Estimate Your Solar Savings in Five Steps

You can build a credible solar savings estimate in about thirty minutes, using nothing but your utility bills and arithmetic — before any installer visits, before any sales pitch frames the numbers for you. That last part is the whole point. Walking into a quote already holding your own estimate flips the conversation: proposals get measured against your number instead of quietly defining reality for you. A projection that lands far from what you calculated now owes you an explanation, and the explanation always points at one specific input you can then go pin down.

The method runs on five moves — pull your usage, find your real rate, translate a system into production, discount for how exports are paid, and turn the result into a payback period. A single running example threads through all of them so you can watch the arithmetic accumulate; every figure in it is an assumption to swap for your own.

Start with a year of usage and your real rate

Solar savings begin with one number: how many kilowatt-hours your home uses in a year. Not a month — a year, because usage swings hard with the seasons and a single bill will mislead you badly in either direction depending on whether you grabbed a July or an April statement. Log into your utility account and find the usage history, where nearly every utility shows twelve to twenty-four months of monthly kWh, and add up the last twelve. If you can only find a handful of bills, average what you have and multiply by twelve, but note the distortion: a summer-only sample runs high in a cooling-dominated climate and a winter-only sample runs high where heating is electric, so a partial year skews in a direction you should at least be aware of. For a sense of whether your total is normal, average home electricity usage lays out the national and regional benchmarks, though your own number is the one that matters. One forward-looking adjustment belongs here and nowhere else: if an EV, a heat pump, or a pool is arriving in the next couple of years, add its expected consumption now, because solar sized to your past will be undersized for your future, and adding capacity later is a more expensive project than sizing it right the first time. In the running example, assume 13,200 kWh over the last twelve months — an even 1,100 kWh a month.

A couple of data snags come up often enough to plan for. If your utility shows only a handful of months online, most will mail or email a full twelve- or twenty-four-month history on request, and it’s worth the short wait rather than extrapolating from a lopsided sample. If you already have solar, or recently added a big load like an EV, the usage figure on your bills reflects that arrangement rather than the home’s underlying appetite, so you may need to reconstruct gross consumption from the generation and net numbers, or from the period before the change, to size honestly. And if you heat or cool with electricity, expect one or two months to tower over the rest — that’s normal, and it’s exactly why a single bill makes such a poor basis for the estimate. The goal at this step is a clean whole-year total that represents a typical year, because every downstream number is a percentage of it, and an error here scales quietly through the entire calculation.

With annual usage in hand, the second number is what a kilowatt-hour actually costs you, and this is where most DIY estimates quietly go wrong in solar’s favor rather than against it. Take a recent bill and divide the total amount due by the kWh billed. That is your all-in rate, and it is almost always higher than the rate printed as your “supply” or “energy” charge, because it captures delivery charges, riders, taxes, and fixed fees spread across your consumption. Solar offsets most of those volumetric charges, not just the advertised supply portion, so the all-in figure is the honest one for savings math — and skipping this step to use the advertised rate instead understates your savings meaningfully, which is one of the few errors in this whole process that works against solar rather than for it. Do the division for two or three different months and average them, since some fee structures shift the effective rate seasonally and a single month can mislead. In the running example, a $198 bill covering 1,100 kWh gives $0.18 per kWh — and holding those two numbers together, 13,200 kWh a year at $0.18, already tells you the home spends roughly $2,376 a year on electricity, which is the ceiling on what solar could ever save and a useful reality check before going further. Everything after this point is about figuring out how much of that ceiling a real system actually reaches, and the honest answer is almost never all of it.

Turn a system into production, then discount for how exports are paid

The third move translates a hypothetical system into annual kilowatt-hours, using a shorthand that gets you close: annual production is roughly the system’s kW times peak sun hours times 365 times 0.8. Peak sun hours is your location’s average daily solar resource, running from about 3.5 in the cloudiest parts of the country to 5.5 or more in the desert Southwest, and a quick search for your state’s figure lands you near enough for an estimate. The 0.8 factor is doing real work — it bundles the real-world losses that separate a lab rating from a roof: inverter conversion, wiring resistance, heat, dust, and the rest. You can run the formula in whichever direction fits your question. To size a system that offsets all your usage, divide your annual kWh by (sun hours times 365 times 0.8); to test a specific quote, push its size through the formula and compare the result against the installer’s production claim, treating any projection wildly above this shorthand as something that owes you an explanation. In the running example, at 4.5 sun hours, offsetting 13,200 kWh needs 13,200 divided by (4.5 × 365 × 0.8), which is about 10 kW, and a 10 kW system produces roughly 13,140 kWh a year. If your roof can’t hold that much, carry whatever fits and use the smaller production number from here on, since a partial offset saves proportionally rather than catastrophically.

