded to whole dollars. Every total is computed from unrounded values, so the rounded figures may not multiply out exactly.
• The 25-year total: The 25-year total assumes panels degrade 0.5% per year and your rate stays flat. The optional rising-rate scenario adds 3% per year, roughly the US average residential price increase from 2013 to 2023 (about 12 to 16 cents per kWh, EIA).
• What it leaves out: real savings depend on weather, placement, shading, your rate plan and how much power your home uses while the sun is out. Batteries and time-of-use rates can be worth more than this flat-rate math shows; they are not modeled here.

Full method, sources and worked examples: How We Calculate Plug-In Solar Savings.