The fourth move is the one most DIY estimates skip entirely, and skipping it is exactly why they tend to run optimistic. Your home does not consume most of its solar the instant it’s generated — a large share exports to the grid, and what that exported power earns depends on your utility’s rules, which can swing the final answer by a quarter or more. Under full net metering, exports earn the retail rate and no discount is needed, so production times rate is your savings and the math stays clean. Under net billing or avoided-cost structures, exports earn less than retail, and only the solar you consume in real time is worth the full rate. A serviceable approximation is to assume 30 to 50% of production is self-consumed — higher if someone is home on weekdays or you can shift laundry, dishwashing, and EV charging into the midday window — and value the remainder at your utility’s export rate, which you find in its net metering or net billing tariff. That ten minutes of searching is worth it, because this single policy fact is often the pivotal variable in the whole estimate. In the running example, assume net billing pays exports $0.08 per kWh with 40% self-consumption. Savings then come to (13,140 × 0.40 × $0.18) plus (13,140 × 0.60 × $0.08), which is about $946 plus $631, or roughly $1,577 a year. Under full net metering the same system would save 13,140 × $0.18, about $2,365 — so the export rule alone is a $788-a-year difference on identical hardware, which is a larger swing than most homeowners would get from moving to a sunnier state. This is the moment the estimate usually reveals which fact actually governs your decision, and for this hypothetical home it is plainly the export policy, not the sunshine or the system price.

From annual savings to a payback you can trust

Annual savings alone can’t answer “is it worth it?” — for that you need the cost alongside it and the ratio between the two. If you have no quotes yet, estimate system cost at a mid-band assumption of $3.00 per watt installed, subtract the 30% federal Residential Clean Energy Credit, and divide the net cost by annual savings to get payback in years. In the running example, 10 kW at $3.00 per watt is $30,000 gross, or $21,000 after the credit. Under net billing, $21,000 divided by $1,577 is about 13.3 years; under full net metering, $21,000 divided by $2,365 is about 8.9 years. Interpreting that result doesn’t take a finance degree: paybacks under roughly 7 years are excellent, 7 to 11 is solid, and anything beyond that is worth a harder look at either your assumptions or your market before you commit, with the full framework laid out in the payback period guide. One deliberate conservatism in this method deserves to be named out loud, because it changes how you should read the number. The calculation values every future kilowatt-hour at today’s rate, and utility rates have historically drifted upward over time, so a system’s year-fifteen savings will likely exceed its year-one savings in nominal dollars. Leaving that escalation out keeps the estimate honest — you aren’t counting chickens from a rate forecast nobody can make reliably — and it means your real payback will tend to arrive somewhat sooner than the figure you calculated, not later. If the estimate clears your bar without assuming any rate increases, it clears it robustly, which is a much more comfortable place to stand than a rosy projection that needs rates to cooperate.

Before you treat any of this as settled, wiggle the soft inputs, because a number that only works under one precise set of assumptions isn’t really an answer. Run the export step at both 30% and 50% self-consumption, and the cost step at both $2.75 and $3.50 per watt. If payback stays attractive across that whole range, you have a genuinely robust case and can stop worrying about precision. If it swings from 8 years to 15 depending on which end of the range you pick, you have learned something more valuable than a single number — you now know exactly which fact to nail down before signing anything, and where to spend your remaining diligence. The solar ROI calculator automates this entire chain and makes the sensitivity checks instant, so you can watch the payback move as you drag each input, and how much solar panels actually save gives you benchmark savings figures to sanity-check your result against a broader sample rather than trusting your own arithmetic in isolation.

Then, and only then, get two or three quotes. With your own estimate in hand you will be evaluating those proposals rather than absorbing them, which is a completely different posture than the one most homeowners bring to a sales appointment. Any quote that lands far from your number owes you a reason, and the reason will always trace back to one of the five moves — your usage figure, your real rate, the production assumption, the export treatment, or the price per watt. Knowing which input is in dispute is what lets you push back on a specific line instead of vaguely distrusting the whole proposal, and that specificity is the entire payoff of spending thirty minutes on the arithmetic yourself.